Atomic Absorption, Atomic Emission, and Flame Emission

Anal. Chem. 1996, 68, 231R-256R

Atomic Absorption, Atomic Emission, and Flame Emission Spectrometry Kenneth W. Jackson* and Guoru Chen

Wadsworth Center, New York State Department of Health, and School of Public Health, State University of New York, Albany, New York 12201-0509 Review Contents Fundamental Data Instrumentation Signal and Data Processing Electrothermal Atomization Instrumentation and Operation Atomization Efficiency and Characteristic Mass Atomization Mechanisms Interferences Modifiers Flame Atomic Absorption and Emission Instrumentation and Operation Atomization and Interferences Laser Techniques Laser-Excited Atomic Fluorescence Spectrometry (LEAFS) Laser Atomic Absorption Spectrometry (LAAS) Laser-Enhance Ionization (LEI) Spectrometry Other Techniques Furnace Atomization plasma Emission Spectrometry (FAPES) Furnace Atomization Nonthermal Excitation Spectrometry (FANES) Coherent Forward Scattering (CFS) Glow Discharge Atomic Absorption and Fluorescence Spectrometry Plasma Atomic Absorption and Fluorescence Spectrometry Analysis of Solids and Slurries Vapor-Phase Sample Introduction Hydride Generation Techniques Cold-Vapor Mercury Determinations Other Gaseous Metals Flow Injection Analysis Sample Preparation Separation Matrix Isolation and Preconcentration Speciation Literature Cited

231R 232R 232R 232R 232R 235R 235R 237R 238R 239R 239R 240R 241R 241R 241R 241R 241R 241R 242R 242R 242R 242R 242R 244R 244R 245R 246R 246R 247R 247R 247R 248R 249R

This review covers literature published during the two-year period since the last review in the series (1). The major emphasis is on analytical atomic absorption and fluorescence spectrometry, but also included are emission techniques that are not covered in the review on plasma emission spectrometry appearing in this issue. We have attempted to provide both a critical and a comprehensive discussion of the significant published fundamental research, but mostly we have avoided reference to papers published in languages other than English. Where foreign language papers have been cited, we have also included the Chemical Abstracts citation with those references in the bibliography. Applied research is not within the scope of this review, and applications papers are discussed only if we believe they are S0003-2700(96)00012-1 CCC: $25.00

© 1996 American Chemical Society

of relevance to related fundamental work. Comprehensive coverage of applied research is provided in the applications reviews that are published every other year in this journal, and in the Updates section of the Journal of Analytical Atomic Spectrometry. An exhaustive listing of fundamental and applied research appears biannually in Atomic Spectroscopy. The most published area of research continues to be electrothermal atomic absorption spectrometry (ETAAS), where major emphasis is being placed on the spatial distribution of gaseous species and their secondary heterogeneous reactions at the atomizer surface. Improved designs of graphite furnace electrothermal atomizers (ETAs) and better understanding of interference mechanisms are facilitating the development of methods that are relatively interference-free. Although slurry-ETAAS remains popular, there is increased interest in glow discharge (GD) atomic absorption and fluorescence techniques for the direct analysis of solids. On-line techniques, such as the use of flow injection (FI), and gas and liquid chromatographs coupled with atomic spectrometric detectors, are continuing to be popular for the speciation of metals in biological and environmental samples. Published books of general interest include Advances in Atomic Spectroscopy, edited by Sneddon (2), and Flame Spectrometry in Environmental Chemical Analysis, by Cresser (3). Two other books have chapters on atomic spectrometry (4, 5). Reference to some specialized books may also be found in the relevant sections of this review. An overview was published (6) of software development for AAS over the past decade. Greenfield (7) provided an historical perspective of atomic fluorescence spectrometry and predicted its possible future development. FUNDAMENTAL DATA In recent years, the availability of more accurate fundamental data, particularly oscillator strengths, has improved considerably. Wiese (8) reviewed the status of atomic spectroscopic databases, with respect to wavelengths, energy levels, and transition probabilities. Also, Doidge (9) presented an extremely useful review that summarized literature data for oscillator strengths of resonance lines of 65 elements. Hannaford (10) presented some revised oscillator strengths. A database designed for astrophysical studies (11) should be of interest in analytical spectrometry, since it provides wavelengths, statistical weights, and oscillator strengths for 2249 atomic and ionic spectral lines. Software displaying the visible emission spectra of more than 80 elements was described (12). The current literature on fundamental reference data continues to be covered in Spectrochimica Acta, Part B. In the first of these reports to appear during this reviewing period (13), the work on fundamental data produced by the Laser Spectroscopy Group at the CSIRO in Australia was reviewed. Of particular relevance to AAS were papers dealing with radiative lifetimes, Analytical Chemistry, Vol. 68, No. 12, June 15, 1996 231R

oscillator strengths, and transition probabilities. Most noteworthy in the second report (14) is information on the NIST Physics Laboratory home page, accessible on the Internet, and including a bibliography of the literature on atomic transition probabilities. INSTRUMENTATION Most references to instrumental developments can be found in the relevant sections of this review. This section is reserved for generic instrumentation that may be applicable to more than one technique. The Updates pages of the Journal of Analytical Atomic Spectrometry featured two reviews with sections on instrumentation (15, 16). Florek and Becker-Ross (17) designed a compact echelle monochromator with external order separation by means of a prism. It was used to build a prototype continuumsource AA spectrometer with a charge-coupled device (CCD) detector. The principles of CCDs and their uses in analytical spectrometry are presented in a book (18). Although there is no reference to their use in AAS, the book will be useful for anyone planning to incorporate these detectors into multielement AA spectrometers. Sneddon et al. (19) reviewed the development, performance and applications of multielement AAS. A continuumsource background correction system was incorporated into a previously described frequency-modulated AA spectrometer (20). Radiation from three hollow cathode lamps and the continuum lamp was multiplexed by fiber optics. Demodulation permitted background correction to be performed on each of three analytes in their respective output channels. Niemax et al. (21) reviewed the application of semiconductor diode lasers in analytical atomic spectrometry. Advantages cited included rapid wavelength tunability, small size, low power consumption, and reliability. Further references to the use of diode lasers in AAS are in the section on Laser Atomic Absorption Spectrometry. SIGNAL AND DATA PROCESSING A further publication by Winefordner et al. (22) on theoretical and practical limits in atomic spectrometry will continue to help create a better understanding of the concepts of limit of detection (LOD), limit of guarantee of purity (LOG), and limit of quantitation (LOQ). The major figures of merit that limit the LOD and LOG were described as the efficiency of detection and efficiency of measurement. These figures of merit were then used, together with noise expressions, to develop expressions for the LOD. Several papers described chemometric experimental designs for optimization of analytical parameters. Lan et al. (23, 24) used an orthogonal array design that was said to combine the advantages of sequential Simplex and simultaneous factorial methods. The approach was illustrated with a typical analytical method, the digestion of biological samples and determination of Se by hydride generation AAS. In the first of two papers (23), a two-level orthogonal design was used with seven experimental variables. This was refined in the second paper (24) with a four-level design that considered the three most significant variables. A composite experimental design was described (25) for comparing two methods for the determination of Mg in chlorophyll. Penninckx et al. (26) described a knowledge-based computer system for the detection of matrix interferences in AAS, by comparing the slopes of a standard addition curve and an aqueous calibration curve. Zehr and Maryott (27) compared dilution schemes in terms of precision and accuracy. 232R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

ELECTROTHERMAL ATOMIZATION Trace element determinations by graphite furnace AAS were reviewed by Slavin (28). Tsalev (29) gave an overview of progress over the past decade in applications of ETAAS to analyses associated with occupational and environmental health practice. Falk (30) discussed the research of the late Klaus Dittrich in graphite furnace techniques. Atomic emission in graphite furnaces was reviewed by Baxter and Frech (31). Instrumentation and Operation. Previously, the use of end caps was shown to reduce diffusional loss in a transversely heated graphite furnace atomizer (THGA). This was further investigated by Hadgu and Frech (32), who found that the best sensitivity increase occurred for elements with high atomization temperatures, where diffusional loss would be more pronounced. End caps with apertures of 3 mm or more did not decrease spectral throughput, nor did they promote high temperature gradients in the tube. A better signal-to-background ratio was seen with end capped tubes, since background absorbance is the same with or without end caps. Smith and Harnly (33) studied the contribution of convection to analyte loss in a THGA by conducting experiments at elevated pressure (6 atm). This required the use of a continuum-source AA spectrometer with linear photodiode array detection, which avoided problems of broadening and spectral line shift that occurs with a line source at high pressure. By comparing experimental data with a simple model, it was concluded that rate of analyte loss by convection at 1 atm pressure was about one-eighth of the rate of loss by diffusion. Improvements in sensitivity of 4-6-fold were realized by operating at 6 atm compared with conventional 1-atm operation. Voigtman et al. (34) modeled a THGA Zeeman AA spectrometer using a polychromatic optical calculus simulation software program, which allowed the effects of various instrumental parameters and their interactions to be studied. Modeling of stray light interference showed that its major contribution is from polychromaticity of the light source, even when the source profile has a relatively narrow bandwidth. Gas temperature measurements of a THGA were conducted (35), using coherent anti-Stokes Raman scattering (CARS) measurements. Applications papers continue to show the advantages of THGA over conventional end heated furnaces. For the determination of Al in blood plasma and urine (36), analyte carryover was said to be less with the THGA, which also permitted a lower atomization temperature and a faster atomization cycle. Use of a THGA permitted lower detection limits for Ni, Co, and Cr in water and urine (37) and eliminated the gas phase and covolatilization interferences of chloride and sulfate that occur in the determination of Ag in an end heated furnace. A THGA was combined with a flow injection (FI) hydride generation system for the determination of organolead compounds (38, 39). Katskov et al. (40) described a two-channel spectrometer with a two-step furnace, designed for the study of high-temperature processes taking place in ETAAS. Further details are provided below in the section on atomization mechanisms. Several instrumental designs for multielement ETAAS have been reported over the years, and from a commercial standpoint the most significant may be a completely new design of instrument with a THGA furnace that uses a high-throughput echelle polychromator, high quantum efficiency solid-state detectors, and longitudinal Zeeman effect background correction (41, 42). Radziuk et al. (43) described the design and performance characteristics of the solid-state detector used in this instrument.

Compared with continuum-source instruments, this spectrometer is limited in the number of elements that can be determined simultaneously, through the need to multiplex hollow cathode lamps. However, even when operated under compromise conditions for the simultaneous determination of Ag, Cd, Pb, and Sb in potable water, instrumental detection limits were at least as good as with conventional single-element operation (44). Harnly and Radziuk (45) studied the compromise conditions that would be best for simultaneous multielement ETAAS with a THGA instrument. Using end caps and a Pd modifier, an atomization temperature of 2500 °C produced the best signal-to-noise ratios (SNR) for Cd, Pb, Cu, Cr, and V, so no compromise would be needed with multielement operation. Edel et al. (46) described a simultaneous multielement system in which three multiplexed hollow cathode lamps were modulated at different frequencies to separate the light of each source from the others. This simplified the optical system by permitting the use of interference filters in place of a monochromator. For the simultaneous determination of three elements, characteristic masses, detection limits, and working ranges were in good agreement with conventional singleelement ETAAS. Also determined with this instrument (47) were several elements in a number of complex matrices. A commercial four-channel spectrometer was used to determine several elements in rocks, lake, and stream sediments by ETAAS (48). Smith and Harnly (49) reported analytical figures of merit for several elements using a continuum-source ETA spectrometer with an echelle polychromator and a linear photodiode array detector. They compared the sensitivity of continuum-source and line-source ETAAS by introducing the concept of intrinsic mass, based on wavelength and time-integrated absorbance, and having units of picometer seconds. For most elements, the intrinsic masses agreed within a factor of 2 and detection limits were similar for continuum-source and line-source ETAAS. Fernando and Jones (50) further developed their previously reported system that used a Xe arc lamp and Czerny-Turner monochromator with a linear photodiode array detector in the focal plane. Multielement determinations were limited to a 10-nm spectral window, but detection limits were generally within a factor of 2 of those with single-element line-source ETAAS. The use of a continuum source for multielement AAS avoids the multiplexing problems of hollow cathode lamps, but the typical Xe arc lamps are noisy in the ultraviolet region of the spectrum. Smith et al. (51) increased the spectral intensities of Xe arc lamps by pulsing them to higher operating currents. Although this shortened the lifetimes of the lamps, the resulting increase in intensity led to a comparable improvement in detection limit, since the instrument is detector noise limited. Becker-Ross et al. (52), using an instrument with a Xe flashlamp continuum source, echelle monochromator, and linear array CCD, corrected for high source fluctuations by using the simultaneously recorded spectral vicinity of the absorption line. Williams and Green (53), used a lower-noise deuterium arc lamp as a continuum source. An oscillating spectrometer entrance slit caused the wavelength of light to vary sinusoidally with time, and the resulting second-harmonic ripple from the output of the photomultiplier tube (PMT) was amplified by a tuned amplifier. This produced a large SNR improvement that was said to compensate for the sensitivity loss in using this relatively low intensity continuum source. Unfortunately, figures of merit were not provided to substantiate this claim. A fast Fourier transform AA spectrometer (54), with multiplexed hollow cathode lamps,

simultaneously determined up to 10 elements. This is an interesting concept, but detection limits were orders of magnitude poorer than conventional ETAAS. An interesting new graphite furnace design was described by Katskov and co-workers (55, 56). On heating the graphite tube, analyte vapors passed through a graphite filter into the light path. This furnace permitted higher sample volumes (up to 100 µL) and reduced drying times. Lower spectral background and chemical interferences were reported, and further results are awaited with interest. Kitagawa et al. (57) used a similar principle to separate matrix components from biological samples. A 20cm-long glassy carbon tube atomizer was packed with activated charcoal. A sample aliquot was dried in a microsampling cup, which was then inserted into the end of the furnace tube. Sample vapors were driven by a flow of Ar through the activated charcoal, and absorbance measurements were made through a hole drilled transversely through the tube. A double-chamber atomizer was used for direct analysis of powdered samples (58). A porous carbon plate, used as a platform for the determination of Se in serum (59), was said to facilitate the interaction of the analyte with a Pd modifier, since they both permeated into the plate. A comparison of probe, platform, and wall atomization (60) revealed that platforms gave the best analytical performance. Electrographite, pyrocoated graphite, and total pyrolytic graphite were compared as surfaces for the atomization of Cr (61). Ohta et al. (62) used a molybdenum tube atomizer for the determination of Au. Control of the H2/Ar ratio in the purge gas led to improved sensitivity, with characteristic mass and detection limits more than 10 times better than in a graphite atomizer. This furnace was also used for the determination of K in biological materials (63). A tungsten tube atomizer provided improvement in sensitivity, compared with a graphite furnace, for the determination of Al (64) and Li (65). Ohta et al. (66) determined Zn using a high-temperature molybdenum capillary tube that vaporized trace metals and sequentially eluted them with flowing Ar into the light path of an AA spectrometer. This eliminated interference by Al when Zn was determined in Al metal. A Ta platform (67), used in a graphite furnace, enabled La to be determined with improved sensitivity compared with a graphite platform. Ma et al. (68) compared the lifetimes of W foil and La foil platforms used in graphite atomizers. A graphite furnace lined with Ta foil was used for the determination of Zr, Hf, and Nb (69). A Ta-coated graphite surface was shown to reduce interference by chloride on the determination of Li and K in biological fluids (70). Monteiro and Curtius (71) found that a W coating in a graphite tube melted at the high temperatures used for atomization of Ba, and hence a pyrocoated tube gave the best sensitivity and lifetime. A comparison (72) of Zr-, Ta-, Mo-, and W-coated tubes for the determination of Si in biological fluids showed the W coating to provide the best performance. Volynsky (73) suggested new terminology for coating of graphite tubes with metal carbides, since the IUPAC-recommended term is too restrictive. The low-cost W coil atomizer, first described by Berndt and co-workers, may be more prone to interferences compared with enclosed graphite furnaces, but two independent studies (74, 75) showed this device to be applicable to the accurate determination of lead in blood. Hence, the viability was demonstrated of incorporating this atomizer into a low-cost dedicated portable instrument for blood lead determinations (75). Tungsten coil atomizers were used for the determination of Ba (76) and Cd and Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

233R

Pb (77, 78). A Ta coil atomizer was used (79) for the determination of Fe (as the pentacarbonyl) in Ar by atomic fluorescence spectrometry (AFS). Saprykin et al. (80) described an instrument for ETAES, using nonthermal excitation by electron flux. A liquid sample was dried on a W coil, which was irradiated with electrons in an evacuated chamber in order to excite the analyte atoms. The absence of a plasma discharge permitted a high SNR, thus providing detection limits similar to those obtained by ETAAS. A transverse Zeeman effect instrument (81) was claimed to offer improved background correction over commercial instruments. Stevens (82) described a field-on-source Zeeman design, which allowed a significant reduction of magnet size compared with conventional Zeeman systems. The design was more effective in a longitudinal Zeeman configuration rather than a transverse configuration which produced an interfering off-set signal as a result of self-absorption. Sholupov et al. (83) used high-frequency modulated light polarization to increase optical transmission and reduce the noise associated with the temporal background correction error. Hence, improved detection limits were realized. Further work on the use of an algorithm to increase the linear dynamic range in Zeeman instruments was reported (84). The linearization of calibration curves by factors of 3-5 was demonstrated for several elements under a range of lamp currents and slit widths. Yuzefovsky et al. (85) later modified this algorithm, by using Newton’s method of successive approximations, to permit improved linearization of the upper end of calibration curves. L’vov et al. (86) described an automated method for applying this algorithm. L’vov et al. (87) theoretically studied photometric errors in Zeeman effect ETAAS, and evaluated their effect on precision, with particular reference to the use of linearization methods. It was shown how the measurement precision and hence detection limits could be improved when the pulse restoration method of linearization was used. This method for evaluation of detection limit, which is based on a theoretical analysis of photometric error, was developed further (88), with inclusion of the effect of background absorption on the measurement error. The method used four experimental determined parameters, namely, the baseline offset compensation time, integration time, light flux, and background absorbance. Good agreement between calculated and experimental detection limits was reported. The rollover effect is generally associated with Zeeman effect background correction, but this was also studied (89) in a Smith-Hieftje system. Comparisons were reported between various background correction systems, including transverse vs longitudinal Zeeman effect (90), Zeeman effect vs continuum source (91), and Zeeman effect vs continuum source and Smith-Hieftje (92). Gilmudtinov et al. (93, 94) investigated the spatial distribution of radiation from hollow cathode lamps, using a photodiode array imaging system. The shape of the radiational intensity distribution was related to hollow cathode diameter. They questioned (93) the ability of conventional detection with PMTs to provide accurate spectral information, since the recorded absorbance will depend on the spatial distribution of atoms in the furnace. The joint effect of radiation and analyte nonuniformity over the furnace cross section was analyzed, and it was proposed that ETA spectrometers should use solid-state detectors rather than PMTs so that spatially resolved absorbance can be measured. Semiconductor diode lasers offer several advantages over hollow cathode lamps, including wavelength tunability, small size, and low cost. In the 234R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

past, however, they were limited in their applicability to wavelengths of >625 nm. Schnuerer-Patschan et al. (95) showed how the wavelength range can be extended by second-harmonic generation of diode laser radiation in nonlinear crystals. This was demonstrated by the determination of Al at 396.1 nm. It was also shown that detection limits could be reduced through the use of a wavelength modulation technique. Halls (96) reviewed ways to minimize analysis time. Included was consideration of fast furnace techniques, such as elimination of the pyrolysis stage, and injection of sample into a preheated furnace. Hoenig and Cilissen (97) compared fast and conventional programs using different commercial instruments and a variety of sample types. Halls (98) developed a low-cost device to allow a commercial autosampler to be started before the furnace program has ended. This reduced sample throughput time by 10-15 s. The applicability of fast programs for the determination of trace metals in plant samples was tested (99). Li et al. (100) applied fast furnace technology to the THGA, showing how sample throughput time could be reduced from the usual 2 or 3 min to less than 1 min. The sample was introduced to a preheated furnace, the cooldown step was avoided, and in many cases the pyrolysis stage was eliminated and no modifier was used. Chernyakhovskiy et al. (101) described a fast program of 36 s. A fast method for Pb and Cu in drinking water was described (102). Several papers described the application of fast furnace techniques to the analysis of slurries (103-106). Second-surface atomization was reported by Hocquellet (107), who inserted a tantalum carbide-treated graphite vault inside a conventional Massmann furnace to trap the analyte initially vaporized from the wall. This was applied to the determination of Cd in tissues, and it was claimed that reatomization was less affected by spectral and chemical interferences than conventional platform atomization. Berndt and Schaldach (108) described a modification of the aerosol deposition technique. The conventional pneumatic nebulizer was replaced with a hydraulic highpressure nebulizer, which was said to substantially improve the efficiency of sample transport. Grimm and Hermann (109) used a low-pressure furnace for isotope determinations. A Simplex optimization technique for establishing furnace operating conditions was described (110). Variables considered were pyrolysis temperature, atomization temperature, atomization ramp time, and modifier amount. Araujo et al. (111) described a multivariate optimization procedure, based on a fractional factorial design, for the determination of Cd. It was applied to the determination of Cd in cocoa beans (112), when the drying, pyrolysis, and atomization factors were evaluated simultaneously. Wienke et al. (113) presented a modified Kalman filter algorithm that included quality control samples. The linear calibration model automatically performed simultaneous calibration and recalibration, detection and correction of drift, outlier rejection, etc. Kale and Voigtman (114) investigated different signal-processing techniques (peak area by gated integration, peak height, matched filtering) for digitally generated ETA absorbance peaks. The effects of various types of noise upon the precision of absorbance measurement was then evaluated by numerical calculation. Perhaps not surprisingly, it was found that peak area measurements invariably produced better precision irrespective of the type and source of noise. A rapid method for measuring instrumental detection limits (115) was based on the use of a simplified furnace heating program and without use of a blank.

Atomization Efficiency and Characteristic Mass. The reliability of theoretical characteristic mass values is obviously dependent on the accuracy of the fundamental data used in their calculations. Therefore, it is particularly interesting that Hannaford (10) presented a modified theory of oscillator strength calculation and thus determined revised characteristic mass values for several elements. As a result, the ratio of theoretical to experimental characteristic mass was reduced for several elements, suggesting that atomization efficiency may be lower than previously assumed. Su et al. (116) studied characteristic mass, effective stray light, Zeeman rollover absorbance, and Zeeman sensitivity ratio as a function of lamp current and slit width and presented mathematical expressions to describe the relationships of these parameters. It was possible, by taking effective stray light and Zeeman sensitivity ratio into account, to calculate a more precise characteristic mass. Zheng (117) determined atomization efficiencies, based on measurements of residence time and peak and integrated absorbance. It was suggested that atomization efficiency depends on the instrumentation used, since results were not generally in agreement with other results in the literature. However, atomization efficiency was often improved by use of a modifier, and this may stress the importance of adhering to stabilized-temperature platform furnace (STPF) conditions if comparisons are to be made between instruments. Torsi (118) pointed out that a limitation of theoretical characteristic mass is its dependence on the geometry of the atomizer. Instead, he proposed the use of the spectroscopic constant in Beer’s law. Yang and Ni (119) found that experimental atomization efficiencies of Mn, Ga, and Cr were lower than the theoretical values and were temperature dependent. For Pb and Bi, the experimental and calculated values were similar. They also found higher atomization efficiency when a Pd modifier was used. Pupyshev et al. (120) used a thermodynamic model to obtain a relationship between atomization efficiency and temperature and found the results to agree well with experimentation. Reports of analyses based on characteristic mass include the determination of Ag and Cd (121); In (122); Tl (123); In, Ag, and Tl in sediment and geochemical samples (124); and Be in environmental and biological samples (125). Atomization Mechanisms. Studies of the spatial distribution of atomic and molecular species in graphite furnaces are providing valuable insight into atomization processes. Gilmutdinov (126), using the technique of shadow spectral filming, noted that maximum atom density for Zn, Cd, and Hg was seen in the gas phase near to the surface where the sample was deposited. For Al (127), free atoms were located near the furnace wall and under the platform, whereas Al compounds were found further from the walls near the tube axis. As a result, a mechanism leading to the formation of Al atoms was postulated. In the case of Ga and In (128) and Tl, Ge, and Be (129), the occurrence of secondary surface reactions was shown, since atoms appeared to originate from the furnace wall when the sample had been deposited on a platform. It was thought that these analytes evaporate from the nitrate as volatile suboxides and are reduced on the wall to atoms. These atoms vaporize, condense on the cooler platform, and are then reatomized. This was also considered to be the process by which atoms propagate toward the ends of the tube due to the temperature gradient. L’vov (130) suggested that the donut cloud formation, previously reported by Gilmutdinov and co-workers when Al is atomized in a graphite furnace, was due to condensation as a result of temperature gradients in the tube. Goltz et al. (131)

used a CCD camera to investigate the spatial distribution of atomic and molecular species of B. Absorption measurements at 254 nm showed no gaseous molecular species near the deposition site, but they were detected near the top and axial center of the tube. It was suggested that the analyte vaporized initially as boron oxides (which do not absorb at 254 nm), which then thermally dissociate in the gas phase to suboxides. Atomization was considered to occur through high-temperature (>2300 °C) reduction of the suboxide at the graphite surface and heterogeneous dissociation of boron carbide at the surface. Masera et al. (132) used a CCD camera to investigate the spatial distribution of atomic and molecular species in a furnace using both absorption and laser-induced fluorescence measurements. An investigation of the interference of Ag on Au atomization showed that the Ag matrix vaporized before Au, so Ag had no measurable effect on the distribution of Au atoms. However, when Ag was in such large amounts that it started to condense at the cooler ends of the furnace tube before the Au AA signal appeared, there was an effect on the spatial distribution of Au atoms. In recent years, there has been increasing interest in reactions occurring on atomizer surfaces. Since atomization is kinetically controlled, the rate-limiting processes may be more likely to occur in the condensed phase or as heterogeneous surface reactions rather than a gas-phase reaction. Surface reactions are also highly complex, as shown by Steele (133), who reviewed the physical interaction of simple molecules with solid surfaces. Reference to graphite surfaces included adsorption of gases and the structures of monolayers formed by O2, CO, CO2, and other gases on graphite. Previous studies on the atomization of Ag indicated that the mechanism of atomization depends on the dispersion of analyte on the atomizer surface; i.e., a first-order release occurs from microdispersed analyte, but a fractional order of release occurs with higher analyte masses, since the release occurs from microdroplets. Chekalin et al. (134) studied the atomization of Ag in a graphite furnace by laser-induced fluorescence (LIF) under reduced-pressure conditions to minimize the effects of diffusion processes. Since the signal was proportional to pressure, it was shown that gas-phase reactions do not influence the production of Ag atoms. Studies of experimental activation energy suggested that Ag atomizes as individual adatoms under low-pressure and low analyte mass conditions, but under higher pressure conditions, two- and three-dimensional microstructures were said to be formed on the graphite surface. Rojas and Olivares (135) described a two-precursor kinetic atomization mechanism for Ag. A characteristic double absorbance peak was explained through release from dispersed particles and also from clusters, whose size increases as the mass of analyte increases. In a second paper in the series (136) Cu, ascorbic acid, and Triton-X were added, since they are known to interact with graphite surfaces and hence should modify the surface. All three modified the atomization mechanism. Ascorbic acid promoted faster Ag atomization due to an increased number of active sites resulting from its pyrolysis, whereas Cu slowed the rate by blocking the active sites. Aller (137) reported that, for low concentrations of Au, interactions with the graphite surface are very strong and release is kinetically first order. However, Au-graphite interactions decrease at higher concentrations and then the order is fractional. Ni and co-workers published several papers describing studies of the influence of the atomizer surface on the kinetics of atomization. In the case of Sn (138), there was evidence of multiple release mechanisms Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

235R

from a pyrolytically coated surface. A single mechanism was observed from a Zr-coated surface, and it was suggested that zirconium carbide facilitates the formation of Sn atoms. A Pdcoated surface gave a very high activation energy, indicating strong interaction between Sn and Pd. First-order release, indicative of highly dispersed analyte species, was seen for In (139) from pyrocoated and uncoated electrographite surfaces at low temperatures. However, at higher temperatures a one-third order of release was observed, suggesting that the analyte formed surface agglomerates. From a Zr surface, a two-thirds-order mechanism was identified, suggesting release of atoms from a sphere of molten In. First-order release of In from a Pd surface indicated desorption as highly dispersed atoms, which is consistent with its simple release from a Pd solid solution. For Pb (140), a first-order release was seen from a pyrocoated surface and a Pd-coated surface, but a higher activation energy for the release from the Pd-coated surface was indicative of stronger interaction between the analyte and the surface. From a Zr-coated surface a fractional order was observed, with a low activation energy, suggesting that Pb formed three-dimensional structures and interaction with the surface was relatively weak. Fonseca et al. (141) compared three methods for the determination of kinetic parameters in ETAs and found them to give comparable results. A new kinetic method was described (142) for the simultaneous determination of order and activation energy during atomization at constant temperature from a W wire probe. The extremely high resolution of atomic force microscopy permits imaging of surface topography on the atomic scale, and this was used by Habicht et al. (143) to study surface changes during the atomization of Cr from a pyrocoated surface. After just a few firings of a new tube, the protrusions of a few micrometer size were replaced by much smaller protrusions ∼500 nm in size. This led to improved surface homogeneity, which may account for the improved reproducibility obtained after a new tube has been “burned-in”. This technique may have the potential of providing important new information on surface reactions and the influence of the surface on atomization behavior. Scanning tunneling microscopy may also be a powerful means of studying surfaces. This technique was used (144), with Auger electron spectroscopy, to study the intercalation of La on a graphite surface. Surface techniques, including X-ray diffraction and X-ray photoelectron spectroscopy, were used to study the atomization of Eu (145), Sm (146), Sb (147), and Ge (148). In the past, it was observed by several researchers that metal nitrates yielded small concentrations of gaseous products at temperatures far below the thermodynamic decomposition temperatures. This led to two very different mechanisms being proposed. Holcombe and co-workers suggested vaporization of oxide molecules through the explosive disintegration of the salt crystals, and L’vov proposed congruent gasification of all salt decomposition products irrespective of their saturated vapor pressures. In an attempt to resolve the issue, these researchers collaborated to study the gaseous products from the low-temperature thermal decomposition of metal nitrates, vaporized from a graphite or tantalum platform, using quadrupole mass spectrometry. However, they were still unable to agree on the interpretation of the experimental data, so they published separately. L’vov and Novichikhin (149, 150) argued that a qualitative interpretation of the temperature dependence of the gaseous ions detected mass spectrometrically supported the gasificatiom theory. Their cal236R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

culated appearance temperatures and Arrhenius activation energies were also in good agreement with their theory, lending further support to the gasification argument. They also concluded (150) that the frequently reported first-order release of Cu in a graphite furnace may occur through Cu being distributed as particles of the same size irrespective of the sample mass, brought about by redistribution during a three-stage decomposition of the nitrate. Holcombe and co-workers (151) were critical of a correction factor for platform temperature introduced by L’vov and Novichikhin. They concluded that the qualitative experimental data did not support the gasification theory, since metal nitrate ions were detected, and the decomposition of several nitrates did not produce a detectable oxygen ion. Their calculated appearance temperatures and activation energies, based on the gasification theory, were not in agreement with experimental results. Since some of the metal nitrates studied were unlikely to be in solid form at the decomposition temperature, they conceded that the “crystal shattering” theory would not be applicable. However, they showed that the expulsion process depends on the physical form of the sample, indicating a physical expulsion mechanism rather than the chemical gasification mechanism. McAllister (152) refuted the gasification mechanism, because in his experiments metal oxide species were not detected mass spectrometrically. However, L’vov and Novichikhin (149) questioned the sensitivity of McAllister’s instrumentation to detect the oxide species. Other researchers have also studied the low-temperature loss of analyte species from graphite furnaces. Sramkova et al. (153) attributed loss of Tl at temperatures as low as 300 °C to volatilization of the nitrate. Mueller-Vogt et al. (154) postulated that loss of Tl and Bi at low pretreatment temperatures occurred through reduction of the oxides to volatile suboxides at active sites on the graphite surface. Then on collision with the tube wall, this suboxide is reduced to the metal. They reduced the number of active sites by preconditioning the furnace with O2, and this stabilized Tl and Bi to higher pretreatment temperatures. Two-step atomizers, in which the sample is vaporized from a cup into a separately heated tube, provide the means to control separately the vaporization and atomization processes. This can provide new insight into the processes occurring in graphite furnaces. Katskov et al. (40) reviewed research conducted in their laboratory using such an instrument. Evaporation characteristics of many elements in several matrices were studied, and the compositions of the vapors were examined through molecular absorption spectrometry. Their results also demonstrated the validity of applying the Langmuir theory of evaporation to studies of atomization mechanisms. Fewer interferences are encountered when this furnace is used compared with conventional furnaces, and in a later paper (155) they examined the reasons for these reduced interferences from a theoretical and experimental standpoint. It was discovered that molecular vapors interact with the graphite of the filter to form stable compounds, hence reducing their rate of diffusion through graphite and permitting temporal resolution of atomic and background absorption signals. Hadgu et al. (156) constructed a model equation to describe the diffusional losses of analyte vapor through the ends of a THGA with end caps. The model was said to be useful for optimizing the geometry of end capped tubes to produce the best SNR. Torsi et al. (157) presented further experimental validation of a previously postulated diffusion mechanism that assumed instan-

taneous atomization. The model was obeyed for the atomization of Cd and Hg. Electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICPMS) has evolved as a powerful instrumental combination for trace metal determinations. In a typical configuration, a Massmann furnace is used to vaporize the sample, which is then swept into the plasma with a flow of Ar gas. Several researchers have used the technique as a means of studying the chemistry and physics of atomization in ETAAS. Byrne et al. (158) found that W salts formed the oxide at lower temperatures and the carbide at higher temperatures. Losses of B were attributed to its vaporization in a molecular form, and its removal from the furnace without dissociation (159). Goltz et al. (160) reported that some U vaporizes as the oxide at temperatures well below the appearance temperature of U atoms. Sodium chloride interfered by causing intercalation of U into the graphite, resulting in partial retention of the analyte during the atomization step. The addition of Freon prevented this intercalation. Gilmudtinov et al. (161) interfaced a graphite furnace to an ICPMS in order to study the kinetic release of C and CO from the furnace when it was heated in the absence of sample. Kantor et al. (162) used ETVFAAS to study the atomization of Ca in a Ga matrix. Several other relevant papers were published. The reduction of oxides by carbon (ROC) atomization model, proposed by L’vov, supposes that the appearance of periodic spikes in the absorbance signal occurs through a thin carbon shell being formed on the surface of the metal oxide, leading to a breakdown of the autocatalytic ROC process. Deng and Gao (163) used various surface analysis techniques to study such shells formed on graphite and Ta probes during the atomization of La, introduced as the nitrate. Their results suggested that the shells were not formed by C and may have been caused by lattice defects in the surface or through the formation of lanthanum hydroxide from the oxide absorbing water vapor. Fonseca et al. (164) used mass spectrometry to monitor the species evolved from a graphite platform when Cr salts were heated. Two dominant mechanisms found were the decomposition of chromium carbide and the thermal desorption of Cr, which had redeposited at active surface sites. The use of air as a sheath gas (165) gave a higher integrated absorbance for Pb in sulfur, compared with Ar sheath gas. Raman spectroscopy was used to study the molecular species of Pb during sample pyrolysis. Pupyshev (166) used thermodynamic simulation to describe the degree of ionization of elements in a graphite furnace. Studies were published on the atomization of B (167), Ga (168), and Al (169). Liang and Ni (170) developed a computer program that was described as a toolbox for theoretical studies in ETAAS. Functions included the fitting and filtering of data, on-line data collection, calculation of Arrhenius activation energy, and modeling of absorbance profiles. Interferences. The most widely studied interferant in ETAAS is undoubtedly chloride, yet there continues to be considerable debate over the most likely mechanisms causing chloride interference. Akman and Doner published several papers describing the use of a dual-cavity platform to physically separate the analyte and chloride salt. In this way it was hoped to differentiate between gas phase and other interference mechanisms. Decreased interference was seen when Zn and Co were separated from a NaCl matrix (171), indicating a condensed-phase interference reaction. Losses were also partly attributed to occlusion of analytes in microcrystals of interferant, which were swept from the furnace

without decomposing. However, the use of separate platform cavities did not completely eliminate the interference, and it was suggested that expulsion of analytes occurred as a result of expansion of gaseous products from the chloride matrix. Interference by CoCl2 on Zn was said to be due mostly to formation of volatile ZnCl2 in the condensed phase during the furnace pretreatment stage (172) and its expulsion from the furnace together with HCl produced from hydrolysis of CoCl2. However, the possibility of gas-phase reactions and expulsion mechanisms, especially at low temperatures, was not ruled out. Nickel chloride interfered in the determination of Zn and Co (173) by forming analyte chlorides both in the condensed phase and by reaction between analyte species and HCl gas produced by hydrolysis of NiCl2. At low pyrolysis temperatures, NiCl2 is not significantly hydrolyzed, and then losses were said to be caused by expulsion of analyte with the salt matrix or gas-phase reactions during the atomization stage. Qiao et al. (174) noted that when Tl is atomized in the presence of a NaCl matrix, there is a sharp dependence of Tl absorbance on pyrolysis temperature, indicating loss of volatile TlCl2 during pyrolysis; i.e., TlCl2 must be formed in the condensed phase. Cabon and Le Bihan (175) studied the interference of a seawater matrix on the determination of Zn. Besides loss of volatile ZnCl2, they reported a Zeeman interference effect from the vaporization of the chloride matrix leading to a systematic background undercompensation. When seawater was modified by adding HNO3, a spectral interference from Zeeman splitting of the NO absorption bands was noted. Pszonicki and Essed (176) studied interference by chloride on the determination of Pb. They reported (177) that Pd alone as a modifier did not remove chloride interference, but strong acidification or addition of Mg or NaNO3 did. The method of standard additions and successive dilutions was used (178) to remove chloride interference on Cd, As, and Pb in the presence of a Pd modifier. Ammonium acetate was used as a modifier to prevent chloride interfering with the determination of Ge (179). Chuang and Huang (180) reported that a phosphate/ HNO3 modifier permitted the interference-free determination of Cd in seawater. Huang and Shih (181) found NH4NO3 to be effective in removing chloride interference on Cu, thus permitting its determination in seawater. Imai et al. (182) studied the effects of organic matrices on the Au absorbance peak shape and position with respect to time. Multiple peaks and both early and late shifts were observed in the presence of various organic compounds. An early shift was attributed to the formation of smaller Au microdroplets through adsorption on the active C resulting from pyrolysis of the matrix. A late shift was attributed to larger size droplets being formed through entrapment of analyte inside thermally stable C residues. Penninckx et al. (183) developed a method for studying changes in absorbance peak shapes due to interferences. A multivariate statistical test was used to monitor changes in peak characteristics compared with a reference peak from the analyte in the absence of the matrix. Thermal derivative analysis and infrared spectroscopy were used (184) to study the interference of an Al matrix on the determination of Cd. It was determined that interference occurs through Al forming a spinel structure with Cd. Zong et al. (185) reported a background overcorrection interference when determining Pb in the presence of phosphate by Zeeman-effect ETAAS. They suggested that the problem was caused by molecular absorption of PO, since PO bands appeared to undergo splitting in the magnetic field. Interestingly, this Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

237R

problem occurred when a longitudinal Zeeman-effect instrument was used, but not a transverse Zeeman-effect instrument. Kurfuerst and Pauwels (186) noted that high concentrations of S caused spectral interference on the Pb line at 283.3 nm, when solid samples were analyzed. This interference was said to be due to molecular absorption by S2. Epstein et al. (187) reported a spectral interference by Pb nonresonance AA on both the primary As lines at 193.7 and 197.2 nm, when analyzing Pb-based alloys. They were able to overcome the interference at 197.2 nm by arranging conditions so that temporal overlap of the As and Pb absorption lines did not occur. Modifiers. Metal salts continue to be the most popular modifiers in ETAAS. They stabilize many analytes during the furnace pretreatment stage, thus permitting the use of higher pyrolysis temperatures when volatile matrix components may be removed. Also, the absorbance signal is frequently delayed in time until the furnace has reached a higher and more stable temperature. Morishige et al. (188) investigated the stabilizing effects of several metallic modifiers on Cd and Sb. The reason for analyte stabilization by Pd during pyrolysis was said to be formation of intermetallic compounds which decrease the thermodynamic activity and vapor pressure of Cd. The addition of Mg to Pd reduced the chloride interference that occurred with Pd alone, and this was thought to be due to reaction of MgCl2 to form HCl. The stabilizing effects of Pt and Ni on Cd were similar to those of Pd. Palladium had a less pronounced stabilizing effect on Sb, perhaps because the intermetallic compounds have lower melting points compared with the Cd-Pd compounds. However, the Ni-Sb alloy had a higher melting point, so Ni stabilized Sb more effectively. Ouishi et al. (189), assuming that metallic modifiers stabilize analytes through alloy formation, used activity coefficients as an indication of the strength of interaction between the analyte and metallic modifier. A second report from the same research group (190) showed the activity coefficient decreasing as the appearance temperature of the absorbance peak increased through stabilization by the modifier. Yasuda et al. (191) used high-resolution transmission electron microscopy to observe vaporization of atoms from a Sn-Pd alloy. The alloy was placed on a heated wire, and as the temperature increased, vaporization of atomic layers was observed. Hirano et al. (192) noted that measured Pb atomic vapor temperatures increased in the presence of Pd, but only if atomization was rapid. They attributed this phenomenon to an endothermic vaporization process through Pb being released from a Pd-Pb alloy. This phenomenon was also noted for As when Pd was used as a modifier (193). A paper that might be of interest to researchers in ETAAS described X-ray absorption studies of the electronic structures of Pd-Ag and PdAu alloys (194). Stabilization of Si by Pd was thought (195) to be due to formation of an intermetallic compound that prevented the formation of volatile siilicon oxide or refractory silicon carbide. The use of X-ray diffraction confirmed the formation of intermetallic compounds between Sn and Pd (196) and Sn and the modifiers Mo, W, Pd, and Zr (197). Compounds formed between In and several metallic modifiers were studied by secondary impact mass spectrometry (SIMS) (198). Liang and Ni (199) measured Arrhenius activation energies for several elements with and without a Pd modifier. Medium-volatility elements, such as Mn and Co, did not show a significant change in activation energy in the presence of Pd, but high-volatility elements such as Ag and Tl did. This is consistent with the improved thermal stabilization 238R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

of these volatile elements by Pd. In the absence of modifier, it has been shown by several researchers that atomization of Au involves a fractional order of release from the graphite surface. Thomaidis et al. (200) showed that release becomes first order in the presence of Pd, Ni, Cu, Rh, and Rh/Re as modifiers. The best sensitivity improvement was seen with Rh/Re, Pd/ascorbic acid, and Rh/ascorbic acid. Aller (137) reported that the fractional order of release of Au became first order in the presence of a V modifier. The presence of Pd was shown (138) to shift the absorbance signal for Sn to a higher temperature and to increase the activation energy. Yan et al. (139), from results of a kinetic study of release of In from a Pd modifier, suggested that atomization occurs simply through release of In from the solid solution of In in Pd. Similar physical effects were suggested by Mahmood et al. (201), in studying the stabilization of Se by Ni or Cu modifier. Although it was acknowledged that chemical interaction between analyte and modifier may be significant, it was considered that the absorbance peak shift to higher temperature during atomization was mainly due to physical effects. This was evidenced by changes in appearance of the modifier deposits on the graphite platform surface during heating. These changes were observed visually and by scanning electron microscopy. It was also observed that replacement of the Ar purge gas by H2 during the drying stage resulted in strong adsorption of H2 on active C sites and this eliminated a characteristic dip that occurred in the pyrolysis curves at 800 °C. Qiao et al. (174) reported that the reduction of chloride interference on Tl is more effective if the Pd is prepyrolyzed. This results in the low-temperature adsorption of Tl on the Pd deposit, where it remains until the NaCl matrix has been volatilized at a pyrolysis temperature of 1100 °C. The thermal stabilization of Tl during the atomization stage was considered to be physical in nature, involving Tl becoming embedded in molten Pd and then being released slowly. An inconvenience with the use of Pd and other metallic modifiers is the need to add the modifier with every sample aliquot, since it is lost through volatilization during the typical furnace cleanout stage. Bulska and Jedral (202) electroplated Pd and Rh onto a tube surface and found both of these modifiers to permit repeated firing of the tubes for the determination of As, Se, and Si (80 firings with Pd and 160 with Rh) without having to replace the modifier. Hanna et al. (203) coated Ir onto the inner surface of a graphite tube simply by pipeting an Ir solution into the tube and heating it to 2200 °C. This coated furnace was used for sequestration of volatile hydrides and could be fired up to 300 times before the coating had to be replaced. Rademeyer et al. (204) showed that Ir can be retained on the graphite surface through at least 700 firings if it has been deposited on the inner surface of the tube by cathodic sputtering in a low-pressure Ar discharge. They found that Ir was deposited uniformly over the inner surface of the graphite tube, whereas a nonuniform layer resulted when Ir was deposited from solution. This modifier was similar to Pd in stabilizing volatile elements to higher pyrolysis temperatures. Matousek and Powell (205) electroplated Pd and analyte onto the inner surface of a graphite tube. A comparison was made between Pd and analyte plated separately and plated together. They reported that analyte stabilization occurred through surface bonding of the analyte to the Pd deposit. A significantly lower mass of modifier (0.25 mg) was required, compared with its conventional introduction by pipet.

Shan and Wen (206) compared Pd, Pd/ascorbic acid, and Pd/ Mg as modifiers for the stabilization of a wide range of analyte elements. This is an interesting study, since the ETA community appears to be divided, with many researchers claiming that a reducing agent such as ascorbic acid is needed, while others claim that Pd can be used alone, or that the addition of Mg aids recovery and produces sharper absorbance peaks. In terms of maximum pyrolysis temperatures, characteristic masses, and removal of chloride and sulfate interferences, all three modifiers were similar. Also, they all produced similar late shifts in atomization times for slurry samples. However, higher background absorbances were reported during the determination of some elements in the presence of the Pd/Mg modifier. Nevertheless, a Pd/Mg mixture continues to be one of the most popular combinations of modifiers. In two separate studies (207, 208), a Pd/Mg mixture was preferred over several other metallic modifier combinations for the determination of Se. Rayson and Fresquez (209) studied the time dependence of Ag and Cu loss during thermal pretreatment in the presence of a Pd/Mg modifier. The use of Pd alone resulted in loss of Ag that increased with pyrolysis time, but a mixture of Pd/Mg did not show this loss. However, for the determination of Cu, the Pd/Mg mixture as well as Pd showed increasing loss with pyrolysis time. Several papers described combinations of Pd with metals other than Mg. Ni et al. (210) compared Pd, La, Sr, Ba, Pd/Sr, Pd/Ba, Pd/Mg/Sr, and Pd/Mg/ Ba for minimizing sulfate interference on the determination of Se. Most effective were Pd/Sr and Pd/Ba. Several noble metals were compared as modifiers for the determination of Sb in biological fluids, using a radioactive tracer technique (211). The preferred modifier was a mixture of Pd, Pt, Rh, Ru, and ascorbic acid, and this mixture was reported to stabilize Sb to a pyrolysis temperature at least 100 °C higher than Pd alone. The addition of Al to Pd permitted the pyrolysis temperature for the determination of Pb to be raised by a further 150 °C, compared with Pd alone (212). Cimadevilla et al. (60) used a mixture of Pd and NH4NO3 for the determination of Cd in seawater, but Pd alone was suitable for the determination of As and Pb. A comparison was reported (213) between HNO3, BaF2, Mg, Pd/Mg, and Pd/ hydroxylamine hydrochloride for the determination of Mo in urine. The performance of Pd as a modifier for group IIIB-VIB elements was compared with other modifiers (214). Several metallic modifiers other than Pd were used. Coating the graphite surface with Zr was compared with the use of a Pd/ Mg modifier for the determination of Sn (215). Arruda et al. (216) preferred Mg over Pd, HF, or H3PO4 for the determination of Al in milk. For the stabilization of Mn, in the presence of a NaNO3/ borate matrix resulting from fusion of rock samples, Ni was preferred over Mg or Pd (217). In terms of accuracy and precision, Ni was a better modifier than Pd for the determination of Te (218), though Pd gave a lower detection limit. Tang et al. (219) compared Ca and Mg as modifiers for the determination of Al, and they preferred Ca for the determination in a bone matrix, where Ca is already a major component. Botelho et al. (220) found Ca to be a more suitable modifier than Ni, Mg, Y, or La for the determination of B. Prepyrolyzed Ca was used for the stabilization of Si (221). The stabilization of B (222) by coating the graphite surface with tungsten or lanthanum carbide, or addition of a Ca/Mg modifier, was attributed to prevention of lowtemperature dissociative desorption of boron oxide at active C sites. In a Ta foil lined graphite furnace (69), La was used as a

modifier for the determination of Nb, and Al as a modifier for the determination of Zr. Yuzefovsky and Michel (223) used Ba as a modifier for the determination of fluoride, as MgF2, by laserexcited molecular fluorescence spectrometry. It was postulated that Ba changed the mechanism of formation of MgF2, making it a more efficient reaction. Cabon and Le Bihan (175) compared several modifiers for the determination of Zn in seawater. Reports of the use of nonmetallic modifiers were relatively few. Volynskii (224) reviewed the use of organic modifiers in ETAAS. Ascorbic acid is perhaps the most popular, and its thermal decomposition was studied (225) using thermal and elemental analysis techniques. It was suggested that the delay in atomization of elements such as Pb, Sn, and Ga occurred through analytes diffusing from the C residue resulting from the decomposition of ascorbic acid. Rojas (136) reported that pyrolysis of ascorbic acid produced a deposit containing increased active sites, and this caused faster atomization of Ag. Imai et al. (226) used ETVICPMS to study the decomposition products of ascorbic acid. Identified were hydrocarbons, CO, and CO2 below 580 K, active C species between 600 and 1100 K, and thermally stable C species between 1200 and 2400 K. Raman spectroscopy showed that the thermally stable C residues were vaporized as active C species at higher pyrolysis temperatures. However, some C residue remained after 2150 K, and this was said to be the cause of delay in the appearance times of analyte atoms; i.e., pyrolysis of ascorbic acid produces a less well oriented pyrocoating with many surface defects compared with the original tube coating. It was reported (227) that the organic matrix in pharmaceuticals generally acts as its own modifier, so additional modifier is unnecessary. Hydrogen was used as a modifier (228) by introducing it as a 10% mixture in the Ar purge gas during the cooldown stage prior to the atomization stage. It reduced smoke and hence decreased the background signal from soil and sewage sludge. Alvardo et al. (229) studied several fluorides as modifiers for increasing the permitted pyrolysis temperature when determining P, and preferred NaF, though they reported the sensitivity to be lower than when Pd was used as a modifier. Additional noteworthy applications of modifiers are presented in Table 1. FLAME ATOMIC ABSORPTION AND EMISSION Instrumentation and Operation. Berndt and Mueller described further developments of high-pressure nebulization (247), which provides improved sensitivity and reduced matrix interferences, compared with conventional pneumatic nebulization. Interferences were reduced further (248) by placing an HPLC column between the sample injection valve and the nebulizer, in order to separate interferents so they did not enter the flame at the same time as the analyte. The pulse nebulization of organic extracts of metals (249, 250) enabled sample volumes of 60 µL or less to be used. Detcheva and Havezov (251) studied controlled-dispersion systems for FAAS, citing the advantage of small sample volumes. Tan et al. (252) developed an atom cell that appears to hold considerable promise for the highly sensitive detection of HPLC eluents. The sample was nebulized by thermospray into a H2-O2 diffusion flame in a heated T-shaped quartz tube that was mounted in the optical path of the spectrometer. Detection limits for Cd and Zn approached those obtainable by ETAAS. The advantages of the NO-C2H2 flame, compared with the more common N2O-C2H2 flame, were described for the determination of refractory elements (253). This flame has a Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

239R

Table 1. Applications of Modifiers analyte Ag B B B Be Cd Cd Cd Cr Cr Ge Mn Mo Pb Pb Sb, Se Se Se Se Se, Te Si Tl V Y

matrix coal fly ash alloys drinking and contaminated water seawater seawater, ref materials blood and urine sediment and botanical material serum and lake water seawater seawater river water sugars and syrups solid biological material serum urine alloys urine alloys sludge

modifier

ref

Pd Ni/Zr Ni(NO3)2 Ca/Mg, Ti/ascorbic acid Al(NO3)3

230 231 159 167 232

(NH4)2HPO4/HNO3 Pd/tartaric acid Pd(NO3)2/Mg(NO3)2 Mg(NO3)2, Pd(NO3)2, Triton-100, NaVO3 V/Mo Co/Al nitrates NaOH Pd Co(NO3)2/NH4H2PO4 Mg(NO3)2 glucose Cu/Pd in Triton-100 Pd/ammonium acetate Pd/Mg/Ba Ni/Pd NiCl2 ascorbic acid Cr(NO3)3 Eu

180 68 233 234 235 179 236 237 238 239 240 241 59 242 243 244 222 245 246

higher temperature and is relatively safe, having a lower combustion rate. In practice, detection limits were similar in the two flames. Woller et al. (254) determined As by flame atomic fluorescence spectrometry (FAFS). Ultrasonic nebulization was used to introduce HPLC effluent into the H2 diffusion flame of a commercial atomizer that is normally used for the introduction of volatile hydrides. Not surprisingly, detection limits were significantly poorer than those obtainable by hydride generation methods. The revival of “atom traps” continues, with reports of moderate sensitivity improvement with a slotted atom trap (255258), and a water-cooled device (259). Madden et al. (260) placed electrodes across an air-C2H2 flame and applied an electric field. Quantification of the resulting reduction in atomic emission and absorption intensity was performed, and reasons for this effect were suggested. It was proposed that the method could be useful for extending the linear range by avoiding dilution of more concentrated solutions. Instead of a hollow cathode lamp, a flameemitting analyte radiation was used as a light source in FAAS (261). This system was offered as a simple and inexpensive alternative to lamps, and it was claimed that detection limits were sometimes as good as with traditional lamps. Of some interest may be a report of X-ray fluorescence for the determination of atomic species in an air-C2H2 flame (262). A Talbot interferometer was used (263) for measuring the temperature profile of a flame and the effects of nebulization on flame temperature. Garcia et al. (264) used gradient dilution of a flowing stream to permit calibration with a single standard solution. A calibration graph was obtained in under 1 min, and it was also shown how a standard additions method of calibration could be performed. Instrument control and data analysis software was described (265) for routine analysis by FAAS. Included was an expert system for detection of errors during unattended operation. Projahn and Keim (266) developed an intelligent flow system for FAAS and illustrated its use through the fully automated analysis of alloys. A compilation of several useful parameters, including detection limit and precision, was supplied in graphical form for many elements (267). A Simplex optimization strategy for FAAS (268) included considerations of air and C2H2 flow rates and burner height. 240R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

Atomization and Interferences. Standardless analysis, using theoretical sensitivity calculations, may be less feasible for FAAS compared with ETAAS (discussed in the preceding section of this review). However, Magyar (269) presented an interesting review of the theoretical calculations and derived a simple formula. Zaranyika (270) presented a model for the degree of ionization of elements during flame spectrometry. Included were considerations of thermal excitation, ionization, collisional excitation, and collisional charge-transfer interactions. Good agreement with experimental values was reported. Further work on the enhancing effects of surfactants in FAAS showed that sensitivity improvement was better when the surfactant chain length was shorter (271). The sensitivity of Ti determination in a N2O-C2H2 flame was improved by addition of 8-hydroxyquinoline dissolved in acetic acid (272). Effects of organic solvents on the determination of Ru were studied (273). Platteau and Carrillo (274) determined trace elements in crude petroleum samples by nebulization of emulsions. A method for the determination of Ni and V in fuel oils, involving direct nebulization of the oil diluted with an organic solvent, was evaluated (275) through an interlaboratory study. Mauri et al. (276) observed different atomization behavior from Ca introduced as solutions and slurries, and this permitted speciation of dissolved and suspended Ca in a slurry sample. An algorithm was described (277) for taking into account both additive and multiplicative interference effects. It was based on a combination of pattern recognition and standard addition techniques. Welz and Luecke (278) studied interference by Al on the alkaline earth elements. Although this well-known interference had been studied extensively in the past, it was not previously understood why Al(NO3)3 interfered more severely than AlCl3, and reasons for this were suggested. Matrix components of aluminum-silicon alloys on the determination of Sr and Mg (279) and Mn (280) were studied, and suitable spectroscopic buffers for reducing these interferences were investigated. A mixture of KCl, HCl, H2SO4, and tartaric acid was used for eliminating interference by Si and Al on the determination of Fe, Ca, and Mg in fused rock samples (281). For the determination of Mn, a mixture of KCl, tartaric acid, and SrCl2 was used. Effective removal of interference by Al on Sr was achieved through the addition of 8-hydroxyquinoline (282). A mixture of La and EDTA was also suitable. Kabil (283) eliminated interferences by a range of organic and inorganic species on the determination of Cr by addition of n-butylamine or N-cyanoacylacetaldehyde hydrazone. A mixture of n-butylamine and thiourea was used (284) for eliminating interference by Cu and other elements on the determination of Se. Triethanolamine eliminated interference by several organic and inorganic species on Au (285). Ascorbic acid, citric acid, and EDTA were effective in reducing interferences by several metals on Pb (286). Ethanolamine was used (287) to eliminate some interferences on the determination of Ag. Luterotti (288) studied matrix interferences on the determination of Zn in biological tissue. Kantor and Ernyei (289, 290) examined the effects of halocarbons, introduced as vapors, on the atomization efficiencies of several elements. In some cases, signal depression was attributed to the formation of monochloride (289) or dichloride (290) species, and in others incomplete solute vaporization (290) was blamed.

LASER TECHNIQUES Omenetto (291) reviewed the use of tunable lasers in atomic spectrometry, with particular reference to laser-excited atomic fluorescence and laser-enhanced ionization spectrometry. It was described how the combined use of the two techniques can be used to obtain information on atomization processes in flames and plasmas. Moulin et al. (292) reviewed applications of various laser spectroscopic techniques to the determination of actinides and radioelements and compared the techniques with ICPMS. Niemax and Groll (21) reviewed the application of semiconductor diode lasers to laser spectroscopic techniques. A book on applied laser spectroscopy (293), although not directed toward analytical applications, may provide a good introduction to those wishing to learn the fundamental principles and basic instrumentation of laser techniques. Laser-Excited Atomic Fluorescence Spectrometry (LEAFS). The most common atom cells continue to be graphite furnaces. Butcher (294) reviewed advances in this technique (ETA-LEAFS) and also laser-excited molecular fluorescence in furnaces. An instrument with a CCD detector was used for the determination of Ni by ETA-LEAFS (295). This permitted the simultaneous detection of several fluorescence wavelengths for improved spectral selectivity and simultaneous monitoring of background signals. The possibility of performing two-dimensional spatial distributions was also cited. This instrument was also used for the determination of Al and Pb in atmospheric aerosol samples (296) and Sb in biological and environmental samples (297). The spatial distribution of Au atoms in a graphite furnace was investigated (132) by using a CCD detector. Of particular interest was the perturbing effect of a silver matrix on the Au fluorescence signal. Chekalin et al. (134) used LEAFS to study atomization processes in ETAs at low pressure, when diffusional effects were minimized. A modified optical system for ETA-LEAFS (298) used an ellipsoidal mirror to provide high collection efficiency and sensitivity. The detection limit for Co was improved 5-fold over the conventional optical system. Petrucci et al. (299) described the technique of double resonance laser-induced fluorescence, in which two different laser photons are absorbed by an atom. In this two-step process, the second laser was tuned to a transition starting from the excited level reached by the first laser, and fluorescence was then measured at a single wavelength. The final fluorescence energy level must be lower than the ionization energy of the element, so the technique is best suited to elements of high ionization energy and Au was chosen for the study. This technique was used with a graphite furnace (ETA-LEAFS) and a flame-furnace combination in which the vapor produced in the furnace was swept into a small flame. Longer residence times and less chance of fluorescence quenching made ETA-LEAFS more sensitive, giving a method detection limit of 15 fg for the determination of Au in atmospheric particulate samples. Cignoli et al. (300) investigated the possibility of absolute concentration measurement of Pb, without the need for calibration, by LEAFS in an air-C2H2 flame. The “wings effect” in saturated laserinduced fluorescence, where different saturation degrees occur within the probe volume, was corrected through the detection of two time-delayed fluorescence signals corresponding to two widely separated zones of the saturation curve. Omenetto and Matveev (301) produced time-resolved fluorescence waveforms for several elements in different atom cells. This permitted a study of the interaction between the laser radiation and the atomic system. A

miniature glow discharge atom cell, requiring only nanoliter sample volumes, was evaluated (302) for the determination of Eu, Y, and Tm by LEAFS. Davies et al. (303) designed an instrument for laser-induced breakdown spectroscopy (LIBS). A laser was used to ablate a solid steel sample via a fiber-optic light guide, and the resulting emitted radiation was transmitted via a second fiber optic into a spectrometer with a photodiode array detector. Concentrations of several trace metals were determined down to a few hundred ppm. Applications of ETA-LEAFS included the determination of Bi (304, 305), Co (306) and Pb (307) in seawater, Tl (308) and Pb (309) in natural water, Hg in soil digests (310), and As and Se in blood (311). Molecular fluorescence of MgF in a graphite furnace was used for the determination of F (223), and Cl was determined by the molecular fluorescence of InCl (312). Laser Atomic Absorption Spectrometry (LAAS). The availability of low-cost diode lasers has fueled interest in this technique, and their application in AAS was reviewed (313). Groll et al. (314) extended previous research on wavelength-modulated LAAS using diode lasers, by the determination of Ti, Cs, and Cr in flames. Detection limits for all three elements were better than those obtained by conventional FAAS with hollow cathode lamps. Zybin et al. (315) determined several halogenated and fluorinated hydrocarbons in a dc gas discharge by monitoring Cl and F. Wavelength-modulated diode laser AAS in a microwave-induced plasma (MIP) was used as a gas chromatography detector for Cl (316, 317). Preliminary detection limits of 1 mg L-1 were reported for halocarbons (316), but subsequent modulation of the plasma permitted the detection limit to be improved to 3 pg L-1 (317). Burakov et al. (318) used an intracavity laser echelle spectrometer with a liquid cell (for molecular absorption measurements) or a graphite furnace atomizer. Extremely low detection limits (< 1 ng L-1) were reported for Ba (318, 319) and Li, Sr, and Mg (319). Laser-Enhance Ionization (LEI) Spectrometry. Chekalin et al. (320) determined Co, Cr, Mn, and Ni in NH4F and NaF, which are used for optical fiber production. A graphite rod in a flame permitted the direct analysis of solid samples. Barker et al. (321) used a combination of LEI and LEAFS in order to measure gas velocities in an air-C2H2 flame seeded with Na. OTHER TECHNIQUES Furnace Atomization Plasma Emission Spectrometry (FAPES). Blades (322) reviewed atmospheric pressure radiofrequency capacitively coupled He plasmas and their use in FAPES. With the current interest in gas-phase spatial studies in ETAAS, it is perhaps not surprising that the technology is being applied to FAPES. Pavski et al. (323) used shadow spectral filming with a CCD imaging system to study the spatial distribution of analyte and background species in a FAPES source. Measurements of He emission showed the most intense plasma near the center electrode and a more diffuse plasma near the tube wall. The distribution of this plasma was dependent on the dc bias of the center electrode. They also observed that Ag species vaporized from the sample deposition site on the furnace wall, condensed on the cooler center electrode, and revaporized as the center electrode heated. However, operation at higher power maintained the electrode at a temperature high enough to avoid condensation. LeBlanc and Blades (324) also observed the most intense He emission near the center electrode, suggesting that the FAPES source behaved as a glow discharge. Using a CCD Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

241R

detector, they made spatially resolved measurements of spectroscopic gas temperature. The temperature increased with increasing rf power and was fairly uniform except for a slight increase near the furnace wall and center electrode. Imai and Sturgeon (325) noted that revaporization of analyte species that had condensed on the center electrode was responsible for the phenomenon of double analyte peaks. For several analytes, the species likely to be migrating from the wall to the electrode were considered. Imai et al. (326) studied the effects of pressure on a FAPES source. The temperature of the center electrode, efficiency of deposition of analyte onto secondary sites, and analyte emission intensity were shown to be pressure dependent. The optimum pressure for analytical work was said to be unique for each element. Imai and Sturgeon (327) studied the effects of easily ionized metal salt matrices on analyte emission. The major sources of interference were considered to be radiative power loss of the plasma due to excitation of the matrix species and alteration of the electron energy distribution. Furnace Atomization Nonthermal Excitation Spectrometry (FANES). Dittrich et al. (328) determined Hg after generating the vapor and sequestering it onto an Ir modifier coated on the inner wall of the graphite tube. Luedke et al. (329) trapped air particulates in a graphite tube by drawing the air sample though the tube wall. Multielement FANES determinations were then made using an echelle spectrometer. Hollow cathode FANES was used for the determination of B, with a detection limit of 71 pg. Coherent Forward Scattering (CFS). Gross and Hermann (330) calculated theoretical line profiles in CFS and discussed various factors affecting the profiles, including atomic and molecular spectral interferences and saturation and rollover effects. Laser ablation was used (331) to generate an aerosol which was then transported into a graphite furnace and deposited by a corona discharge in the tube, followed by multielement CFS determination. Glow Discharge Atomic Absorption and Fluorescence Spectrometry. Interest in these techniques, particularly glow discharge atomic absorption spectrometry (GDAAS) has increased. Previously (1), GD was discussed in the section on solids and slurries, but the increased research activity now merits a separate section in this review. Leis and Steers (332) reviewed the suitability and use of various types of boosted glow discharges in atomic spectrometry, including AAS. A book chapter by Piepmeier (333) reviewed AAS and AFS using GD sources. Pavski and Chakrabarti (334) used a Fourier transform spectrometer to investigate atomic line profiles in commercial hollow cathode lamps and a dc GD source to be used for AAS. Low gas temperatures in the GD resulted in narrow absorbance profiles compared with the usual high-temperature atom cells used in AAS. Consequently, the sensitivity and linear dynamic range of GDAAS will be more strongly affected by the hollow cathode lamp operating conditions, and low lamp currents must be used to reduce broadening and self-absorption. Parker and Marcus (335) suggested that rf powering of GD sources should be considered as an alternative to conventional dc powering, in order to allow the analysis of nonconducting samples. Consequently, they studied the effects of discharge power and pressure, and limiting orifice diameter, on the production of free atoms in a rf GD source used for AAS. Absorbance signals increased with the size of the limiting orifice diameter and with discharge power, but at higher power there was a compromise between ablation rate and source 242R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

stability. Absalan et al. (336) concluded that the rf source sustained a plasma stable enough for sequential AAS and depth profiling of metal samples. Inhomogeneity of the glow discharge was noted, and independent control of flow rate and pressure of the sputtering gas was critical for controlling the spatial distribution of atoms. Choi and Kim (337) designed a GDAAS system and studied and optimized the discharge parameters. They also noted that absorbance increased with power, but the system became less stable at high power, so compromise power conditions were required. Walden et al. (338) discussed the applicability of the GD source for AFS. Using pulsed Xe flash lamp continuum sources, improved signal-to-background ratios were obtained for multielement analysis, compared with GDAES. A miniature GD was used as an atom reservoir for LEAFS (302). Optimum operating pressures and currents, and factors affecting the transient fluorescence signal profiles for Eu, Y, and Tm, were described. Headrick et al. (339) noted severe matrix interferences in the use of a commercial cathodic sputtering atomizer for the analysis of solutions by AAS. A matrix such as NaCl was found to arc and thus prevent the formation of a stable glow discharge. Also observed were chemical reaction between the sample solution residue and the cathode substrate and changes in the spatial distribution of the solution residue on the cathode surface due to viscosity and surface tension effects. Plasma Atomic Absorption and Fluorescence Spectrometry. Several papers on microwave-induced plasma atomic absorption spectrometry (MIP-AAS) were published by Duan and co-workers. An instrument was described (340), comprising an ultrasonic nebulizer, a desolvation system with condenser and desiccator, and both L-shaped and T-shaped plasma tubes. Analytical figures of merit for several elements were similar to those obtained by FAAS. The influence of operating parameters on the desolvation system was studied (341), and Mn was determined on this instrument (342). A heated Ta filament was used for electrothermal vaporization of samples introduced to a low-power surface-wave MIP (surfatron), viewed in the axial position (343). This instrument was used for the determination of Ca (344). There were two reports on the use of wavelength-modulated diode laser MIP-AAS for the gas chromatographic detection of Cl (316, 317). An instrument with ultrasonic nebulization was described for MIP-AFS (345). Rigin (346) determined Fe, Co, and Ni, after converting them to their volatile carbonyls, by MIPAFS. An ac plasma was used as an atom source for AAS (347). Radiation from a hollow cathode lamp was passed coaxially through a quartz discharge tube containing the plasma, which was generated across two copper electrodes. Detection limits for 11 elements were similar to those obtained with other plasma sources in AAS but several orders of magnitude poorer than FAAS. ANALYSIS OF SOLIDS AND SLURRIES It is our possibly biased opinion that the most successful trace atomic spectrometric technique for directly analyzing solid samples involves their introduction as slurries into graphite furnaces for AAS determination (slurry-ETAAS). Reasons for the superiority of this technique include the high atomization efficiency of ETAAS, permitting calibration with aqueous standards, and the ability to easily transfer the sample to the atom cell with 100% efficiency using autosamplers with ultrasonic agitation. Several interesting applications of slurry-ETAAS are shown in Table 2.

Table 2. Direct Analysis of Solids and Slurries by ETAAS matrix animal feed biological material biological material biological material food gallium gold human hairs lignite molybdenum metal and molybdenum silicide muscle tissue PVC teeth airborne particles biological material biscuits, bread, cereals biscuits, bread, cereals

boron nitride diatomaceous earth edible oils and fats environmental ref material foods foods fruit

element

Solids Cu aqueous calibration Ag, Cu, Fe, aqueous calibration Mn, Pb, Zn Se cup-in-tube, Cu/Pd modifier Co, Ni, Mn, preashing method Pb Fe aqueous calibration Sn Mo modifier Si aqueous calibration As Pd/Mg modifier, aqueous calibration V calibration by standard addition Cu, K, Mg, cup and boat technique Mn, Na, Zn Pb Mg aqueous calibration Cd, Pb aqueous calibration Slurries Sb, Ni, V Pd/Mg modifier Ag, Cu, Fe, aqueous calibration Mn, Pb, Zn Cu aqueous calibration Cd, Pb phosphate modifier for Pb, Pd/Cu modifier for Cd, aqueous calibration Si Mg modifier, calibration by both standard addition and aqueous standards Cd, Zn, Mn NH4/phosphate modifier Fe, Ni ultrasonic autosampler Be preconcentration

glass

Al Co, Ni Cd, Cu, Fe, Pb, Se Cu

marine sediment

Pb

milk molybdenum oxide mussel

Al Li As

plant material respirable and inhalable aerosols silica gel

As Cd, Pb

titanium dioxide, zirconium dioxide wheat flour

comment

Cd, Pb, Cu, Fe, Al, Ti Si Se

Mg(NO3)2 modifier aqueous calibration comparison with acid/ digestion method suspended in 10% glycerin solution Pd/Mg modifier, aqueous calibration various modifiers tungsten tube atomizer Pd/Mg modifier, calibration by aqueous standards and standard additions Pd modifier comparison with FAAS method calibration by standard additions radiotracers, calibration by standard addition Pd/Mg modifier

ref 348 349 241 350 351 352 353 354 355 356 357 358 359 360 349 104 361

362 105 363 364 365 366 367 368 369 216 65 370 371 372 373 374 375

There is now a substantial body of literature demonstrating that a wide range of elements can be determined accurately by slurry-ETAAS in a large number of matrices. However, the literature also shows that matrix interferences may be more problematic unless methods based on modern furnace technology (the use of platforms, appropriate modifiers, fast recording of integrated absorbance, efficient background correction, etc.) are followed. This was seen in a collaborative study by Miller-Ihli (376) on the determination of Pb and Cr in water and sediment reference materials, where most laboratories reported accurate results, and inaccuracies could generally be traced to the laboratory’s specific methodology. Miller-Ihli (377) discussed the influence of slurry preparation on accuracy when ultrasonic sample mixing was used. Considered were the number of sample particles introduced to the atomizer, particle size, and fraction of analyte extracted into the liquid phase of the suspension. Arruda et al. (378) reviewed the direct analysis of foods as slurries by

atomic spectrometric techniques, and particularly ETAAS. Tittarelli and co-workers studied the formation of gas-phase molecular species when slurries of pure compounds of Si and Ge (379) and Pb and Sn (380) were vaporized. All the elements formed the monoxides, and in the presence of sulfur-containing compounds the monosulfides, in competition with free atoms. Friese and Krivan (381), in the determination of several elements in silicon nitride powders, obtained double absorbance peaks in the absence of a modifier, the first peak probably resulting from analyte near the surface of the particle, and the second from analyte occluded in the particle. A single peak resulted from the use of an NH4H2PO4/Mg(NO3)2 modifier, and this was attributed to reaction of the sample with the modifier during thermal pretreatment. The modifier also facilitated the removal of siliceous residue during the high-temperature cleanout step. The buildup of carbonaceous residues can be a problem in the analysis of plant and animal tissues by slurry-ETAAS. Vinas et al. added H2O2 to avoid this problem when determining Cu, Pb, Zn, Fe, and Cr in chewing gum (382), Pb and Cd in vegetables (106), and Al and Cr in vegetables (103). A mild calcination step was also required prior to the determination of metals in chewing gum by slurryETAAS (382, 383). Several researchers analyzed adsorbents directly by slurry-ETAAS after their use in preconcentrating trace metals from solution. Included were the analysis of activated carbon after preconcentrating In (384) and Se (385), solid chelating resins after preconcentrating Te (386) and Cu (387), polymeric organic sorbents after preconcentrating several metals (388), and bacterial cells after preconcentrating Au (389). An electrospark system was used for producing a colloidal suspension of metallic samples, followed by direct analysis by ETAAS (390). Slurry-FAAS may suffer from inefficient transport of the solid particles through the nebulizer and inefficient atomization resulting from the short residence time of the particles in the flame. However, if the trace metals can be leached from the solid material so they are in the liquid phase of the slurry sample, complete recovery may be obtained. This was seen when slurries of diatomaceous earth (391) and cement (392) were prepared in solutions containing mineral acid and HF, and trace elements were determined using aqueous calibration standards. Vinas et al. (393) determined several elements in vegetable slurries by slurryFAAS. Mauri et al. (276) noted that Ca exhibited different atomization behavior when it was in the liquid phase of a slurry compared with the solid phase. This formed the basis of a method to speciate the solid and dissolved forms of Ca in slurries. Another means of introducing solid samples involves directly weighing the dry solid into the atom cell. Some applications are listed in Table 2. Herber (394) reviewed direct weighing-ETAAS for the determination of trace metals in tissue. Herber and Grobecker (395) reported results of a collaborative study, where participants determined Pb, Cd, Cu, and Hg in several materials, including plant and animal tissue, milk, and dust. Most results were satisfactory, except for Cu in bovine liver and Cd in dust, where poor interlaboratory precision was observed. Byrd and Butcher (349) noted that the precision of direct weighing was poor compared with the slurry technique for the determination of trace elements in biological reference materials. A frequently cited difficulty with direct weighing-ETAAS is the inability to calibrate with aqueous standards. A new method of calibration, know as extrapolation to zero matrix, was described (396). The ratio of peak height to sample mass decreases in a linear manner Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

243R

with increasing sample mass. Hence, extrapolation to zero sample mass was considered to provide a signal free from matrix effects. A similar linear relationship was seen when the ratio of peak height to analyte mass was plotted against the analyte mass. Evaluation of this calibration method (397) showed it to be less accurate than the method of standard additions and the use of matrix-matched solid standards, but it was satisfactory for some applications. Minami et al. (398) reported on the use of a threepoint estimation standard addition method of calibration for the determination of Cr in powdered biological samples. This involved plotting a series of straight line graphs of absorbance vs sample mass, each line representing a different amount of spiked analyte. The absorbance value at a fixed mass (1 mg) from each graph was then used to plot a standard additions graph. Another difficulty with the direct-weighing method is the inability to dilute samples that have analyte concentrations greater than the upper limit of the linear dynamic range. This is exacerbated by the small dynamic range of AAS and frequently necessitates the use of lesssensitive nonresonance analytical wavelengths. An alternative strategy, described by Holcombe and Wang (399), was to use pressure-regulated ETAAS, when the sensitivity could be controlled by varying the pressure in the atomizer. Hence, all measurements could be made at the primary resonance line, and several elements were determined in fly ash by this technique. Wang and Holcombe (400) noted that the vaporization of Pb from a solid copper alloy produced three absorbance peaks when the atomizer was operated at reduced pressure. It was suggested that the first peak originated from the release of Pb from the surface, the second from grains, and the third from Pb in the bulk of the sample. This presented the feasibility of differentiating between near-surface and bulk analyte concentrations in solid samples. Hinds et al. (401) studied the release of Si from solid samples of gold. The gold matrix first melted, but release of Si was then too fast to occur from simple diffusion through the gold matrix. Results indicated that Si migrated to the surface of the droplet with the aid of convection currents. Mass spectrometric measurements (402) showed different Si species evolving from aqueous and solid gold samples. In the case of aqueous samples, SiC2 was detected, implying atomization via dissociative adsorption on the graphite surface. From gold samples, Si2 was seen, and this was consistent with the previously proposed mechanism of convection of Si species to the surface of the gold droplet. Nakamura et al. (403) determined Cu and Pb in Si3N4 and SiC and reported accurate results using aqueous standards, in spite of a double absorbance peak being obtained for Cu. From X-ray diffraction and scanning electron microscope studies, it was believed that the first and second peaks occurred through Cu vaporizing from Si and SiC, respectively. Radiotracer experiments were used (404) to investigate the efficiency of vaporization of several volatile trace elements from Mo powder. Vaporization efficiencies of >98% indicated the feasibility of direct analysis by ETAAS. Kurfuerst and Pauwels (186) identified a spectral interference due to S2 molecules when Pb was determined at 283.3 nm in solid samples high in sulfur content. It was possible to remove the interference through controlled atomization conditions. Kurfuerst (405) discussed how the precision of directweighing ETAAS can be used to assess the distribution of trace metals in the solid material. Solid-sampling ETAAS was used for evaluating the homogeneity of reference materials (406, 407) and for examining the distribution of Pb and Cd in liver (408, 409). 244R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

Nakamura et al. (410) co-precipitated trace metals from natural water with Zr(OH)4 and analyzed the precipitate directly by solidsampling ETAAS using aqueous calibration standards. Laser ablation coupled with ETAAS was used for the determination of Cu and Pb in solid geological samples (411). VAPOR-PHASE SAMPLE INTRODUCTION Yan and Ni (412) reviewed vapor generation AAS techniques, discussing the principles, instrumentation, and applications. Hydride Generation Techniques. The book Hydride Generation Atomic Absorption Spectrometry, by Dedina and Tsalev (413) is a comprehensive treatise, discussing the theory, instrumentation, and applications. A separate chapter is devoted to each hydride-forming element. Nakahara (414) also reviewed hydride generation techniques in atomic spectrometry, including the development of the techniques, details of atomization of elements from their hydrides, and interferences. They listed comparative detection limits for the use of hydride generation with various spectrometric techniques. Cervera and Montoro (415) discussed atomic spectrometric techniques, including hydride generation atomic absorption spectrometry (HGAAS), for the determination of As in foods. Tao and Fang (416) reviewed FI-HGAAS. Radiotracers were used (417) to compare the characteristics of two types of gas-liquid separator used, in conjunction with a cold trap, for continuous-flow HGAAS. Only the classical separator based on gravity was satisfactory, and not the separator that works on diffusion though a permeable tube. Torralba et al. (418) showed that a continuous-flow HGAAS system gave better precision than a FI system for the determination of As(III). Ebdon et al. (419), using a continuous-flow vapor generation system, obtained a 1500-fold improvement in sensitivity for Tl compared with a batch system. The volatile species, produced from the reduction of Tl with tetrahydroborate, was thought to be TlH. Stockwell and Corns (420) discussed the role of hydride generation atomic fluorescence spectrometry (HGAFS) in automated environmental analysis. An experimental design, including seven factors, was described for optimization of a HGAAS method (421). The influence of each analysis variable on peak area, repeatability, and SNR was evaluated. Rubio et al. (422) described an on-line ultraviolet photolytic system to transform complex organoarsenic species into simpler species that are amenable to reduction and determination by HGAAS. Chen et al. (423) examined the efficacy of 22 chelating agents in enhancing the signal for Pb in HGAAS. Preferred was 1-(2-pyridylazo)-2-naphthol-6-sulfonic acid, and it was thought that enhancement may be due to plumbane being generated from the chelated Pb(II) instead of the metastable Pb(IV). Also used to enhance the sensitivity of Pb determination was Nitroso-R-salt (424). Fernandez de la Campa et al. (425) reviewed the use of surfactants to improve the characteristics of vapor generation techniques, including HGAAS. Effects discussed included enhancements in efficiency of formation and transport of the volatile species, increases in the reaction kinetics, improved stabilization of some species, and changes in the selectivity of some reactions. The effects of pH and tetrahydroborate concentration on the sensitivity of As determination were investigated (426). Ni and He (427) examined various acid conditions for the optimum generation of GeH4. Among several acids investigated, HClO4 gave optimum hydride generation efficiency over the widest pH range. On-line reduction of Se(VI) to Se(IV) with HCl was improved substantially by heating the reduction coil to 140 °C

(428). Hill et al. (429) used HGAFS to study the kinetics of the reduction of Se(VI) to Se(IV) by heating with HCl. The time required for the reduction and its activation energy were calculated. Cabrera et al. (430) used a FI system with on-line microwave digestion for the analysis of food slurries by HGAAS. Garcia et al. (431) generated AsH3 from slurries of fly ash and diatomaceous earth. Chikuma and Aoki (432) adsorbed Pb from solution and treated the resulting resin slurry with a tetrahydroborate solution to produce PbH4. Treatment with the resin in this way mostly eliminated interferences by transition metals. An interesting way to determine Hg involved reduction of methylmercury compounds with tetrahydroborate to CH3HgH (433). After cryogenic trapping, Hg was determined by AFS. Cold trapping was also used during the determination of tributyltin in seafood by HGAAS (434). Foster and Howe (435) developed a proposed international standard method for the determination of As in workplace air by HGAAS. Bettinelli et al. (436) compared HGAAS with ETAAS for the determination of As, Bi, Sb, and Se in alloys. Several advantages of HGAAS were cited, particularly detection limits that were 1 order of magnitude lower than ETAAS. Blaylock and James (437) reported that HGAAS gave lower detection limits than colorimetric and ion chromatographic methods for the determination of Se species in environmental samples. Haygarth et al. (438) compared several techniques for the determination of total Se in environmental samples. For routine analysis, hydride generation inductively coupled plasma AES and HGAAS were both considered suitable, but especially FI-HGAAS. For measuring very low concentrations of Se, hydride generation inductively coupled plasma mass spectrometry and neutron activation analysis were preferred. Interferences continue to be a serious consideration in hydride generation techniques. Martin et al. (439) studied the interferences likely to occur in the determination of organotin compounds in a harbor sediment. Organic and inorganic matrix constituents were systematically spiked into solutions of the analyte compounds. The organic compounds interfered little, but predictably the inorganic species (metals) caused severe signal depression. The interfering effects of 12 metals were studied further (440), when the magnitude of interference was found to depend on the particular organotin compound being determined. Interferences were said to occur in both the solution and gas phases of the hydride generation process and were mostly suppressed by addition of ethylenedinitrilotetraacetic acid (EDTA). However, EDTA reacted with two of the organotin species, and L-cysteine was then preferred for interference suppression. Ni and He (427) used EDTA to suppress interferences by transition metals on the determination of Ge. Wickstrom et al. (441) pointed out that the use of chelating agents to mask interfering metal ions is limited by the acidity of the solution owing to protonation of the ligands. For the determination of Se they used an alkaline solution of tetrahydroborate, when interference by Co and Ni was suppressed by EDTA and diethylenetriaminepentaacetic acid (DTPA), and interference by Fe and Cr was suppressed by tartrate. Hydride was then generated after making the solution acidic in a flow system. This method was effective in suppressing interferences when applied to the determination of Se in nickel alloys, but some interference persisted when steel was analyzed (442). They suggested that this was a gas-phase interference, possibly from hydrides of Sn and As. Welz and Sucmanova (443) used L-cysteine as a reducing and releasing agent for the determination

of As and Sb. It was a better reducing agent than the previously used KI, since lower acid and reagent concentrations were required. L-Cysteine was also more effective than KI in reducing interferences by Ni and Cu when As and Sb were determined in copper and steel (444). Alexandrov et al. (445) investigated the interference by Cd and Zn salts on the determination of hydrideforming elements. Walcerz et al. (446) noted that As, Sb, Se, and Sn interfere with other hydride-forming elements in the gas phase, and As and Sb may also spectrally interfere through molecular absorption in the 190-235 nm-wavelength range. Interferences by Bi and Te were said to occur also in the liquid phase. Although speciation is frequently required, especially in the analysis of environmental samples, it may be necessary instead to determine the total concentration of an element. Le et al. (447) pointed out the errors that may occur when compromise operating conditions are used for the determination of total As, since at a given pH value, identical responses are not obtained from arsenite, arsenate, monomethylarsonic acid, and dimethylarsinic acid. However, in the presence of cysteine, all species provided optimum response in the same pH range, so the error was removed. It was suggested that cysteine reduces all the species to As(III) species which react to the same extent with tetrahydroborate High analytical sensitivity for hydride-forming elements is achieved by sequestering the volatile hydride on a suitable metal substrate coated on the inner surface of a graphite furnace for subsequent ETAAS determination. Besides providing a preconcentration step, this removes matrix interferences that might otherwise occur in conventional ETAAS. Tao and Fang (448) described a FI method for the determination of Sn, where the generated hydride was separated from the liquid phase in a gasliquid separator and transferred to a Pd-coated graphite tube at 300 °C. Also sequestered onto Pd for ETAAS determination were As and Sb (449), As and Se (450), Ge (427), and Te (451). Hanna et al. (203) sequestered Se onto a graphite tube coated with an Ir modifier. Unlike Pd, which had to be renewed after each firing of the furnace, Ir was retained for 300 firings. Ni et al. (452) found Ag to be a satisfactory and less expensive alternative to Pd for coating a graphite surface in order to trap Se and Te. Haug and Yiping (453) trapped Sn on graphite tubes coated with a carbideforming element (Zr, Nb, Ta, or W). These coatings were found to be more stable than a Pd/Ir coating during 400 successive firings. Also, higher atomization and clean-out temperatures were possible compared with Pd/Ir-coated tubes. Sayayam et al. (454) generated stannane and collected it in a solution of AgNO3. Aliquots were then pipeted into a graphite furnace for determination by ETAAS. Cold-Vapor Mercury Determinations. Cold vapor atomic fluorescence spectrometry (CVAFS) is becoming increasingly competitive with more traditional cold vapor atomic absorption spectrometry (CVAAS). Morita et al. (455) reviewed the determination of Hg by AFS. Included were the principles of the determination, losses and interferences, preconcentration methods, and applications. Saouter and Blattmann (456) described a semiautomated instrument for determining total and organic Hg by CVAFS. For total Hg, reduction and Au amalgamation steps were used prior to AFS measurement. Organic Hg was first derivatized using tetrahydroborate and then trapped on carbon before transfer to a gas chromatographic column coupled to an AF spectrometer. Complete analysis took only 10 min. A Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

245R

commercial continuous-flow CVAFS instrument was used for the determination of Hg in urine (457), and CVAFS was also used for the determination of Hg in water (458). A portable CVAAS instrument, using Au amalgamation to trap Hg, was described (459). In an on-line CVAAS system, organomercury compounds were destroyed through ultraviolet irradiation (460). A Wickbold combustion apparatus was incorporated into a FI-CVAAS system (461). A user interface for instrument control of a commercial FI-CVAAS instrument was described (462). Gold and other noble metals are commonly used for trapping Hg in CV systems. Frech et al. (463) studied noble metal collectors for the amalgamation of Hg from natural gas. A comparison of Ag, Au and Au/Pt showed Au/Pt to quantitatively trap Hg at higher flow rates. Although this study was aimed at developing the optimum collection procedure for Hg from natural gas, the work should be of interest to those using amalgamation as a preconcentration method. Bergdahl et al. (464) used amalgamation on a Au wire for preconcentration, releasing Hg by inductive heating of the wire. Kwokal et al. (465) avoided transporting seawater samples to the laboratory by collecting Hg on a Au wire during sample collection. The wires were then returned to the laboratory for analysis by CVAAS. There were several other reports on trapping with Au (466-468) and Ag (469). Sinemus et al. (470) sequestered Hg vapor onto the inside surface of an Ir-coated graphite tube and determined Hg by ETAAS. This provided a marginally lower detection limit compared with trapping on a Au amalgam. Also, Hg was sequestered onto the Ir-coated inner surface of a graphite tube for subsequent determination by FANES, with a detection limit of 0.9 ng L-1 (328). Garcia et al. (431) generated Hg vapors directly from slurries of fly ash and diatomaceous earth. A FICVAAS method was developed for the determination of Hg in ores (471). Fernandez et al. (472) studied the enhancing effects of different surfactants on Hg generation. A 3-fold improvement in detection limit by CVAAS was obtained by generating Hg in the presence of didodecyldimethylammonium bromide. At about the same time, a second group from Spain performed similar studies. Gutierrez et al. (473) reported a 50% improvement in analytical signal in the presence of sodium dioctylsulfosuccinate. A possible mechanism for the enhancement was proposed. Madrid et al. (474) reduced interferences from transition metals, As, Sb, and Se, by generating the Hg vapor in micellar media. Fernandez de la Campa et al. (425) reviewed the use of surfactants in vapor generation techniques, including CVAAS. Pszonicki et al. (475) applied the methods of standard additions and successive dilutions for the reduction of interferences in CVAAS. A masking agent containing 1,10-phenanthroline and thiosemicarbazide in HCl was used for reducing interferences by several metals (476). Lind et al. (477) discussed the problem of overestimation of the concentration of inorganic Hg in a sample when methylmercury is also present. This was said to occur through partial degradation of methylmercury during formation of the analytical signal, and they proposed a means of correcting this error. Tanaka et al. (478) extended previous work showing that iodide interference was minimized if Hg vapor was generated through reduction with Cr(II) instead of Sn(II). In this work, they evaluated their method further by determining Hg in several sample types. Daniels and Wigfield (479) investigated the intermittent second peak that occurs in CVAAS and presented evidence that it is due to HCl gas. They suggested its elimination by adsorption on glass wool. 246R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

Other Gaseous Metals. Willie (480), using an automated FI system, generated volatile tetraethyllead by reaction with tetraethylborate and trapped it in a graphite furnace for subsequent determination by ETAAS. Ebdon et al. (481) described a similar continuous-flow system and reported higher sensitivity for Pb compared with HG methods. Sanz-Medel et al. (482) showed that Cd can form a cold vapor under appropriate conditions. In the presence of a surfactant, reduction with tetrahydroborate produced a volatile Cd compound that was thought to be the hydride. Their detection limit by CVAAS was 80 ng L-1. Guo and Guo (483) found that addition of thiourea and Co2+ greatly enhanced the generation efficiency of the volatile Cd species. They reported a detection limit of 8 ng L-1 by CVAFS. Interferences by other metals were controlled by masking with KCN. Baaske et al. (484) determined Fe in HCl gas by passing the gas into an uncoated graphite furnace tube for analysis by ETAAS. Rigin (346) converted Fe, Co, and Ni into their volatile carbonyls for determination by MIP-AAS. FLOW INJECTION ANALYSIS This section describes instrumental developments, fundamental studies, and selected applications in FI analysis. The reader will also find numerous references to FI in the sections on vaporphase sample introduction, sample preparation, and separation. Welz and Sperling (485), in a review of FI, described it as the ultimate approach to automation in analytical spectroscopy. Sample introduction devices in FI analysis were reviewed by Burguera and Burguera (486). Kuban (487) discussed the various manifolds and other instrumental components of FI systems that have been used for continuous precipitation and filtration to permit on-line preconcentration and separation. Other reviews discussed FI for the automation of hydride generation systems (416) and for the indirect analysis of drugs (488). Garcia et al. (489, 490) designed computer-controlled FI manifolds for controlling the volume of sample and thus extending the dynamic range of FAAS. Dialyzer units (491, 492) were used for on-line dilution in FAAS. An additional advantage cited (491) for the analysis of industrial effluents was the removal of solid particles that would otherwise block the nebulizer. A dialyzer unit (493, 494) was used for automatic dilution and also to split the sample stream into two components for the simultaneous analysis of two metals in separate flame spectrometers. Xu and Fang (495) used a zone penetration technique for on-line dilution in FI-FAAS, with dilution factors as high as 27 000 and delivering 45 samples/ h. A randomized factorial central composite design was described (496) for optimizing flow rate and length of the dispersion coil used for on-line dilution. Simplex and univariate optimization methods were used in the determination of Sn by FI-HGAAS (497). Xu and Fang (498) developed a sample introduction system for FAAS based on air-transported sample loading and hydrodynamic injection. Only 0.8 mL of sample was required for triplicate analysis. Detcheva and Havezov (251) showed how sample consumption in FAAS can be reduced by a factor of 3 using controlled-dispersion FI. Gluodenis and Tyson (499) constructed a FI system with a stopped-flow microwave-heated reactor for the on-line digestion of samples pumped as slurries. Continuous-flow microwave digestion systems, with samples introduced as slurries, were used for the determination of Hg in environmental samples by CV-AFS (500) and Pb in food by HG-AAS (430). Welz et al. (501) described a FI manifold for on-line microwave-assisted

digestion, followed by reduction of As(V) to As(III), and its subsequent determination by HGAAS. Pitts et al. (502) described the use of microwave energy to reduce Se(VI) to Se(IV). On-line reduction of As(V) to As(III) was accomplished by mixing with reducing agent in a knotted reactor and storing it in a sample loop for 40 s (503). On-line preconcentration was achieved by adsorbing metal chelates on the wall of a PTFE knotted reactor (504) and by coprecipitation and collection of the precipitate on the inner walls of a knotted reactor (505). Tao and Fang (448) developed a FI system for the determination of Sn in various samples by on-line ion-exchange and in situ preconcentration by sequestration into a graphite tube for subsequent determination by ETAAS. Organolead compounds were analyzed (38) by ETAAS using FI with a THGA. The use of FI-FAAS for the transport and direct analysis of slurries of vegetables (393), clay (391), and cement was described (392, 506). Arruda et al. (365, 507) determined Al in food slurries with a FI system that fed an autosampler for determination by ETAAS. Bettinelli et al. (436) showed that FI-HGAAS gave lower detection limits and was easier and faster than ETAAS for the determination of As, Bi, Sb, and Se in steels. McIntosh (467) incorporated amalgamation onto a gold gauze for the determination of Hg by FI-CVAAS. Nafion drying tubes were used for removal of moisture when FI was used with chemical vapor generation methods (508). Krieger et al. (509) offered discontinuous-flow analysis as an alternative technique to FI. A programmable cam-driven pumping system combined mixtures of samples, standards, diluents, and reagents. This permitted direct calibration by incremental mixing of a standard and diluent, and standard additions calibration by combining sample, standard, and diluent. SAMPLE PREPARATION Batch process and on-line microwave digestion procedures have become commonplace, and therefore methods based on their use are generally no longer within the scope of this review. Of interest, however, are reports of the complete automation of analytical systems incorporating microwave digestion. Burguera et al. (510) described a completely automated FI system for the digestion and determination of Fe and Zn in biological samples by ETAAS. An automated on-line system for the microwaveassisted digestion of slurries and analysis by FAAS was described (511). A robotic system incorporating microwave digestion was used for the determination of trace metals in soil by FAAS (512). Also of interest is a report of the microwave digestion of air filters directly in the autosampler cups of a graphite furnace AA spectrometer (360). Lan et al. (23, 24, 513) described orthogonal array chemometric designs for the optimization of microwave digestion of biological samples. White and Lawrence (514) used a high-frequency induced O2Ar-F2 plasma for the low-temperature decomposition of botanical and biological samples. D’Ulivo et al. (515) studied the breakdown of organoselenium compounds in a HBr-Br2 digestion system. Bombach et al. (516) used thermal evaporation for separating Hg from soils and sediments. Horvat et al. (517) isolated methylmercury compounds from sediments by distillation. This method was superior to alkaline digestion, which suffered from matrix effects, and acid leaching, which gave incomplete recovery. Silva et al. (518) stabilized Pb and Cu in a mixture of kerosene and water by addition of propanol to create a three-

component solution, thus permitting their direct determination by ETAAS. SEPARATION Matrix Isolation and Preconcentration. In this section we have been particularly selective by not including reference to straightforward applications describing the use of “new” chelating agents, existing ion-exchange or coprecipitation technologies, etc. On-line systems for the preconcentration and speciation of trace metals in environmental samples were reviewed (519). Kuban (487) reviewed continuous precipitation techniques in FI analysis. Continuous-flow methods for trace metal chelation and sorption onto a solid substrate have become almost commonplace, and it was necessary to be selective in this review. However, of interest is a comparison by Lancaster et al. (520) of several chelates used with a C18 support to provide preconcentration factors of 50-100. Welz et al. (521) described a fully automated FI column preconcentration system, with enrichment factors of >60, for the determination of Cd and Pb by ETAAS. On-line systems, using C18 microcolumns, were described for the determination of Pb, Cd, and Cu (522) and Pb in seawater (523). Naghmush et al. (524) found functionalized cellulose sorbents more suitable than C18, and Gallego et al. (525) compared fullerenes with C18 as online sorbents. Several dialkyldithio phosphates were compared as chelating agents for FI sorbent extraction methods (526). Chelation and on-line sorption on the walls of a knotted reactor allowed enrichments of 120-fold for Cu (504) and 66-fold for Cd (527). There were several reports of on-line coprecipitation methods, involving collection of the precipitate on the walls of a knotted reactor and its subsequent dissolution in an organic solvent. Included were the coprecipitation of Ag with Fe(II)diethyldithiocarbamate (528), Mo with pyrrolydinedithiocarbamate (529), Pb and Cu with pyrrolydinedithiocarbamate (530), Cd with hexamethylene-ammonium hexamethylene dithiocarbamate (531), and Ni with 1-nitroso-2-naphthol (532). Tao and Hansen (505) coprecipitated Se(IV) with La(OH)3 and eluted the precipitate from the knotted reactor with HCl. Trace metal chelation and adsorption on an active carbon column was used for the separation and preconcentration of Pb (533), Cd (534), and Cu (535). Other sorbents used were an aminopyridine resin for the determination of Pd, Pt, and Ir (536) and sulfhydryl cotton for Ag (537). Maquieira et al. (538) used immobilized bacteria on glass (538) and yeast (539) for the preconcentration of several metals. Elmahadi and Greenway used immobilized algae (540) and immobilized cysteine on glass (541). An on-line dialysis preconcentration system was described (542). Papantoni et al. (543) used a supported liquid membrane system, where metals were enriched in a flow system, for subsequent off-line determination by FAAS. Off-line or batch preconcentration methods continue to be popular, and reported enhancement factors are often higher than those claimed with on-line systems. Saxena et al. (544) functionalized Amberlite XAD-2 by coupling it with salicylic acid and reported preconcentration factors of 140 for Pb and 180 for Zn. Amberlite XAD-1180 resin was used for the preconcentration of Cu, Cd, Pb, and Bi (545). A cross-linked poly(dithiocarbamate) resin was used for several metals (546). Silica gel was impregnated with a mixture of Aliquat 336 and Zincon for the sorption preconcentration of 14 metals (547). This could be especially useful for preconcentration prior to ETAAS determination, since Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

247R

interfering alkali metals were not retained on the sorbent. Hiraide et al. (548) immobilized 1-nitroso-2-naphthol on surfactant-coated alumina for the collection of Co(II). Sedykh et al. (388) described the use of polymeric organic sorbents for the preconcentration of several metals, followed by direct analysis of the sorbent by slurry-ETAAS. Trioctylamine and trioctylmethylammonium chloride anion exchangers were used for the separation of Cr (549). The chelating ion exchanger Spheron Thiol was used for Hg (550). Luo and Hou (551) used a polyurethane foam in a column for the preconcentration of Tl. Donnan dialysis with a tubular cation-exchange membrane was described for the preconcentration of Au (552). A remarkable preconcentration factor of 10 000 for Ag and Cd was described (553), using a combination of ion exchange and chelation/solvent extraction prior to determination by FAAS. Activated carbon is highly effective in adsorbing and preconcentrating trace metals, provided they have first been chelated with organic ligands. Examples were the preconcentration of Mo (554), Cd and Pb (555), V (556), Ag (557), Co (558), In (384), and Se (385). The feasibility was investigated of using activated charcoal loaded with Zr as an adsorbent for As, Se, and Hg (559). Bacterial cells were used for the preconcentration of Be (364) and Au (389). An adsorbent consisting of dried lichen and seaweed biomass entrapped on silica gel was said to be comparable to a commercial chelating resin for the preconcentration of several metals (560). Ashino et al. (561, 562) separated Se and Te from alloys by reductive coprecipitation with Pd in ascorbic acid. For subsequent ETAAS determination, the Pd was also an effective modifier. Nakamura et al. (410) coprecipitated several trace metals with Zr(OH)4 and then introduced the solid precipitate directly into a graphite furnace for analysis by ETAAS. Reductive precipitation with hydrazine was described (563) for the determination of several trace metals in TeO2 and Bi2O3. Ghazy and co-workers described the use of oleic acid for the flotation separation of Ag (287), Au (285), Pb (564), and Cu (565). Tri-n-octylamine and monobutanediimide were used for the flotation separation of Pd (566). A chelating resin on a Pt-wire matrix was used for the collection and preconcentration of Pb (567). The wire was then inserted into a flame for analysis by FAAS. Matousek and Powell (568) used electrolysis at uncontrolled potentials to deposit trace metals from solution onto graphite platforms for analysis by ETAAS. They also electroplated Pd and analyte onto the inner surface of a graphite tube (205). Supercritical fluid extraction was used for separating Hg from seafoods (569) and to characterize Cd, Cu, and Zn bound to metallothioneins (570). Speciation. Sanz-Medel (571) reviewed the problems of speciation in biological systems and discussed some specific methods used for the speciation of Hg, As, and Sn. The use of atomic spectrometric detectors on-line with gas or liquid chromatography provides the most versatile means of trace metal speciation. Uden (572) reviewed these “hyphenated techniques”, including both FAAS and ETAAS detection of GC and HPLC effluents. Donard and Ritsema (573) reviewed the application of these techniques for the speciation of organometallic compounds in environmental samples. Lobinski et al. (574) reviewed the speciation of organolead compounds by GC with atomic spectrometric detection, and Ritsema and Martin (575) reviewed GC-HGAAS for speciation analysis. Thermospray interfaces were described for HPLC-FAAS (252, 576) and for HPLC-ETAAS with periodic deposition of sample onto a heated 248R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

platform in the furnace (577). Weber and Berndt (578, 579) described a high-pressure nebulization system as an interface between HPLC and FAAS. Silica T-tube atomizers were used for As and Se (580) and organolead and organotin compounds (581). An HPLC-flame nebulizer interface was described (582). An HPLC-FAFS system for As speciation incorporated ultrasonic nebulization (254). Hydride generation of Se usually requires the element to be in its +4 oxidation state. An HPLC-HGAAS system incorporated an on-line microwave system to oxidize trimethylselenium and reduce Se(VI) to Se(IV), thus allowing all three species to be monitored (583). A similar device was used to convert Se(VI) to Se(IV) in an HPLC-HGAFS instrument (584). Johansson et al. (585) used a deconvolution procedure for improving the accuracy of quantitation of overlapping chromatographic peaks in GC-AAS. It was applied to the determination of organolead compounds. They also studied the operating conditions and quartz tube atomizer dimensions providing the best SNRs for coupled GC-HGAAS systems (586). A factorial experimental design was used (587) to optimize the experimental conditions for a GC-AAS system for the speciation of organotin compounds, using both hydride generation and ethylation as derivatization procedures. Sarradin et al. (588) discussed the optimization of experimental conditions for the speciation of organotin compounds by GC-HGAAS. Martin and Donard (440) studied mechanisms of interferences from metals during the speciation of organotin compounds in an on-line system incorporating cryogenic trapping, GC separation, and HGAAS detection. Masking agents were suggested to reduce the interferences, which were seen to occur in both the liquid and gas phases. In a later paper they studied interferences likely to occur in the determination of tributyltin in sediment (439). Szpunar-Lobinska et al. (589) studied interferences in the speciation of organotin and organolead compounds by GC-AAS. Koelbl (590) compared FAAS, ETAAS, and ICPMS used on-line with HPLC for the determination of Se species in natural water. Speciation of Se was described using HPLC-HGAAS (591-593), and HPLCETAAS (593, 594). Also speciated by HPLC-HGAAS were As (595-597) and Sb (598). Butyltin compounds were speciated by GC-quartz furnace AAS (599, 600) and by HPLC-ETAAS (601). Schulze and Lehmann (602) used ion-exchange chromatography coupled with HGAAS for butyltin compounds. Several papers described the determination of Hg species by coupled HPLC-CVAAS (460, 603-606), and in two of them a postcolumn ultraviolet irradiation step was incorporated to convert organomercury compounds to elemental Hg (460, 606). Filipelli et al. (607) separated methylmercury, after its reduction to methylmercury hydride, from dimethylmercury with a purge and trap system on-line with a GC-CVAAS instrument. Several applications of coupled GC-CVAFS were also described for the speciation of Hg (456, 608-610). The use of HPLC-ETAAS was described for the speciation of Al and Fe in proteins (611). There were several examples of chromatographic separation of species followed by atomic spectrometric measurement off-line. For the separation of As(III) and As(V), a water sample was passed through a column containing a modified C18 stationary phase (612). Following elution, the species were determined off-line by FAAS or ETAAS. Ion chromatography was used for As speciation followed by off-line hydride formation (613). The hydride was then sequestered onto the inside surface of a graphite furnace for measurement by ETAAS. Hasegawa et al. (614) used

Table 3. Use of AAS for Indirect Analysis species aliphatic aldehydes antazoline, hydralazine, amiloride hydrochlorides, quinine sulfate atenolol carbonate, dichromate, oxalate carbonate, chloride, chromate, oxalate clindamycin cocaine, papaverine, strychnine cyanide, thiocyanate diethyldithiocarbamate diphenhydramine hydrochloride indomethacin iodide iodide iodine lignocaine lincomycin malathion metallothionein metallothionein nitrogenous compounds nitrophenols ondansetron phosphate saccharin salicylic acid sodium dodecyl sulfate sulfate sulfur tannins thiosulfate

matrix

analyte

ref

aqueous solution pharmaceuticals

Hg Co

645 646

pharmaceuticals aqueous solution aqueous solution

Cu various Ag

647 648 649

drugs pharmaceuticals aqueous solution fungicide aqueous solution aqueous solution tap water aqueous solution aqueous solution pharmaceuticals pharmaceuticals aqueous solution freshwater mussel urine pharmaceuticals aqueous solution drugs aqueous solution artificial sweeteners, pharmaceuticals pharmaceuticals toothpastes, liquid detergents, wastewater aqueous solution soil wines, tea tap water

Cu Bi Ag Cu Co Ag, Cu Hg As Hg Cu Cu Bi Cd various Co Cu Pb Mg Ag

650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668

Co Co

669 670

Cr Ba Cu Ag

671 672 673 674

chelation and chromatographic separation to separate several As species including trivalent methylarsenals prior to HGAAS. Separation by HPLC followed by off-line ETAAS measurement was used for the determination of Al and Si species in serum (615). Appropriate solvent extraction procedures were used for the speciation of As(III), As(V), and total organic As in soil, followed by HGAAS measurement (616). Torralba et al. (617) speciated As(III), As(V), monomethylarsonate, and dimethylarsinate through differential reduction to the hydride by control of the acid conditions. Selective chelation and solvent extraction of As(III) in the presence of other species, followed by ETAAS, was described (618). Several papers described FI-HGAAS systems for the speciation of As (418, 619-622). Rondon et al. (623) selectively determined Sb(III) and Sb(V) by control of the acid conditions during hydride generation. Complexation and pH adjustment were used for the selective retention of Sb(III) and Sb(V) on alumina (624). Reddy et al. (625) speciated Se(IV) and Se(VI) in groundwater by their selective adsorption onto CuO particles, followed by HGAAS. Several ion-exchange resins were compared for the separation of inorganic Se species from water samples (626). Coupled FI-HGAAS systems were described for the speciation of Se (502, 627). Butyltin compounds extracted from shellfish were converted to their hydrides and speciated by cryogenic trapping and selective volatilization into a HGAAS atom cell (628). By selecting appropriate acid digestion procedures for blood (464), it was possible to separately determine inorganic Hg and total Hg (inorganic Hg + methylmercury). On-line speciation methods were described for inorganic Hg and methylmercury, involving retention on chromatography stationary phases (433, 629) and on yeast biomass (630). Jian et al. (631) used a microcolumn FI system. Bombach et al. (516) used

thermal evaporation to obtain qualitative speciation of organomercury compounds and metallic Hg in soils and sediments. As usual, there were several reported methods for the speciation of Cr(III) and Cr(VI), involving differential adsorption (540, 632-634), ion-exchange (635, 636), coprecipitation (637), flotation (283), complexation (638), and selective digestion procedures (639). Sorbent extraction preconcentration was used (640) to selectively adsorb an Fe(II) complex, while Fe(III) was unretained. Speciation of Tl(I) and Tl(III) was achieved (641) by selective on-line complexation of Tl(III) on an immobilized chelating agent. Naghmush et al. (524) investigated several adsorbents for the speciation of Pb in a FI-FAAS system. Functionalized cellulose sorbents gave the best retention characteristics and allowed differentiation between tetraalkyllead and other forms of Pb. Davidson et al. (642) evaluated a three-stage sequential extraction procedure for the speciation of heavy metals in sediment and applied the method to the determination of Cr, Mn, Ni, and V (643). Wang and Marshall (644) determined As, Cd, Cu, Mn, Pb, Se, and Zn by FAAS in a silica tube after supercritical fluid extraction of the metal chelates. The technique was then used for the characterization of protein-bound Cd, Cu, and Zn (570). ACKNOWLEDGMENT

We are grateful to Thomas Flynn who performed several on-line literature searches. Kenneth W. Jackson attended Imperial College, London, where he received the degrees of M.Sc. (1970) and Ph.D. (1972), both in analytical chemistry. In 1984 he was appointed as Professor of Analytical Chemistry at the University of Saskatchewan, Canada. He joined the Wadsworth Center of the New York State Department of Health as a Research Scientist in 1987, where he also has a joint appointment as Professor in the School of Public Health of the State University of New York at Albany. Professor Jackson’s major research interest is in electrothermal atomic absorption spectrometry, particularly fundamental studies of atomization from slurry samples and mechanisms of modification in graphite furnaces. He has carried out research on ion chromatography for trace metal determinations, and he is also interested in environmental sampling and preconcentration techniques for trace metals. Guoru Chen is a graduate student in the School of Public Health of the State University of New York at Albany. He hopes to obtain his Ph.D. degree in environmental analytical chemistry, in the spring of 1997, under the direction of Professor K. W. Jackson. He received the degrees of B.Sc. (1982) and M.Sc. (1988) in analytical chemistry, from Hebei University in the P. R. China. He was a lecturer at Hebei University from 1989 to 1992, and then spent one year as a visiting scholar at Bristol University, United Kingdom, where he conducted research in environmental analytical chemistry. Mr. Chen is currently performing research on mechanisms of atomization and modification in electrothermal atomic absorption spectrometry.

LITERATURE CITED (1) Jackson, K. W.; Mahmood, T. M. Anal. Chem. 1994, 66, 252R79R. (2) Sneddon, J., Ed. Advances in Atomic Spectroscopy; JAI Press: Greenwich, CT, 1995; Vol. 2. (3) Cresser, M. S. Flame Spectrometry in Environmental Chemical Analysis: A Practical Guide; Royal Society of Chemistry: Cambridge, U.K., 1994. (4) Vandecastelle, C.; Block, C. B. Modern Methods of Trace Element Determination; John Wiley & Sons: New York, 1993. (5) Alfassi, Z. B., Ed. Determination of Trace Elements; VCH: New York, 1994. (6) Liang, Y.-Z.; Ni, Z.-M.; Yang, P.-Y. J. Anal. At. Spectrom. 1995, 10, 699-702. (7) Greenfield, S. Trends Anal. Chem. 1995, 14, 435-42. (8) Wiese, W. L. In Atomic and Plasma-Material Interaction Processes in Controlled Thermonuclear Fusion; Ratko, K., Drawin, H. W., Eds.; Elsevier, Amsterdam, 1993; pp 355-70. (9) Doidge, P. S. Spectrochim. Acta, Part B 1995, 50B, 209-63. (10) Hannaford, P. Spectrochim. Acta, Part B 1994, 49B, 1581-93. (11) Verner, D. A.; Barthel, P. D.; Tytler, D. Astron. Astrophys., Suppl. Ser. 1994, 108, 287-340. (12) Surovell, T. J. Chem. Educ.: Software Ser. C 1994, 6C, 23. (13) Farnsworth, P. B. Spectrochim. Acta, Part B 1994, 49B, 8114. (14) Wiese, W. L. Spectrochim. Acta, Part B 1995, 50B, 917-8.

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

249R

(15) Hill, S. J.; Dawson, J. B.; Price, W. J.; Riby, P.; Shuttler, I. L.; Tyson, J. F. J. Anal. At. Spectrom. 1994, 9, 213R-47R. (16) Hill, S. J.; Dawson, J. B.; Price, W. J.; Shuttler, I. L.; Tyson, J. F. J. Anal. At. Spectrom. 1995, 10, 199R-228R. (17) Florek, S.; Becker-Ross, H. J. Anal. At. Spectrom. 1995, 10, 1457. (18) Sweedler, J. V., Ratzlaff, K. L., Denton, M. B., Eds. ChargeTransfer Devices in Spectroscopy; VCH: Weinheim, Germany, 1994. (19) Sneddon, J.; Farah, B. D.; Farah, K. S. Microchem. J. 1993, 48, 318-25. (20) Lehnert, R.; Quick, L.; Rump, T.; Winter, F.; Cammann, K. Fresenius’ J. Anal. Chem. 1993, 346, 392-5. (21) Niemax, K.; Groll, H.; Schnuerer-Patschan, C. Spectrochim. Acta Rev. 1993, 15, 349-77. (22) Winefordner, J. D.; Petrucci, G. A.; Stevenson, C. L.; Smith, B. W. J. Anal. At. Spectrom. 1994, 9, 131-43. (23) Lan, W. G.; Wong, M. K.; Chen, N.; Sin, Y. M. Analyst 1994, 119, 1659-67. (24) Lan, W. G.; Wong, M. K.; Chen, N.; Sin, Y. M. Analyst 1994, 119, 1669-75. (25) Araujo, P. W.; Kavianpour, K.; Brereton, R. G. Analyst 1995, 120, 295-8. (26) Penninckx, W.; Vankeerberghen, P.; Massart, D. L.; SmeyersVerbeke, J. J. Anal. At. Spectrom. 1995, 10, 207-14. (27) Zehr, B. D.; Maryott, M. A. Spectrochim. Acta, Part B 1993, 48B, 1275-80. (28) Slavin, W. Tech. Instrum. Anal. Chem. 1994, 15, 53-85. (29) Tsalev, D. L. J. Anal. At. Spectrom. 1994, 9, 405-14. (30) Falk, H. Spectrochim. Acta, Part B 1995, 50B, 907-15. (31) Baxter, D. C.; Frech, W. Spectrochim. Acta, Part B 1995, 50B, 655-706. (32) Hadgu, N.; Frech, W. Spectrochim. Acta, Part B 1994, 49B, 445-57. (33) Smith, C. M. M.; Harnly, J. M. J. Anal. At. Spectrom. 1995, 10, 187-95. (34) Voigtman, E.; Yuzefovsky, A. I.; Michel, R. G. Spectrochim. Acta, Part B 1994, 49B, 1629-41. (35) Hertzberg, J.; Kozlov, D.; Rieck, C.; Loosen, P.; Sperling, M.; Welz, B.; Marowsky, G. Appl. Phys. B: Lasers Opt. 1995, B61, 201-5. (36) Bradley, C.; Leung, F. Y. Clin. Chem. 1994, 40, 431-4. (37) Bozsai, G.; Melegh, M. Microchem. J. 1995, 51, 39-45. (38) Bettmer, J.; Cammann, K. Appl. Organomet. Chem. 1994, 8, 615-20. (39) Erber, D.; Bettmer, J.; Cammann, K. Fresenius’ J. Anal. Chem. 1994, 349, 738-42. (40) Katskov, D. A.; Shtepan, A. M.; McCrindle, R. I.; Marais, P. J. J. G. J. Anal. At. Spectrom. 1994, 9, 321-31. (41) Radziuk, B.; Roedel, G.; Stenz, H.; Becker-Ross, H.; Florek, S. J. Anal. At. Spectrom. 1995, 10, 127-36. (42) Schulze, H. LaborPraxis 1995, 19, 30, 32, 34, 37-9; Chem. Abstr. 1995, 122, 229457. (43) Radziuk, B.; Roedel, G.; Zeiher, M.; Mizuno, S.; Yamamoto, K. J. Anal. At. Spectrom. 1995, 10, 415-22. (44) Latino, J. C.; Sears, D. C.; Portala, F.; Shuttler, I. L. At. Spectrosc. 1995, 16, 121-6. (45) Harnly, J. M.; Radziuk, B. J. Anal. At. Spectrom. 1995, 10, 197206. (46) Edel, H.; Quick, L.; Cammann, K. Fresenius’ J. Anal. Chem. 1995, 351, 479-83. (47) Edel, H.; Quick, L.; Cammann, K. Anal. Chim. Acta 1995, 310, 181-7. (48) Sen Gupta, J. G.; Bouvier, J. L. Talanta 1995, 42, 269-81. (49) Smith, C. M. M.; Harnly, J. M. Spectrochim. Acta, Part B 1994, 49B, 387-98. (50) Fernando, R.; Jones, B. T. Spectrochim. Acta, Part B 1994, 49B, 615-26. (51) Smith, C. M. M.; Harnly, J. M.; Moulton, G. P.; O’Haver, T. C. J. Anal. At. Spectrom. 1994, 9, 419-25. (52) Becker-Ross, H.; Florek, S.; Tischendorf, R.; Schmecher, G. R. J. Anal. At. Spectrom. 1995, 10, 61-4. (53) Williams, D. T.; Green, A. E. S. Rev. Sci. Instrum. 1994, 65, 3339-43. (54) Kitagawa, K.; Shimazaki, Y. Anal. Sci. 1993, 9, 663-9. (55) Vasil’eva, L. A.; Greenshtein, I. L.; Katskov, D. A. Zh. Anal. Khim. 1993, 48, 1345-54; Chem. Abstr. 119, 285412. (56) Katskov, D. A.; Schwarzer, R.; Marais, P. J. J. G.; McCrindle, R. I. J. Anal. At. Spectrom. 1994, 9, 431-6. (57) Kitagawa, K.; Ohta, M.; Kaneko, T.; Tsuge, S. J. Anal. At. Spectrom. 1994, 9, 1273-7. (58) Oreshkin, V. N.; Tsizin, G. I.; Vnukovskaya, G. L. Zh. Anal. Khim. 1994, 49, 755-9; Chem. Abstr. 121, 186627. (59) Itai, K.; Takikawa, S.; Tsunoda, H. Environ. Sci. (Tokyo) 1995, 3, 113-24. (60) Cimadevilla, E. A.-C.; Wrobel, K.; Sanz-Medel, A. J. Anal. At. Spectrom. 1995, 10, 149-54. (61) Alvarez-Cabal Cimadevilla, E.; Wrobel, K.; Marchante Gayon, J. M.; Sanz-Medel, A. J. Anal. At. Spectrom. 1994, 9, 117-23. (62) Ohta, K.; Isiyama, T.; Yokoyama, M.; Mizuno, T. Talanta 1995, 42, 263-7. (63) Ohta, K.; Mizutani, K.; Itoh, S.; Mizuno, T. Mikrochem. J. 1993, 48, 184-91. (64) Ohta, K.; Yokoyama, M.; Itoh, S.; Kaneco, S.; Mizuno, T. Anal. Chim. Acta 1994, 291, 115-20. 250R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

(65) Docekal, B.; Krivan, V. Spectrochim. Acta, Part B 1993, 48B, 1645-9. (66) Ohta, K.; Inui, S.; Yokoyama, M.; Mizuno, T. Anal. Chim. Acta 1994, 285, 53-6. (67) Skroce, A.; McCormick, M.; Meehan, B.; Dolic, V.; Peverill, K. Spectrochim. Acta, Part B 1993, 48B, 1639-44. (68) Ma, Y.; Li, Z.; Wang, X.; Wang, J.; Li, Y. J. Anal. At. Spectrom. 1994, 9, 679-83. (69) Bel’skii, N. K.; Timashuk, E. L.; Ochertyanova, L. I.; Garmash, A. V. Zh. Anal. Khim. 1994, 49, 825-9; Chem. Abstr. 1995, 122, 45251. (70) Shalmi, M.; Kibble, J.; Day, J. P.; Christensen, P.; Ahterton, J. C. Am. J. Physiol. 1994, 267, F695-701. (71) Monteiro, M. I. C.; Curtius, A. J. J. Anal. At. Spectrom. 1995, 10, 329-34. (72) Perez, J. M.; Sanz-Medel, A. J. Anal. At. Spectrom. 1994, 9, 1116. (73) Volynsky, A. B. Spectrochim. Acta, Part B 1995, 50B, 1417-9. (74) Krug, F. J.; Silva, M. M.; Oliveira, P. V.; Nobrega, J. A. Spectrochim. Acta, Part B 1995, 50B, 1469-74. (75) Parsons, P. J.; Qiao, H.; Aldous, K. M.; Mills, E.; Slavin, W. Spectrochim. Acta, Part B 1995, 50B, 1475-80. (76) Silva, M. M.; Silva, R. B.; Krug, F. J.; Nobrega, J. A.; Berndt, H. J. Anal. At. Spectrom. 1994, 9, 861-5. (77) Bruhn, C. G.; Ambiado, F. E.; Cid, H. J.; Woerner, R.; Tapia, J.; Garcia, R. Anal. Chim. Acta 1995, 306, 183-92. (78) Ostrega, P.; Bulska, E.; Hulanicki, A. Chem. Anal. (Warsaw) 1993, 38, 779-88. (79) Khvostikov, V. A.; Grazhulene, S. S.; Golloch, A.; Kirschner, S.; Telgheder, U. J. Anal. At. Spectrom. 1995, 10, 161-6. (80) Saprykin, Y. A.; Golovko, B. M.; Pazderskiy, Y. A. Spectrochim. Acta, Part B 1995, 50B, 815-21. (81) El-Gohary, Z. Egypt. J. Phys. 1994, 23, 25-32. (82) Stephens, R. J. Anal. At. Spectrom. 1994, 9, 675-8. (83) Sholupov, S. E.; Ganeev, A. A.; Timofeev, A. D.; Ivankov, V. M. J. Anal. Chem. (Transl. of Zh. Anal. Khim.) 1995, 50, 589-94. (84) Su, E. G.; Yuzefovsky, A. I.; Michel, R. G.; McCaffrey, J. T.; Slavin, W. Spectrochim. Acta, Part B 1994, 49B, 367-85. (85) Yuzefovsky, A. I.; Su, E. G.; Michel, R. G.; Slavin, W.; McCaffrey, J. T. Spectrochim. Acta, Part B 1994, 49B, 1643-56. (86) L’vov, B. V.; Polzik, L. K.; Novichikhin, A. V.; Fedorov, P. N.; Borodin, A. V. Spectrochim. Acta, Part B 1993, 48B, 1625-32. (87) L’vov, B. V.; Polzik, L. K.; Borodin, A. V.; Fedorov, P. N.; Novichikhin, A. V. Spectrochim. Acta, Part B 1994, 49B, 160927. (88) L’vov, B. V.; Polzik, L. K.; Borodin, A. V.; Dyakov, A. O.; Novichikhin, A. V. J. Anal. At. Spectrom. 1995, 10, 703-9. (89) Aller, A. J. Anal. Chim. Acta 1993, 284, 361-6. (90) Jotov, T. Anal. Lab. 1993, 2, 127-30. (91) Qiao, H.; Parsons, P. J.; Slavin, W. Clin. Chem. 1995, 41, 14514. (92) Sadagov, Y. M. J. Anal. Chem. (Transl. of Zh. Anal. Khim.) 1995, 50, 220-5. (93) Gilmutdinov, A. K.; Nagulin, K. Y.; Zakharov, Y. A. J. Anal. At. Spectrom. 1994, 9, 643-50. (94) Gilmutdinov, A. K.; Radziuk, B.; Sperling, M.; Welz, B.; Nagulin, K. Y. Appl. Spectrosc. 1995, 49, 413-24. (95) Schnuerer-Patschan, C.; Zybin, A.; Groll, H.; Niemax, K. J. Anal. At. Spectrom. 1993, 8, 1103-7. (96) Halls, D. J. J. Anal. At. Spectrom. 1995, 10, 169-75. (97) Hoenig, M.; Cilissen, A. Spectrochim. Acta, Part B 1993, 48B, 1003-12. (98) Halls, D. J. J. Anal. At. Spectrom. 1994, 9, 1177-8. (99) Hoenig, M.; Dheere, O. Analusis 1994, 22, 135-40. (100) Li, Z.; Carnrick, G.; Slavin, W. Spectrochim. Acta, Part B 1993, 48B, 1435-43. (101) Chernyakhovskiy, V.; Chernyakhovskaya, S.; Cirillo, A. At. Spectrosc. 1994, 15, 250-3. (102) Schneider, C. A.; McCaffrey, J. T. At. Spectrosc. 1993, 14, 6570. (103) Vinas, P.; Campillo, N.; Lopez Garcia, I.; Hernandez Cordoba, M. Talanta 1995, 42, 527-33. (104) Vinas, P.; Campillo, N.; Garcia, I. L.; Cordoba, M. H. J. Agric. Food Chem. 1993, 41, 2024-7. (105) Garcia, I. L.; Cortez, J. A.; Cordoba, M. H. Anal. Chim. Acta 1993, 283, 167-74. (106) Vinas, P.; Campillo, N.; Lopez Garcia, I.; Hernandez Cordoba, M. Food Chem. 1994, 50, 317-21. (107) Hocquellet, P. Analusis 1995, 23, 159-63. (108) Berndt, H.; Schaldach, G. J. Anal. At. Spectrom. 1994, 9, 3944. (109) Grimm, W.; Hermann, G. Spectrochim. Acta, Part B 1994, 49B, 925-39. (110) Pergantis, S. A.; Cullen, W. R.; Wade, A. P. Talanta 1994, 41, 205-9. (111) Araujo, P. W.; Gomez, C. V.; Marcano, E.; Benzo, Z. Fresenius’ J. Anal. Chem. 1995, 351, 204-8. (112) Araujo, P. W.; Marcano, E.; Gomez, C. V.; Benzo, Z. An. Quim. 1994, 90, 343-7. (113) Wienke, D.; Vijn, T.; Buydens, L. Anal. Chem. 1994, 66, 8419. (114) Kale, U.; Voigtman, E. Spectrochim. Acta, Part B 1995, 50B, 1531-41. (115) Piso, S.; de Boer, J. L. M. At. Spectrosc. 1994, 15, 220-22. (116) Su, E. G.; Yuzefovsky, A. I.; Michel, R. G.; McCaffrey, J. T.; Slavin, W. Microchem. J. 1993, 48, 278-302.

(117) (118) (119) (120) (121) (122) (123) (124) (125) (126) (127) (128) (129) (130) (131) (132) (133) (134) (135) (136) (137) (138) (139) (140) (141) (142) (143) (144) (145) (146) (147) (148) (149) (150) (151) (152) (153) (154) (155) (156) (157) (158) (159) (160) (161) (162) (163) (164) (165) (166) (167) (168)

Zheng, Y. Spectrosc. Lett. 1994, 27, 353-65. Torsi, G. Spectrochim. Acta, Part B 1995, 50B, 707-12. Yang, W.-M.; Ni, Z.-M. J. Anal. At. Spectrom. 1995, 10, 493-9. Pupyshev, A. A.; Vasil’eva, N. L.; Kalennikova, N. V.; Muzgin, V. N. Zh. Anal. Khim. 1994, 49, 1083-91. Zheng, Y.; Su, X. Can. J. Appl. Spectrosc. 1993, 38, 109-13. Zheng, Y.; Xingguang, S. Mikrochim. Acta 1994, 112, 237-43. Zheng, Y.; Wang, Y. Spectrosc. Lett. 1994, 27, 817-27. Zheng, Y.; Wang, Y. Talanta 1995, 42, 361-4. Yang, W.-M.; Ni, Z.-M. Spectrochim. Acta, Part B 1994, 49B, 1067-79. Gilmutdinov, A. K.; Zakharov, Y., A.; Ivanov, V. P.; Voloshin, A. V. Zh. Anal. Khim. 1993, 48, 813-21; Chem. Abstr. 1994, 120, 68027. Gilmutdinov, A. K.; Zakharov, Y. A.; Ivanov, V. P.; Voloshin, A. V.; Dittrich, K. Zh. Anal. Khim. 1994, 49, 361-9; Chem. Abstr. 1994, 120, 337960. Gilmutdinov, A. K.; Zakharov, Y. A.; Ivanov, V. P.; Voloshin, A. V.; Dittrich, K. Zh. Anal. Khim. 1994, 49, 150-6; Chem. Abstr. 1994, 120, 288977. Gilmutdinov, A. K.; Zakharov, Y. A.; Ivanov, V. P.; Voloshin, A. V. Zh. Anal. Khim. 1994, 49, 157-64; Chem. Abstr. 1994, 120, 288978. L’vov, B. V. Spectrochim. Acta, Part B 1993, 48B, 1633-7. Goltz, D. M.; Chakrabarti, C. L.; Sturgeon, R. E.; Hughes, D. M.; Gregoire, D.C. Appl. Spectrosc. 1995, 49, 1006-16. Masera, E.; Mauchien, P.; Lerat, Y. J. Anal. At. Spectrom. 1995, 10, 137-44. Steele, W. Chem. Rev. (Washington D.C.) 1993, 93, 2355-78. Chekalin, N.; Marunkov, A.; Axner, O. Spectrochim. Acta, Part B 1994, 49B, 1411-35. Rojas, D.; Olivares, W. Spectrochim. Acta, Part B 1995, 50B, 1011-30. Rojas, D. Spectrochim. Acta, Part B 1995, 50B, 1031-44. Aller, A. J. Anal. Chim. Acta 1994, 292, 317-24. Quan, Z.; Ni, Z.; Yan, X. Can. J. Appl. Spectrosc. 1994, 39, 549. Yan, X.; Ni, Z.; Yang, X.; Hong, G. Talanta 1993, 40, 1839-46. Yan, X.; Ni, Z. Spectrochim. Acta, Part B 1993, 48B, 1315-23. Fonseca, R. W.; Pfefferkorn, L. L.; Holcombe, J. A. Spectrochim. Acta, Part B 1994, 49B, 1595-608. Liang, Y.-Z.; Ni, Z.-M.; Yan, X.-P. Spectrochim. Acta, Part B 1995, 50B, 725-37. Habicht, J.; Prohaska, T.; Friedbacher, G.; Grassbauer, M.; Ortner, H. M. Spectrochim. Acta, Part B 1995, 50B, 713-23. Prudnikova, G. V.; Vjatkin, A. G.; Ermakov, A. V.; Shikin, A. M.; Adamchuk, V.K. J. Electron Spectrosc. Relat. Phenom. 1994, 68, 427-30. Deng, B. Fenxi Huaxue 1994, 22, 1002-5; Chem. Abstr. 1995, 122, 177086. Gao, Y.-G.; Deng, B. Gaodeng Xuexiao Huaxue Xuebao 1994, 15, 809-12; Chem. Abstr. 1995, 122, 45042. Liu, Q.; Deng, B. Taiyuan Gongye Daxue Xuebao 1993, 24, 759; Chem. Abstr. 121, 25647. Liu, Q.; Deng, B. Fenxi Huaxue 1993, 21, 1258-62; Chem. Abstr. 1994, 120, 181705. L’vov, B. V.; Novichikhin, A. V. Spectrochim. Acta, Part B 1995, 50B, 1427-48. L’vov, B. V.; Novichikhin, A. V. Spectrochim. Acta, Part B 1995, 50B, 1459-68. Jackson, J. G.; Fonseca, R. W.; Holcombe, J. A. Spectrochim. Acta, Part B 1995, 50B, 1449-57. McAllister, T. J. Anal. At. Spectrom. 1994, 9, 427-30. Sramkova, J.; Kotrly, S.; Dolezalova, K. J. Anal. At. Spectrom. 1995, 10, 763-8. Mueller-Vogt, G.; Hahn, L.; Mueller, H.; Wendl, W.; JacquiersRoux, D. J. Anal. At. Spectrom. 1995, 10, 777-83. Katskov, D. A.; Schwarzer, R.; Marais, P. J. J.; McCrindle, R. I. Spectrochim. Acta, Part B 1995, 50B, 763-80. Hadgu, N.; Ohlsson, K. E. A.; Frech, W. Spectrochim. Acta, Part B 1995, 50B, 1077-93. Torsi, G.; Fagioli, F.; Locatelli, C.; Reschiglian, P. Ann. Chim. (Rome) 1993, 83, 397-404. Byrne, J. P.; Hughes, D. M.; Chakrabarti, C. L.; Gregoire, D. C. J. Anal. At. Spectrom. 1994, 9, 913-7. Byrne, J. P.; Gregoire, D. C.; Goltz, D. M.; Chakrabarti, C. L. Spectrochim. Acta, Part B 1994, 49B, 433-43. Goltz, D. M.; Chakrabarti, C. L.; Gregoire, D. C.; Byrne, J. P. Spectrochim. Acta, Part B 1995, 50B, 803-14. Gilmutdinov, A. K.; Staroverov, A. E.; Gregoire, D. C.; Sturgeon, R. E.; Chakrabarti, C. L. Spectrochim. Acta, Part B 1994, 49B, 1007-26. Kantor, T.; Radziuk, B.; Welz, B. Spectrochim. Acta, Part B 1994, 49B, 875-91. Deng, B.; Gao, Y. Spectrochim. Acta, Part B 1994, 49B, 53744. Fonseca, R. W.; Wolfe, K. I.; Holcombe, J. A. Spectrochim. Acta, Part B 1994, 49B, 399-408. Koshino, Y.; Barukawa, A. Analyst 1994, 119, 2473-5. Pupyshev, A. A.; Muzgin, V. N. Zh. Anal. Khim. 1994, 49, 107782; Chem. Abstr. 1995, 122, 150241. Wiltshire, G. A.; Bolland, D.; Littlejohn, D. J. Anal. At. Spectrom. 1994, 9, 1255-62. Imai, S.; Ibe, T.; Tanaka, T.; Hayashi, Y. Anal. Sci. 1994, 10, 901-5.

(169) Zhang, Y.-F.; Zhang, K.; Fang, Z.; Wang, Y.-Z. J. Anal. At. Spectrom. 1995, 10, 359-62. (170) Liang, Y. Z.; Ni, Z. J. Anal. At. Spectrom. 1994, 9, 669-73. (171) Akman, S.; Doner, G. Spectrochim. Acta, Part B 1994, 49B, 66575. (172) Doner, G.; Akman, S. J. Anal. At. Spectrom. 1994, 9, 333-6. (173) Akman, S.; Doner, G. Spectrochim. Acta, Part B 1995, 50B, 97584. (174) Qiao, H.; Mahmood, T. M.; Jackson, K. W. Spectrochim. Acta, Part B 1993, 48B, 1495-503. (175) Cabon, J. Y.; Le Bihan, A. J. Anal. At. Spectrom. 1994, 9, 47781. (176) Pszonicki, L.; Essed, A. M. Chem. Anal. (Warsaw) 1993, 38, 759-70. (177) Pszonicki, L.; Essed, A. M. Chem. Anal. (Warsaw) 1993, 38, 771-8. (178) Pszonicki, L.; Skwara, W.; Dudek, J. Chem. Anal. (Warsaw) 1995, 40, 351-60. (179) Matsusaki, K.; Okada, K.; Oishi, T.; Sata, T. Anal. Sci. 1994, 10, 281-91. (180) Chuang, H.; Huang, S.-D. Spectrochim. Acta, Part B 1994, 49B, 283-8. (181) Huang, S. D.; Shih, K. Y. Spectrochim. Acta, Part B 1993, 48B, 1451-60. (182) Imai, S.; Okuhara, K.; Tanaka, T.; Hayashi, Y.; Saito, K. J. Anal. At. Spectrom. 1995, 10, 37-41. (183) Penninckx, W.; Smeyers-Verbeke, J.; Hartmann, C.; Bourguignon, B.; Massart, D. L. Chemom. Intell. Lab. Syst. 1994, 23, 137-48. (184) Popova, S.; Raouf, A.; Pankova, M. Anal. Lab. 1993, 2, 17683. (185) Zong, Y. Y.; Parsons, P. J.; Slavin, W. Spectrochim. Acta, Part B 1994, 49B, 1667-80. (186) Kurfuerst, U.; Pauwels, J. J. Anal. At. Spectrom. 1994, 9, 5314. (187) Epstein, M. S.; Turk, G. C.; Yu, L. J. Spectrochim. Acta, Part B 1994, 49B, 1681-8. (188) Morishige, Y.; Hirokawa, K.; Yasuda, K. Fresenius’ J. Anal. Chem. 1994, 350, 410-2. (189) Ouishi, K.; Yasuda, K.; Morishige, Y.; Hirokawa, K. Fresenius’ J. Anal. Chem. 1994, 348, 195-200. (190) Yasuda, K.; Hirano, Y.; Kamino, T.; Hirokawa, K. Anal. Sci. 1994, 10, 623-31. (191) Yasuda, K.; Hirano, Y.; Hirokawa, K.; Kamino, T.; Yaguchi, T. Anal. Sci. 1993, 9, 529-32. (192) Hirano, Y.; Yasuda, K.; Hirokawa, K. Bunseki Kagaku 1994, 43, 813-6; Chem. Abstr. 1995, 122, 229495. (193) Hirano, Y.; Yasuda, K.; Hirokawa, K. Anal. Sci. 1994, 10, 4814. (194) Meitzner, G.; Sinfelt, J. H. Catal. Lett. 1995, 30, 1-10. (195) Zhuang, Z.; Yang, P.; Wang, X.; Deng, Z.; Huang, B. J. Anal. At. Spectrom. 1993, 8, 1109-11. (196) Gong, B.; Li, H.; Ochiai, T.; Zheng, L. T.; Matsumoto, K. Anal. Sci. 1993, 9, 723-6. (197) Sahayam, A. C.; Tyagi, A. K.; Gangadharan, S. Fresenius’ J. Anal. Chem. 1993, 347, 461-2. (198) Hirano, Y.; Yasuda, K.; Hirokawa, K. Bunseki Kagaku 1995, 44, 521-7; Chem. Abstr. 123, 101624. (199) Liang, Y.-Z.; Ni, Z.-M. Spectrochim. Acta, Part B 1994, 49B, 22941. (200) Thomaidis, N. S.; Piperaki, E. A.; Efstathiou, C. E. J. Anal. At. Spectrom. 1995, 10, 221-6. (201) Mahmood, T. M.; Qiao, H.; Jackson, K. W. J. Anal. At. Spectrom. 1995, 10, 43-7. (202) Bulska, E.; Jedral, W. J. Anal. At. Spectrom. 1995, 10, 49-53. (203) Hanna, C. P.; Carnrick, G. R.; McIntosh, S. A.; Guyette, L. C.; Bergemann, D.E. At. Spectrosc. 1995, 16, 82-5. (204) Rademeyer, C. J.; Radziuk, B.; Romanova, N.; Skaugset, N. P.; Skogstad, A.; Thomassen, Y. J. Anal. At. Spectrom. 1995, 10, 739-45. (205) Matousek, J. P.; Powell, H. K. J. Spectrochim. Acta, Part B 1995, 50B, 857-72. (206) Shan, X.-Q.; Wen, B. J. Anal. At. Spectrom. 1995, 10, 791-8. (207) Johannessen, J. K.; Gammelgaard, B.; Joens, O.; Hansen, S. H. J. Anal. At. Spectrom. 1993, 8, 997-1004. (208) Deaker, M.; Maher, W. J. Anal. At. Spectrom. 1995, 10, 42331. (209) Rayson, G. D.; Fresquez, M. R. Can. J. Appl. Spectrosc. 1993, 38, 114-9. (210) Ni, Z.-M.; He, B.; Han, H.-B. Spectrochim. Acta, Part B 1994, 49B, 947-53. (211) Dahl, K.; Thomassen, Y.; Martinsen, I.; Radziuk, B.; Salbu, B. J. Anal. At. Spectrom. 1994, 9, 1-5. (212) Choi, J.-M.; Choi, H.-S.; Kim, Y.-S. J. Korean Chem. Soc. 1995, 39, 204-12. (213) Pita Calvo, C.; Bermejo Barrera, P.; Bermejo Barrera, A. Anal. Chim. Acta 1995, 310, 189-98. (214) Matsumoto, K. Anal. Sci. 1993, 9, 447-53. (215) Bermejo-Barrera, P.; Soto-Ferreiro, R. M.; Aboal-Somoza, M.; Bermejo-Barrera, A. Quim. Anal. (Barcelona) 1994, 13, 15-8. (216) Arruda, M. A. Z.; Quintela, M. J.; Gallego, M.; Valcarcel, M. Analyst 1994, 119, 1695-9. (217) Koshino, Y.; Narukawa, A. Analyst 1993, 118, 1027-30. (218) Tsai, S. J. J.; Jan, C. C. Analyst 1993, 118, 1183-91. (219) Tang, S.; Parsons, P. J.; Slavin, W. J. Anal. At. Spectrom. 1995, 10, 521-6.

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

251R

(220) Botelho, G. M. A.; Curtius, A. J.; Campos, R. C. J. Anal. At. Spectrom. 1994, 9, 1263-7. (221) Mizutani, A. M.; Yoshida, M. Bunseki Kagaku 1995, 44, 2315; Chem. Abstr. 1995, 122, 229514. (222) Saraswati, R.; Desikan, N. R.; Rao, T. H. J. Anal. At. Spectrom. 1994, 9, 1289-91. (223) Yuzefovsky, A. I.; Michel, R. G. J. Anal. At. Spectrom. 1994, 9, 1203-7. (224) Volynskii, A. B. J. Anal. Chem. (Transl. of Zh. Anal. Khim.) 1995, 50, 2-29. (225) Volynsky, A. B.; Tikhomirov, S. V.; Senin, V. G.; Kashin, A. N. Anal. Chim. Acta 1993, 284, 367-77. (226) Imai, S.; Nishiyama, Y.; Tanaka, T.; Hayashi, Y. J. Anal. At. Spectrom. 1995, 10, 439-42. (227) Arpadjan, S.; Alexandrova, A. J. Anal. At. Spectrom. 1995, 10, 799-802. (228) Ebdon, L.; Fisher, A. S.; Hill, S. J. Anal. Chim. Acta 1993, 282, 433-6. (229) Alvardo, J.; Cristiano, A. R.; Curtius, A. J. J. Anal. At. Spectrom. 1995, 10, 483-6. (230) Bhattacharyya, S. S. At. Spectrosc. 1994, 15, 242-4. (231) Liu, Y.; Gong, B.; Xu, Y.; Li, Z.; Lin, T. Anal. Chim. Acta 1994, 292, 325-8. (232) Cernohorsky, T.; Kotrly, S. J. Anal. At. Spectrom. 1995, 10, 15560. (233) Moreira, M. R.; Curtius, A. J.; Calixto de Campos, R. Analyst 1995, 120, 947-50. (234) Chakraborty, R.; Das, A. K.; Cervera, M. L.; de la Guardia, M. J. Anal. At. Spectrom. 1995, 10, 353-8. (235) Manzoori, J. L.; Saleemi, A. J. Anal. At. Spectrom. 1994, 9, 3379. (236) Lan, C. R.; Alfassi, Z. B. Analyst 1994, 119, 1033-5. (237) Beceiro Gonzalez, E.; Bermejo Barrera, P.; Bermejo Barrera, A. Quim. Anal. (Barcelona) 1992, 11, 17-24. (238) Shirasaki, T.; Nakamura, H.; Hiraki, K. Bunseki Kagaku 1994, 43, 1149-54; Chem. Abstr. 1995, 122, 88887. (239) Miller-Ihli, N. J. J. AOAC Int. 1994, 77, 1288-92. (240) Perez-Corona, M. T.; de la Calle-Guntinas, M. B.; Madrid, Y.; Camara, C. J. Anal. At. Spectrom. 1995, 10, 321-4. (241) Oilunkaniemi, R.; Peramaki, P.; Lajunen, L. H. J. At. Spectrosc. 1994, 15, 126-30. (242) Drake, E. N.; Hain, T. D. Anal. Biochem. 1994, 220, 336-9. (243) Gong, B.; Liu, Y.; Li, Z.; Lin, T. Anal. Chim. Acta 1995, 304, 115-20. (244) Kobayashi, R.; Imaizumi, K.; Kudo, M. Anal. Sci. 1995, 11, 2679. (245) Chakraborty, R.; Das, A. K. Fresenius’ J. Anal. Chem. 1994, 349, 774-5. (246) Jie, Z.; Sixuan, G. Analyst 1995, 120, 1661-4. (247) Berndt, H.; Mueller, A. Fresenius’ J. Anal. Chem. 1993, 345, 18-24. (248) Schaldach, G.; Berndt, H. Fresenius’ J. Anal. Chem. 1994, 350, 481-6. (249) Kojima, I.; Inagaki, K.; Kondo, S. J. Anal. At. Spectrom. 1994, 9, 1161-5. (250) Kojima, I.; Nomura, S. Anal. Sci. 1995, 11, 17-21. (251) Detcheva, A.; Havezov, I. Analusis 1994, 22, 453-7. (252) Tan, Y.; Momplaisir, G.-M.; Wang, J.; Marshall, W. D. J. Anal. At. Spectrom. 1994, 9, 1153-9. (253) Pupyshev, A. A.; Gubanova, A. N.; Byl’chenko, K. F. J. Anal. Chem. (Transl. of Zh. Anal. Khim.) 1995, 50, 158-60. (254) Woller, A.; Mester, Z.; Fodor, P. J. Anal. At. Spectrom. 1995, 10, 609-13. (255) Burns, D. T.; Chimpalee, N.; Harriott, M. Fresenius’ J. Anal. Chem. 1994, 349, 527-9. (256) Burns, D. T.; Chimpalee, N.; Harriott, M. Fresenius’ J. Anal. Chem. 1994, 349, 530-2. (257) Burns, D. T.; Chimpalee, N.; Harriott, M. Fresenius’ J. Anal. Chem. 1994, 348, 248-9. (258) Burns, D. T.; Chimpalee, N.; Harriott, M. Anal. Chim. Acta 1995, 311, 93-7. (259) Turner, A. D.; Roberts, D. J.; Le Corr, Y. J. Anal. At. Spectrom. 1995, 10, 721-5. (260) Madden, S. P.; Hueber, D. M.; Smith, B. W.; Winefordner, J. D.; Turk, G. C. Appl. Spectrosc. 1995, 49, 1137-41. (261) Calloway, C. P., Jr.; Jones, B. T. Spectrochim. Acta, Part B 1994, 49B, 1707-15. (262) Nakai, I.; Terada, Y.; Nomura, M.; Uchida, T. Physica B (Amsterdam) 1995, 208, 209, 209-11. (263) Shakher, C.; Daniel, A. J. P.; Angra, S. K. Opt. Eng. 1994, 33, 2663-9. (264) Garcia, I. L.; Vinas, P.; Cordoba, M. H. J. Anal. At. Spectrom. 1994, 9, 553-61. (265) Lahiri, S.; Yuan, B.; Stillman, M. J. Anal. Chem. 1994, 66, 295463. (266) Projahn, H. D.; Keim, A. LaborPraxis 1995, 19, 48-50, 53. (267) Zehr, B. D.; VanKuren, J. P. Spectrochim. Acta, Part B 1994, 49B, 627-32. (268) Echeverria, J.; Arcos, M. T.; Ferrandez, M. J.; Garrido Segovia, J. Quim. Anal. (Barcelona) 1992, 11, 25-33. (269) Magyar, B. Mikrochim. Acta 1993, 111, 1-36. (270) Zaranyika, M. F. Fresenius’ J. Anal. Chem. 1993, 345, 3-7. (271) Rondon, C. E.; Burguera, M.; Burguera, J. L.; Brunetto, M. R.; Carrero, P. J. Trace Elem. Med. Biol. 1995, 9, 49-54. (272) Malcheva, V.; Koleva, S.; Momchilova, E. Anal. Lab. 1993, 2, 205-9. 252R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

(273) Kauppinen, M.; Smolander, K. Anal. Chim. Acta 1994, 296, 195-203. (274) Platteau, O.; Carrillo, M. Fuel 1995, 74, 761-7. (275) Bettinelli, M.; Tittarelli, P. J. Anal. At. Spectrom. 1994, 9, 80512. (276) Mauri, A. R.; Martinez-Avila, R.; Salvandor, A.; de la Guardia, M. Ciencia (Maracaibo) 1994, 2, 87-95. (277) Nikolyukin, A. S.; Kuz’menko, N. E. Anal. Lett. 1993, 26, 203160. (278) Welz, B.; Luecke, W. Spectrochim. Acta, Part B 1993, 48B, 1703-11. (279) Momchilova, E.; Malcheva, V.; Koleva, S. Anal. Lab. 1994, 3, 38-44. (280) Momchilova, E.; Koleva, S.; Malcheva, V. Anal. Lab. 1994, 3, 105-9. (281) Chattopadhyay, P.; Mistry, M. Microchem. J. 1994, 50, 78-87. (282) Luecke, W. J. Anal. At. Spectrom. 1994, 9, 105-9. (283) Kabil, M. A. J. Anal. At. Spectrom. 1995, 10, 733-8. (284) El-Defrawy, M. M.; Kabil, M. A.; Ghazy, S. E. Analusis 1994, 22, 350-2. (285) Ghazy, S. E.; Kabil, M. A.; Mostafa, M. A. J. Anal. At. Spectrom. 1994, 9, 857-60. (286) Abdel-Halim, S. H.; Ibrahim, A. M. A. Ann. Chim. (Rome) 1994, 84, 275-81. (287) Mostafa, M. A.; Ghazy, S. E.; Kabil, M. A. Analusis 1994, 22, 440-4. (288) Luterotti, S. Analyst 1995, 120, 925-30. (289) Kantor, T. Spectrochim. Acta, Part B 1994, 49B, 1717-32. (290) Kantor, T.; Ernyei, L. Spectrochim. Acta, Part B 1994, 49B, 1733-44. (291) Omenetto, N. J. Trace Microprobe Tech. 1992, 10, 277-93. (292) Moulin, C.; Briand, A.; Decambox, P.; Fleurot, B.; Lacour, J. L.; Mauchien, P.; Remy, B. Radioprotection 1994, 29, 517-38. (293) Andrews, D. L., Ed. Applied Laser Spectroscopy: Techniques, Instrumentation, and Applications; VCH: New York, 1993. (294) Butcher, D. J. Adv. At. Spectrosc. 1995, 2, 1-62. (295) Marunkov, A.; Chekalin, N.; Enger, J.; Axner, O. Spectrochim. Acta, Part B 1994, 49B, 1385-410. (296) Enger, J.; Malmsten, Y.; Ljungberg, P.; Axner, O. Analyst 1995, 120, 635-41. (297) Enger, J.; Marunkov, A.; Chekalin, N.; Axner, O. J. Anal. At. Spectrom. 1995, 10, 539-49. (298) Yuzefovsky, A. I.; Lonardo, R. F.; Michel, R. G. Anal. Chem. 1995, 67, 2246-55. (299) Petrucci, G. A.; Beissler, H.; Matveev, O.; Cavalli, P.; Omenetto, N. J. Anal. At. Spectrom. 1995, 10, 885-90. (300) Cignoli, F.; Benecchi, S.; Zizak, G. Spectrochim. Acta, Part B 1995, 50B, 847-55. (301) Omenetto, N.; Matveev, O. I. Spectrochim. Acta, Part B 1994, 49B, 1519-35. (302) Davis, C. L.; Smith, B. W.; Bolshov, M. A.; Winefordner, J. D. Appl. Spectrosc. 1995, 49, 907-16. (303) Davies, C. M.; Telle, H. H.; Montgomery, D. J.; Corbett, R. E. Spectrochim. Acta, Part B 1995, 50B, 1059-75. (304) Bolshov, M. A.; Rudnev, S. N.; Brust, J. Spectrochim. Acta, Part B 1994, 49B, 1437-44. (305) Bolshov, M. A.; Rudnev, S. N.; Candelone, J.-P.; Boutron, C. F.; Hong, S. Spectrochim. Acta, Part B 1994, 49B, 1445. (306) Yuzefovsky, A.; Lonardo, R. F.; Wang, M.; Michel, R. G. J. Anal. At. Spectrom. 1994, 9, 1195-202. (307) Cheam, V.; Lechner, J.; Sekerka, I.; Desrosiers, R. J. Anal. At. Spectrom. 1994, 9, 315-20. (308) Axner, O.; Chekalin, N.; Ljungberg, P.; Malmsten, Y. Int. J. Environ. Anal. Chem. 1993, 53, 185-93. (309) Cheam, V.; Lechner, J.; Desrosiers, R.; Sekerka, I.; Nriagu, J.; Lawson, G. Int. J. Environ. Anal. Chem. 1993, 53, 13-27. (310) Pagano, S. T.; Smith, B. W.; Winefordner, J. D. Talanta 1994, 41, 2073-8. (311) Heitmann, U.; Sy, T.; Hese, A.; Schoknecht, G. J. Anal. At. Spectrom. 1994, 9, 437-42. (312) Su, E. G.; Michel, R. G. J. Anal. At. Spectrom. 1994, 9, 501-8. (313) Schnuerer-Patschan, C.; Groll, H.; Zybin, A.; Niemax, K. J. Phys. IV 1994, 4, C4/639-C4/642. (314) Groll, H.; Schnuerer-Patschan, C.; Kuritsyn, Y.; Niemax, K. Spectrochim. Acta, Part B 1994, 49B, 1463-72. (315) Zybin, A.; Schnuerer-Patschan, C.; Niemax, K. Spectrochim. Acta, Part B 1993, 48B, 1713-8. (316) Schnuerer-Patschan, C.; Niemax, K. Spectrochim. Acta, Part B 1995, 50B, 963-9. (317) Zybin, A.; Schnuerer-Patschan, C.; Niemax, K. J. Anal. At. Spectrom. 1995, 10, 563-7. (318) Burakov, V. S.; Isaevich, A. V.; Misakov, P. Y.; Naumenkov, P. A.; Raikov, S.N. J. Anal. At. Spectrom. 1994, 9, 307-9. (319) Burakov, V. S.; Isaevich, A. V.; Misakov, P. Y.; Raikov, S. N. Proc. SPIE-Int. Soc. Opt. Eng. 1995, 2370, 617-26. (320) Chekalin, N.; Khalmanov, A.; Marunkov, A. G.; Vlasov, I. I.; Malmsten, Y.; Axner, O.; Dorofeev, V. S.; Glukhan, E. Spectrochim. Acta, Part B 1995, 50B, 753-61. (321) Barker, P.; Thomas, A.; Rubinsztein-Dunlop, H.; Ljungberg, P. Spectrochim. Acta, Part B 1995, 50B, 1301-10. (322) Blades, M. W. Spectrochim. Acta, Part B 1994, 49B, 47-57. (323) Pavski, V.; Chakrabarti, C. L.; Sturgeon, R. E. J. Anal. At. Spectrom. 1994, 9, 1399-409. (324) Le Blanc, C. W.; Blades, M. W. Spectrochim. Acta, Part B 1995, 50B, 1395-408. (325) Imai, S.; Sturgeon, R. E. J. Anal. At. Spectrom. 1994, 9, 493-9.

(326) Imai, S.; Sturgeon, R. E.; Willie, S. N. J. Anal. At. Spectrom. 1994, 9, 759-64. (327) Imai, S.; Sturgeon, R. E. J. Anal. At. Spectrom. 1994, 9, 76572. (328) Dittrich, K.; Franz, T.; Wennrich, R. Spectrochim. Acta, Part B 1994, 49B, 1695. (329) Luedke, C.; Hoffmann, E.; Skole, J. J. Anal. At. Spectrom. 1994, 9, 685-9. (330) Gross, M.; Hermann, G. Spectrochim. Acta, Part B 1993, 48B, 1079-91. (331) Hermann, G.; Kling, B.; Koch, B. Spectrochim. Acta, Part B 1994, 49B, 1657-66. (332) Leis, F.; Steers, E. B. M. Spectrochim. Acta, Part B 1994, 49B, 289-325. (333) Piepmeier, E. H. In Glow Discharge Spectroscopies; Marcus, R. K., Ed.; Plenum: New York, 1993; pp 67-112. (334) Pavski, V.; Chakrabarti, C. L. Appl. Spectrosc. 1995, 49, 92738. (335) Parker, M.; Marcus, R. K. Appl. Spectrosc. 1994, 48, 623-9. (336) Absalan, G.; Chakrabarti, C. L.; Hutton, J. C.; Back, M. H.; Lazik, C.; Marcus, R. K. J. Anal. At. Spectrom. 1994, 9, 45-52. (337) Choi, S.-K.; Kim, H.-J. Anal. Sci. Technol. 1995, 8, 69-77. (338) Walden, W. O.; Harrison, W. W.; Smith, B. W.; Winefordner, J. D. J. Anal. At. Spectrom. 1994, 9, 1039-43. (339) Headrick, K. L.; Chakrabarti, C. L.; Bicheng, Z.; Marchand, B. Spectrochim. Acta, Part B 1994, 49B, 975-88. (340) Duan, Y.; Hou, M.; Du, Z.; Jin, Q. Appl. Spectrosc. 1993, 47, 1871-9. (341) Duan, Y.; Zhang, H.; Huo, M.; Jin, Q. Spectrochim. Acta, Part B 1994, 49B, 583-92. (342) Hou, M.; Du, Z.; Duan, Y.; Liu, J.; Jin, Q. Fenxi Shiyanshi 1993, 12, 64-7; Chem. Abstr. 1994, 120, 288871. (343) Duan, Y.; Li, X.; Jin, Q. J. Anal. At. Spectrom. 1993, 8, 1091-6. (344) Duan, Y.; Huo, M.; Liu, J.; Jin, Q. Fresenius’ J. Anal. Chem. 1994, 349, 277-82. (345) Li, Y.; Duan, Y.; Liu, J.; Jin, Q. Chin. Chem. Lett. 1993, 4, 6158. (346) Rigin, V. Anal. Chim. Acta 1993, 283, 895-901. (347) Ombaba, J. M.; Sneddon, J.; Barry, E. F. Anal. Lett. 1993, 26, 2473-89. (348) Anzano, J. M.; Paz Martinez-Garbayo, M.; Belarra, M. A.; Castillo, J. R. J. Anal. At. Spectrom. 1994, 9, 125-8. (349) Byrd, E. D.; Butcher, D. J. Spectrosc. Lett. 1993, 26, 1613-24. (350) Atsuya, I.; Minami, H.; Zhang, Q. Fresenius’ J. Anal. Chem. 1993, 346, 1054-7. (351) Anzano, J. M.; Paz, M.-G. M.; Belarra, M. A.; Castillo, J. R. Quim. Anal. (Barcelona) 1994, 13, 73-7. (352) Sahayam, A. C.; Gangadharan, S. At. Spectrosc. 1993, 14, 834. (353) Hinds, M. W.; Kogan, V. V. J. Anal. At. Spectrom. 1994, 9, 4515. (354) Koons, R. D.; Peters, C. A. J. Anal. Toxicol. 1994, 18, 36-40. (355) Ergenoglu, B.; Olcay, A. Fuel 1994, 73, 629-31. (356) Docekal, B.; Krivan, V. Spectrochim. Acta, Part B 1995, 50B, 517. (357) Luecker, E.; Thorius-Ehrler, S. Fresenius’ J. Anal. Chem. 1993, 346, 1072-6. (358) Belarra, M. A.; Lavilla, I.; Castillo, J. R. Anal. Sci. 1995, 11, 651-6. (359) Nakamura, T.; Kusata, T.; Matsumoto, H.; Sato, J. Anal. Biochem. 1995, 226, 256-62. (360) Carneiro, M. C.; Campos, R. C.; Curtius, A. J. Talanta 1993, 40, 1815-22. (361) Vinas, P.; Campillo, N.; Lopez Garcia, I.; Hernandez Cordoba, M. Fresenius’ J. Anal. Chem. 1994, 349, 306-10. (362) Hauptkorn, S.; Krivan, V. Spectrochim. Acta, Part B 1994, 49B, 221-8. (363) van Dalen, G.; de Galan, L. Spectrochim. Acta, Part B 1994, 49B, 1689. (364) Robles, L. C.; Aller, A. J. J. Anal. At. Spectrom. 1994, 9, 871-9. (365) Arruda, M. A. Z.; Gallego, M.; Valcarcel, M. Anal. Chem. 1993, 65, 3331-5. (366) Vinas, P.; Campillo, N.; Garcia, I. L.; Cordoba, M. H. At. Spectrosc. 1995, 16, 86-9. (367) Cabrera, C.; Lorenzo, M. L.; Lopez, M. C. J. AOAC Int. 1995, 78, 1061-7. (368) Morita, Y.; Tsukada, H.; Isozaki, A. Bunseki Kagaku 1993, 42, 551-6; Chem. Abstr. 1994, 120, 60521. (369) Bermejo-Barrera, P.; Barciel-Alonso, C.; Aboal-Somoza, M.; Bermejo-Barrera, A. J. Anal. At. Spectrom. 1994, 9, 469-75. (370) Bermejo-Barrera, P.; Lorenzo-Alonso, M. J.; Aboal-Somoza, M.; Bermejo-Barrera, A. Mikrochim. Acta 1994, 117, 49-64. (371) Kukier, U.; Sumner, M. E.; Miller, W. P. Commun. Soil Sci. Plant Anal. 1994, 25, 1149-59. (372) Milacic, R.; Dolinsek, F. Int. J. Environ. Anal. Chem. 1994, 57, 329-37. (373) Mierzwa, J.; Dobrowolski, R. Fresenius’ J. Anal. Chem. 1994, 348, 422-5. (374) Hauptkorn, S.; Schneider, G.; Krivan, V. J. Anal. At. Spectrom. 1994, 9, 463-8. (375) Bendicho, C.; Sancho, A. At. Spectrosc. 1993, 14, 187-90. (376) Miller-Ihli, N. J. Spectrochim. Acta, Part B 1995, 50B, 477. (377) Miller-Ihli, N. J. J. Anal. At. Spectrom. 1994, 9, 1129-34. (378) Arruda, M. A. Z.; Gallego, M.; Valcarcel, M. Quim. Anal. (Barcelona) 1995, 14, 17-26.

(379) Tittarelli, P.; Biffi, C.; Kmetov, V. J. Anal. At. Spectrom. 1994, 9, 443-9. (380) Anselmi, A.; Tittarelli, P.; Biffi, C.; Kmetov, V. Riv. Combust. 1994, 48, 53-65. (381) Friese, K.-C.; Krivan, V. Anal. Chem. 1995, 67, 354-9. (382) Vinas, P.; Campillo, N.; Lopez Garcia, I.; Hernandez Cordoba, M. Analyst 1994, 119, 1119-23. (383) Vinas, P.; Campillo, N.; Lopez Garcia, I.; Hernandez Cordoba, M. Fresenius’ J. Anal. Chem. 1995, 351, 695-6. (384) Wei, J.; Liu, Q.; Okutani, T. Anal. Sci. 1994, 10, 465-8. (385) Kubota, T.; Suzuki, K.; Okutani, T. Talanta 1995, 42, 949-55. (386) Chikuma, M.; Aoki, H.; Tanaka, T.; Tanaka, H. Biomed. Res. Trace Elem. 1993, 4, 53-4. (387) Minamisawa, H.; Arai, N.; Okutani, T. Bunseki Kagaku 1993, 42, 767-71; Chem. Abstr. 1994, 120, 123679. (388) Sedykh, E. M.; Tatsy, Y. G.; Ishmiyarova, G. R.; Ostronova, M. M. At. Spectrosc. 1994, 15, 245-9. (389) Carlos Robles, L.; Garcia-Olalla, C.; Javier Aller, A. J. Anal. At. Spectrom. 1993, 8, 1013-22. (390) Bendicho, C. Fresenius’ J. Anal. Chem. 1994, 348, 353-5. (391) Garcia, I. L.; Cortez, J. A.; Cordoba, M. H. Talanta 1993, 40, 1677-85. (392) Bautista, M. A.; Perez Sirvent, C.; Lopez Garcia, I.; Hernandez Cordoba, M. Fresenius’ J. Anal. Chem. 1994, 350, 359-64. (393) Vinas, P.; Campillo, N.; Lopez Garcia, I.; Hernandez Cordoba, M. Anal. Chim. Acta 1993, 283, 393-400. (394) Herber, R. F. M. Microchem. J. 1995, 51, 46-52. (395) Herber, R. F. M.; Grobecker, K. Fresenius’ J. Anal. Chem. 1995, 351, 577-82. (396) Pauwels, J.; Hofmann, C.; Vandecasteele, C. Fresenius’ J. Anal. Chem. 1994, 348, 411-7. (397) Hofmann, C.; Pauwels, J.; Vandecasteele, C. Fresenius’ J. Anal. Chem. 1994, 349, 779-83. (398) Minami, H.; Zhang, Q.; Itoh, H.; Atsuya, I. Microchem. J. 1994, 49, 126-35. (399) Holcombe, J. A.; Wang, P. Fresenius’ J. Anal. Chem. 1993, 346, 1047-53. (400) Wang, P.; Holcombe, J. A. Appl. Spectrosc. 1994, 48, 713-9. (401) Hinds, M. W.; Brown, G. N.; Styris, D. L. J. Anal. At. Spectrom. 1994, 9, 1411-6. (402) Brown, G. N.; Styris, D. L.; Hinds, M. W. J. Anal. At. Spectrom. 1995, 10, 527-531. (403) Nakamura, T.; Noike, Y.; Koizumi, Y.; Sato, J. Analyst 1995, 120, 89-94. (404) Docekal, B.; Krivan, V.; Franek, M. Spectrochim. Acta, Part B 1994, 49B, 577-82. (405) Kurfurst, U. Fresenius’ J. Anal. Chem. 1993, 346, 556-9. (406) Pauwels, J.; Hofmann, C.; Vandecasteele, C. Fresenius’ J. Anal. Chem. 1994, 348, 418-21. (407) Kramer, G. N.; Grobecker, K. H.; Pauwels, J. Fresenius’ J. Anal. Chem. 1995, 352, 125-30. (408) Luecker, E.; Meuthen, J.; Kreuzer, W. Fresenius’ J. Anal. Chem. 1993, 346, 1068-71. (409) Luecker, E.; Gerbig, C.; Kreuzer, W. Fresenius’ J. Anal. Chem. 1993, 346, 1062-7. (410) Nakamura, T.; Oka, H.; Ishii, M.; Sato, J. Analyst 1994, 119, 1397-401. (411) Tao, S.; Lin, S.; Kumamaru, T. Anal. Sci. 1994, 10, 59-63. (412) Yan, X.-P.; Ni, Z.-M. Anal. Chim. Acta 1994, 291, 89-105. (413) Dedina, J.; Tsalev, D. L. Hydride Generation Atomic Absorption Spectrometry; John Wiley & Sons: New York, 1995. (414) Nakahara, T. Adv. At. Spectrosc. 1995, 2, 139-78. (415) Cervera, M. L.; Montoro, R. Fresenius’ J. Anal. Chem. 1994, 348, 331-40. (416) Tao, G.; Fang, Z. Lab. Rob. Autom. 1993, 5, 299-306. (417) Van Elteren, J. T.; Das, H. A.; Bax, D. J. Radioanal. Nucl. Chem. 1993, 174, 133-44. (418) Torralba, R.; Bonilla, M.; Palacios, A.; Camara, C. Analusis 1994, 22, 478-82. (419) Ebdon, L.; Goodall, P.; Hill, S. J.; Stockwell, P.; Thompson, K. C. J. Anal. At. Spectrom. 1995, 10, 317-20. (420) Stockwell, P. B.; Corns, W. T. J. Autom. Chem. 1993, 15, 7984. (421) Lespes, G.; Seby, F.; Sarradin, P.-M.; Potin-Gautier, M. J. Anal. At. Spectrom. 1994, 9, 1433-9. (422) Rubio, R.; Alberti, J.; Padro, A.; Rauret, G. Trends Anal. Chem. 1995, 14, 274-9. (423) Chen, H.; Tang, F.; Gu, C.; Brindle, I. D. Talanta 1993, 40, 1147-55. (424) Liu, X.; Xu, S.; Fang, Z. At. Spectrosc. 1994, 15, 229-33. (425) Fernandez de la Campa, M. R.; Segovia Garcia, E.; Valdes-Hevia y Temparano, M. C.; Aizpun Fernandez, B.; Marchante Gayon, J. M.; Sanz-Medel, A. Spectrochim. Acta, Part B 1995, 50B, 377. (426) Wentzell, P. D.; Sundin, N. G.; Hogeboom, C. Analyst 1994, 119, 1403-11. (427) Ni, Z.-M.; He, B. J. Anal. At. Spectrom. 1995, 10, 747-51. (428) Cobo Fernandez, M. G.; Palacios, M. A.; Camara, C. Anal. Chim. Acta 1993, 283, 386-92. (429) Hill, S. J.; Pitts, L.; Worsfold, P. J. Anal. At. Spectrom. 1995, 10, 409-11. (430) Cabrera, C.; Madrid, Y.; Camara, C. J. Anal. At. Spectrom. 1994, 9, 1423-6. (431) Garcia, I. L.; Cortez, J. A.; Cordoba, M. H. At. Spectrosc. 1993, 14, 144-7. (432) Chikuma, M.; Aoki, H. Microchem. J. 1994, 49, 368-77.

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

253R

(433) Ritsema, R.; Donard, O. F. X. Appl. Organomet. Chem. 1994, 8, 571-5. (434) Pannier, F.; Astruc, A.; Astruc, M. Appl. Organomet. Chem. 1994, 8, 595-600. (435) Foster, R. D.; Howe, A. M. J. Anal. At. Spectrom. 1994, 9, 27380. (436) Bettinelli, M.; Spezia, S.; Baroni, U.; Bizzarri, G. At. Spectrosc. 1994, 15, 115-21. (437) Blaylock, M. J.; James, B. R. J. Environ. Qual. 1993, 22, 8517. (438) Haygarth, P. M.; Rowland, A. P.; Sturup, S.; Jones, K. C. Analyst 1993, 118, 1303-8. (439) Martin, F. M.; Tseng, C.-M.; Belin, C.; Quevauviller, P.; Donard, O. F. X. Anal. Chim. Acta 1994, 286, 343-55. (440) Martin, F. M.; Donard, O. F. X. J. Anal. At. Spectrom. 1994, 9, 1143-51. (441) Wickstrom, T.; Lund, W.; Bye, R. J. Anal. At. Spectrom. 1995, 10, 803-8. (442) Wickstrom, T.; Lund, W.; Bye, R. J. Anal. At. Spectrom. 1995, 10, 809-13. (443) Welz, B.; Sucmanova, M. Analyst 1993, 118, 1417-23. (444) Welz, B.; Sucmanova, M. Analyst 1993, 118, 1425-32. (445) Alexandrov, S.; Gafur, I.; Mandjukov, P.; Nedeltchev, O. Fresenius’ J. Anal. Chem. 1993, 347, 303-4. (446) Walcerz, M.; Bulska, E.; Hulanicki, A. Fresenius’ J. Anal. Chem. 1993, 346, 622-6. (447) Le, X.-C.; Cullen, W. R.; Reimer, K. J. Anal. Chim. Acta 1994, 285, 277-85. (448) Tao, G.; Fang, Z. Talanta 1995, 42, 375-83. (449) Walcerz, M.; Garbos, S.; Bulska, E.; Hulanicki, A. Fresenius’ J. Anal. Chem. 1994, 350, 662-6. (450) Veber, M.; Cujes, K.; Gomiscek, S. J. Anal. At. Spectrom. 1994, 9, 285-90. (451) Grotti, M.; Mazzucotelli, A. J. Anal. At. Spectrom. 1995, 10, 325-7. (452) Ni, Z.; He, B.; Han, H. J. Anal. At. Spectrom. 1993, 8, 995-8. (453) Haug, H. O.; Yiping, L. Spectrochim. Acta, Part B 1995, 50B, 1311-24. (454) Sahayam, A. C.; Natarajan, S.; Gangadharan, S. Fresenius’ J. Anal. Chem. 1993, 346, 961-3. (455) Morita, H.; Tanaka, H.; Shimomura, S. Spectrochim. Acta, Part B 1995, 50B, 69-84. (456) Saouter, E.; Blattmann, B. Anal. Chem. 1994, 66, 2031-7. (457) Corns, W. T.; Stockwell, P. B.; Jameel, M. Analyst 1994, 119, 2481-4. (458) Knox, R.; Kammin, W. R.; Thomson, D. J. Autom. Chem. 1995, 17, 65-71. (459) Urba, A.; Kvietkus, K.; Sakalys, J.; Xiao, Z.; Lindqvist, O. Water Air 1995, 80, 1305. (460) Falter, R.; Schoeler, H. F. J. Chromatogr. A 1994, 675, 253. (461) Erber, D.; Quick, L.; Winter, F.; Roth, J.; Cammann, K. Fresenius’ J. Anal. Chem. 1994, 349, 502-9. (462) Barnett, W. B.; Porter, M. K.; Kreitzberg, C. At. Spectrosc. 1994, 15, 156-60. (463) Frech, W.; Baxter, D. C.; Dyvik, G.; Dybdahl, B. J. Anal. At. Spectrom. 1995, 10, 769-75. (464) Bergdahl, I. A.; Schutz, A.; Hansson, G. Analyst 1995, 120, 1205-9. (465) Kwokal, Z.; May, K.; Branica, M. Sci. Total Environ. 1994, 154, 63-9. (466) Bruhn, C. G.; Rodriguez, A. A.; Barrios, C.; Jaramillo, V. H.; Becerra, J.; Gonzalez, U.; Gras, N. T.; Reyes, O.; Salud, S. J. Anal. At. Spectrom. 1994, 9, 535-41. (467) McIntosh, S. At. Spectrosc. 1993, 14, 47-9. (468) Chan, C. C. Y.; Sadana, R. S. Anal. Chim. Acta 1993, 282, 10915. (469) Yemenicioglu, S.; Salihoglu, I. Turk. J. Biol. 1994, 18, 261-72. (470) Sinemus, H. W.; Stabel, H. H.; Radziuk, B.; Kleiner, J. Spectrochim. Acta, Part B 1993, 48B, 1719-22. (471) Saraswati, R.; Beck, C. M.; Epstein, M. S. Talanta 1993, 40, 1477-80. (472) Fernandez, A. B.; Fernandez de la Campa, M. R.; Sanz-Medel, A. J. Anal. At. Spectrom. 1993, 8, 1097-102. (473) Gutierrez, J. M.; Madrid, Y.; Camara, C. Spectrochim. Acta, Part B 1993, 48B, 1551-8. (474) Madrid, Y.; Gutierrez, J. M.; Camara, C. Spectrochim. Acta, Part B 1994, 49B, 163-70. (475) Pszonicki, L.; Skwara, W.; Dudek, J. Chem. Anal. (Warsaw) 1994, 39, 205-15. (476) Szymd, E.; Baranowska, I. Fresenius’ J. Anal. Chem. 1994, 350, 178-80. (477) Lind, B.; Holmgran, E.; Friberg, L.; Vahter, M. Fresenius’ J. Anal. Chem. 1994, 348, 815-9. (478) Tanaka, H.; Morita, H.; Shimomura, S.; Okamoto, K. Anal. Sci. 1993, 9, 859-61. (479) Daniels, R. S.; Wigfield, D. C. J. Anal. Toxicol. 1993, 17, 1968. (480) Willie, S. N. At. Spectrosc. 1994, 15, 205-8. (481) Ebdon, L.; Goodall, P.; Hill, S. J.; Stockwell, P.; Thompson, K. C. J. Anal. At. Spectrom. 1994, 9, 1417-21. (482) Sanz-Medel, A.; Valdes-Hevia y Temprano, M. C.; Bordel Garcia, N.; Fernandez de la Campa, M. R. Anal. Chem. 1995, 67, 221623. (483) Guo, X.; Guo, X. Anal. Chim. Acta 1995, 310, 377-85. (484) Baaske, B.; Golloch, A.; Telgheder, U. J. Anal. At. Spectrom. 1994, 9, 867-70. 254R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

(485) Welz, B.; Sperling, M. Pure Appl. Chem. 1993, 65, 2465-72. (486) Burguera, J. L.; Burguera, M. J. Anal. At. Spectrom. 1995, 10, 473-8. (487) Kuban, V. Fresenius’ J. Anal. Chem. 1993, 346, 873-81. (488) Zamora, L. L.; Calatayud, J. M.; Danet, A. F.; Cheregi, M. Roum. Chem. Q. Rev. 1995, 3, 51-61. (489) Garcia, I. L.; Vinas, P.; Cordoba, M. H. J. Anal. At. Spectrom. 1994, 9, 1167-72. (490) Garcia, I. L.; Vinas, P.; Campillo, N.; Cordoba, M. H. Anal. Chim. Acta 1995, 308, 85-95. (491) Van Standen, J. F.; Hattingh, C. J. J. Anal. At. Spectrom. 1995, 10, 727-32. (492) Ferreira, A. M. R.; Rangel, A. O. S. S.; Lima, J. L. F. C. Commun. Soil Sci. Plant Anal. 1995, 26, 183-95. (493) Ferreira, A. M. R.; Lima, J. L. F. C.; Rangel, A. O. S. S. Soil Sci. 1995, 159, 331-6. (494) Araujo, A. N.; Lima, J. L. F. C. Quim. Anal. (Barcelona) 1992, 11, 55-63. (495) Xu, S.; Fang, Z. Microchem. J. 1994, 50, 145-50. (496) Matousek, A.; Burguera, J. L.; Burguera, M.; Rivas, C. Talanta 1995, 42, 701-9. (497) Burguera, M.; Burguera, J. L.; Rivas, C.; Carrero, P.; Brunetto, R.; Gallignani, M. Anal. Chim. Acta 1995, 308, 339-48. (498) Xu, S.; Fang, Z. Microchem. J. 1995, 51, 360-6. (499) Gluodenis, T. J., Jr.; Tyson, J. F. J. Anal. At. Spectrom. 1993, 8, 697-704. (500) Morales-Rubio, A.; Mena, M. L.; McLeod, C. W. Anal. Chim. Acta 1995, 308, 364-70. (501) Welz, B.; He, Y.; Sperling, M. Talanta 1993, 40, 1917-26. (502) Pitts, L.; Worsfold, P. J.; Hill, S. J. Analyst 1994, 119, 2785-8. (503) Xu, S.; Fang, Z. Fenxi Shiyanshi 1994, 13, 20-2; Chem. Abstr. 1994, 121, 98625. (504) Chen, H.; Xu, S.; Fang, Z. Anal. Chim. Acta 1994, 298, 16773. (505) Tao, G.; Hansen, E. H. Analyst 1994, 119, 333-7. (506) Salvador, A.; Martinez-Avila, R.; Carbonell, V.; de la Guardia, M. Fresenius’ J. Anal. Chem. 1993, 347, 356-60. (507) Arruda, M. A. Z.; Gallego, M.; Valcarcel, M. J. Anal. At. Spectrom. 1995, 10, 55-9. (508) Sudin, N. G.; Tyson, J. F.; Hanna, C. P.; McIntosh, S. A. Spectrochim. Acta, Part B 1995, 50B, 369. (509) Krieger, B. L.; Kimber, G. M.; Selby, M.; Smith, F. O.; Turak, E. E.; Petty, J. D.; Peachey, R. M. J. Anal. At. Spectrom. 1994, 9, 267-72. (510) Burguera, J. L.; Burguera, M.; Carrero, P.; Rivas, C.; Gallignani, M.; Brunetto, M. R. Anal. Chim. Acta 1995, 308, 349-56. (511) de la Guardia, M.; Carbonell, V.; Morales-Rubio, A.; Salvador, A. Talanta 1993, 40, 1609-17. (512) Torres, P.; Ballesteros, E.; Luque de Castro, M. D. Anal. Chim. Acta 1995, 308, 371-7. (513) Lan, W. G.; Wong, M. K.; Chen, N.; Sin, Y. M. Analyst 1995, 120, 1115-24. (514) White, R. T., Jr.; Lawrence, C. W. J. AOAC Int. 1995, 78, 99109. (515) D’Ulivo, A.; Lampugnani, L.; Sfetsios, I.; Zamboni, R.; Forte, C. Analyst 1994, 119, 633-40. (516) Bombach, G.; Bombach, K.; Klemm, W. Fresenius’ J. Anal. Chem. 1994, 350, 18-20. (517) Horvat, M.; Bloom, N. S.; Liang, L. Anal. Chim. Acta 1993, 281, 135-52. (518) Silva, I. A.; Campos, R. C.; Curtius, A. J.; Sella, S. M. J. Anal. At. Spectrom. 1993, 8, 749-54. (519) Wang, X.; Zhuang, Z.; Yang, P.; Huang, B. Microchem. J. 1995, 51, 88-98. (520) Lancaster, H. L.; Marshall, G. D.; Gonzalo, E. R.; Ruzicka, J.; Christian, G.D. Analyst 1994, 119, 1459-65. (521) Welz, B.; Sperling, M.; Sun, X. Fresenius’ J. Anal. Chem. 1993, 346, 550-5. (522) Ma, R.; Mol, W. V.; Adams, F. Anal. Chim. Acta 1994, 285, 33-43. (523) Liu, Z.-S.; Huang, S.-D. Spectrochim. Acta, Part B 1995, 50B, 197-203. (524) Naghmush, A. M.; Pyrzynska, K.; Trojanowicz, M. Talanta 1995, 42, 851-60. (525) Gallego, M.; Petit de Pena, Y.; Valcarcel, M. Anal. Chem. 1994, 66, 4074-8. (526) Ma, R.; Van Mol, W.; Adams, F. Anal. Chim. Acta 1995, 309, 395-403. (527) Fang, Z.; Xu, S.; Dong, L.; Li, W. Talanta 1994, 41, 2165-72. (528) Pei, S.; Fang, Z. Anal. Chim. Acta 1994, 294, 185-93. (529) Chen, H.; Xu, S.; Fang, Z. J. Anal. At. Spectrom. 1995, 10, 5337. (530) Zhuang, Z.; Wang, X.; Yang, P.; Yang, C.; Huang, B. Can. J. Appl. Spectrosc. 1994, 39, 101-7. (531) Min, R. W.; Hansen, E. H. Chem. Anal. (Warsaw) 1995, 40, 361-71. (532) Mendes, P. C. S.; Santelli, R. E.; Gallego, M.; Valcarcel, M. J. Anal. At. Spectrom. 1994, 9, 663-6. (533) De Pena, Y. P.; Gallego, M.; Valcarcel, M. Talanta 1995, 42, 211-8. (534) Petit de Pena, Y.; Gallego, M.; Valcarcel, M. J. Anal. At. Spectrom. 1994, 9, 691-6. (535) Santelli, R. E.; Gallego, M.; Valcarcel, M. Talanta 1994, 41, 817-23. (536) Di, P.; Davey, D. E. Talanta 1995, 42, 685-92.

(537) Gomez, M. M. G.; Garcia, M. M. H.; Corvillo, M. A. P. Analyst 1995, 120, 1911-5. (538) Maquieira, A.; Elmahadi, H. A. M.; Puchades, R. Anal. Chem. 1994, 66, 3632-8. (539) Maquieira, A.; Elmahadi, H. A. M.; Puchades, R. Anal. Chem. 1994, 66, 1462-7. (540) Elmahadi, H. A. M.; Greenway, G. M. J. Anal. At. Spectrom. 1994, 9, 547-51. (541) Elmahadi, H. A. M.; Greenway, G. M. J. Anal. At. Spectrom. 1993, 8, 1009-14. (542) van Staden, J. F.; Hattingh, C. J. Anal. Chim. Acta 1995, 308, 214-21. (543) Papantoni, M.; Djane, N.-K.; Ndung’u, K.; Joensson, J.; Mathiasson, L. Analyst 1995, 120, 1471-7. (544) Saxena, R.; Singh, A. K.; Rathore, D. P. S. Analyst 1995, 120, 403-5. (545) Tokalioglu, S.; Kartal, S.; Elci, L. Anal. Sci. 1994, 10, 779-82. (546) Yebra Biurrun, M. C.; Bermejo Barrera, A.; Bermejo Barrera, M. P. Quim. Anal. (Barcelona) 1992, 11, 299-307. (547) Kocjan, R. Analyst 1994, 119, 1863-5. (548) Hiraide, M.; Ohta, Y.; Kawaguchi, H. Fresenius’ J. Anal. Chem. 1994, 350, 648-50. (549) Kim, S. W.; Kim, D. M.; Kim, Y. S.; Lim, H. B. Anal. Sci. Technol. 1994, 7, 387-93. (550) Stresko, V.; Polakovicova, J.; Kubova, J. J. Anal. At. Spectrom. 1994, 9, 1173-5. (551) Luo, F.; Hou, X. At. Spectrosc. 1994, 15, 216-9. (552) Pyrzynska, K. Talanta 1994, 41, 381-6. (553) Tao, S.; Shijo, Y.; Wu, L.; Lin, L. Analyst 1994, 119, 1455-8. (554) Boaventura, G. R.; Hirson, J.; Santelli, R. E. Fresenius’ J. Anal. Chem. 1994, 350, 651-2. (555) Yaman, M.; Guecer, S. Analyst 1995, 120, 101-5. (556) Yaman, M.; Guecer, S. Fresenius’ J. Anal. Chem. 1994, 350, 504. (557) Avila, A. K.; Curtius, A. J. J. Anal. At. Spectrom. 1994, 9, 5436. (558) Yaman, M.; Gucer, S. Analusis 1995, 23, 168-71. (559) Peraeniemi, S.; Hannonen, S.; Mustalahti, H.; Ahlgren, M. Fresenius’ J. Anal. Chem. 1994, 349, 510-5. (560) Ramelow, G. J.; Liu, L.; Himel, C.; Fralick, D.; Zhao, Y.; Tong, C. Int. J. Environ. Anal. Chem. 1993, 53, 219-32. (561) Ashino, T.; Takada, K.; Hirokawa, K. Anal. Chim. Acta 1994, 297, 443-51. (562) Ashino, T.; Takada, K. Anal. Chim. Acta 1995, 312, 157-63. (563) Karadjova, I.; Jordanova, L.; Arpadjan, S. Fresenius’ J. Anal. Chem. 1995, 352, 604-5. (564) Ghazy, S. E.; Kabil, M. A. Analusis 1995, 23, 117-9. (565) Ghazy, S. E.; Kabil, M. A. Asian J. Chem. 1993, 5, 1078-83. (566) Wang, Z.; Li, Y.; Guo, Y. Anal. Lett. 1994, 27, 957-68. (567) Lu, G.; Xia, W.; Wan, J.; Shong, F.; Xu, H. Y. Talanta 1995, 42, 557-60. (568) Matousek, J. P.; Powell, H. K. J. Talanta 1993, 40, 1829-31. (569) Holak, W. J. AOAC Int. 1995, 78, 1124-5. (570) Wang, J.; Marshall, W. D. Analyst 1995, 120, 623-8. (571) Sanz-Medel, A. Analyst 1995, 120, 799-807. (572) Uden, P. C. J. Chromatogr. A 1995, 703, 393-416. (573) Donard, O. F. X.; Ritsema, R. Tech. Instrum. Anal. Chem. 1993, 13, 549-606. (574) Lobinski, R.; Dirkx, W. M. R.; Szpunar-Lobinska, J.; Adams, F. C. Anal. Chim. Acta 1994, 286, 381-90. (575) Ritsema, R.; Martin, F. M.; Quevauviller, P. Tech. Instrum. Anal. Chem. 1995, 17, 489-503. (576) Chang, P. P.; Robinson, J. W. Spectrosc. Lett. 1993, 26, 201735. (577) Bendicho, C. Anal. Chem. 1994, 66, 4375-81. (578) Weber, G.; Berndt, H. Int. J. Environ. Anal. Chem. 1993, 52, 195-202. (579) Berndt, H. GIT Fachz. Lab. 1995, 39, 100-2, 104. (580) Momplaisir, G.-M.; Lei, T.; Marshall, W. D. Anal. Chem. 1994, 66, 3533-9. (581) Forsyth, D. S.; Hayward, S. Fresenius’ J. Anal. Chem. 1995, 351, 403-9. (582) Wichems, D. N.; Jones, B. T. Appl. Spectrosc. 1994, 48, 273-5. (583) Cobo-Fernandez, M. G.; Palacios, M. A.; Chakraborti, D.; Quevauviller, P.; Camara, C. Fresenius’ J. Anal. Chem. 1995, 351, 438-42. (584) Pitts, L.; Fisher, A.; Worsfold, P.; Hill, S. J. J. Anal. At. Spectrom. 1995, 10, 519-20. (585) Johansson, M.; Berglund, M.; Baxter, D. C. Spectrochim. Acta,Part B 1993, 48B, 1393-409. (586) Johansson, M.; Baxter, D. C.; Frech, W. J. Anal. At. Spectrom. 1995, 10, 711-20. (587) Martin, F. M.; Donard, O. F. X. Fresenius’ J. Anal. Chem. 1995, 351, 230-6. (588) Sarradin, P. M.; Leguille, F.; Astruc, A.; Pinel, R.; Astruc, M. Analyst 1995, 120, 79-83. (589) Szpunar-Lobinska, J.; Ceulemans, M.; Dirkx, W.; Witte, C.; Lobinski, R.; Adams, F. C. Mikrochim. Acta 1994, 113, 28798. (590) Koelbl, G. Mar. Chem. 1995, 48, 185-97. (591) Matni, G.; Azani, R.; Van Calsteren, M. R.; Bissonnette, M. C.; Blais, J. S. Analyst 1995, 120, 395-401. (592) Lei, T.; Marshall, W. D. Appl. Organomet. Chem. 1995, 9, 14958. (593) Koelbl, G.; Kalcher, K.; Irgolic, K. J. Anal. Chim. Acta 1993, 284, 301-10.

(594) Tsunoda, K.; Takano, Y.; Matsumoto, K.; Kawamoto, H.; Akaiwa, H. Microchem. J. 1994, 49, 282-90. (595) Lopez-Gonzalvez, M. A.; Milagros Gomez, M.; Camara, C.; Palacios, M. A. J. Anal. At. Spectrom. 1994, 9, 291-5. (596) Martin, I.; Lopez-Gonzalvez, M. A.; Gomez, M.; Camara, C.; Palacios, M. A. J. Chromatogr. B 1995, 666, 101-9. (597) Rubio, R.; Padro, A.; Rauret, G. Fresenius’ J. Anal. Chem. 1995, 351, 331-3. (598) Smichowski, P.; Madrid, Y.; Guntinas, M. B.; Camara, C. J. Anal. At. Spectrom. 1995, 10, 815-821. (599) Dirkx, W. M. R.; de la Calle, M. B.; Ceulemans, M.; Adams, F. C. J. Chromatogr., A 1994, 683, 51-8. (600) Cai, Y.; Rapsomanikis, S.; Andreae, M. O. Talanta 1994, 41, 589-94. (601) Astruc, A.; Dauchy, X.; Pannier, F.; Potin-Gautier, M.; Astruc, M. Analusis 1994, 22, 257-62. (602) Schulze, G.; Lehmann, C. Anal. Chim. Acta 1994, 288, 21520. (603) Schickling, C.; Broekaert, J. A. C. Appl. Organomet. Chem. 1995, 9, 29-36. (604) Aizpun, B.; Fernandez, M. L.; Blanco, E.; Sanz-Medel, A. J. Anal. At. Spectrom. 1994, 9, 1279-84. (605) Sarzanini, C.; Sacchero, G.; Aceto, M.; Abollino, O.; Mentasti, E. Anal. Chim. Acta 1993, 284, 661-7. (606) Falter, R.; Schoeler, H. F. Fresenius’ J. Anal. Chem. 1995, 353, 34-8. (607) Filippelli, M. Appl. Organomet. Chem. 1994, 8, 687-91. (608) Alli, A.; Jaffe, R.; Jones, R. J. High Resolut. Chromatogr. 1994, 17, 745-8. (609) Liang, L.; Horvat, M.; Bloom, N. S. Talanta 1994, 41, 371-9. (610) Liang, L.; Bloom, N. S.; Horvat, M. Clin. Chem. 1994, 40, 6027. (611) Van Landeghem, G. F.; D’Haese, P. C.; Lamberts, L. V.; De Broe, M. E. Anal. Chem. 1994, 66, 216-22. (612) Russeva, E.; Havezov, I.; Detcheva, A. Fresenius’ J. Anal. Chem. 1993, 347, 320-3. (613) Han, H.; Liu, Y.; Mou, S.; Ni, Z. J. Anal. At. Spectrom. 1993, 8, 1085-90. (614) Hasegawa, H.; Sohrin, Y.; Matsui, M.; Hojo, M.; Kawashima, M. Anal. Chem. 1994, 66, 3247-52. (615) Wrobel, K.; Blanco Gonzalez, E.; Sanz-Medel, A. Analyst 1995, 120, 809-15. (616) Chappell, J.; Chiswell, B.; Olszowy, H. Talanta 1995, 42, 3239. (617) Torralba, R.; Bonilla, M.; Perez-Arribas, L. V.; Palacios, A.; Camara, C. Spectrochim. Acta, Part B 1994, 49B, 893-9. (618) Bermejo-Barrera, P.; Barciela-Alonso, M. C.; Ferron-Novais, M.; Bermejo-Barrera, A. J. Anal. At. Spectrom. 1995, 10, 247-52. (619) Jimenez de Blas, O.; Gonzalez, S. V.; Garrido, R. M.; Pascual, A. M.; Martin, M. A. S. Quim. Anal. (Barcelona) 1994, 13, 13843. (620) Hanna, C. P.; Tyson, J. F.; McIntosh, S. Clin. Chem. 1993, 39, 1662-7. (621) Ruede, T. R.; Puchelt, H. Fresenius’ J. Anal. Chem. 1994, 350, 44-8. (622) Jimenez de Blas, O.; Gonzalez, S. V.; Rodriguez, R. S.; Mendez, J. H. J. AOAC Int. 1994, 77, 441-5. (623) Rondon, C.; Burguera, J. L.; Burguera, M.; Brunetto, M. R.; Gallignani, M.; Petit de Pena, Y. Fresenius’ J. Anal. Chem. 1995, 353, 133-6. (624) Smichowski, P.; Camara, C. Spectrochim. Acta, Part B 1994, 49B, 1049-55. (625) Reddy, K. J.; Zhang, Z.; Blaylock, M. J.; Vance, G. F. Environ. Sci. Technol. 1995, 29, 1754-9. (626) Pyrzynska, K. Analyst 1995, 120, 1933-6. (627) Larraya, A.; Cobo-Fernandez, M. G.; Palacios, M. A.; Camara, C. Fresenius’ J. Anal. Chem. 1994, 350, 667-70. (628) Pannier, F.; Astruc, A.; Astruc, M. Anal. Chim. Acta 1994, 287, 17-24. (629) Fernandez Garcia, M.; Pereiro Garcia, R.; Bordel Garcia, N.; Sanz-Medel, A. Talanta 1994, 41, 1833-9. (630) Madrid, Y.; Cabrera, C.; Perez-Corona, T.; Camara, C. Anal. Chem. 1995, 67, 750-4. (631) Jian, W.; Mena, M. L.; McLeod, C. W.; Rollins, J. Int. J. Environ. Anal. Chem. 1994, 57, 99-106. (632) Kubrakova, I.; Kudinova, T.; Formanovsky, A.; Kuz’min, N.; Tsysin, G.; Zolotov, Y. Analyst 1994, 119, 2477-80. (633) Naghmush, A. M.; Pyrzynska, K.; Trojanowicz, M. Anal. Chim. Acta 1994, 288, 247-57. (634) Pasullean, B.; Davidson, C. M.; Littlejohn, D. J. Anal. At. Spectrom. 1995, 10, 241-6. (635) Oktavec, D.; Lehotay, J.; Hornackova, E. At. Spectrosc. 1995, 16, 92-6. (636) Milacic, R.; Stupar, J. Analyst 1994, 119, 627-32. (637) Boughriet, A.; Deram, L.; Wartel, M. J. Anal. At. Spectrom. 1994, 9, 1135-42. (638) Fung, W. S.; Sham, W. C. Analyst 1994, 119, 1029-32. (639) Soares, M. E.; Bastos, M. L.; Ferreira, M. A. J. Anal. At. Spectrom. 1994, 9, 1269-72. (640) Krekler, S.; Frenzel, W.; Schulze, G. Anal. Chim. Acta 1994, 296, 115-7. (641) Mohammad, B.; Ure, A. M.; Littlejohn, D. Mikrochim. Acta 1994, 113, 325-37. (642) Davidson, C. M.; Thomas, R. P.; McVey, S. E.; Perala, R.; Littlejohn, D.; Ure, A. M. Anal. Chim. Acta 1994, 291, 27786.

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

255R

(643) Belazi, A. U.; Davidson, C. M.; Keating, G. E.; Littlejohn, D.; McCartney, M. J. Anal. At. Spectrom. 1995, 10, 233-40. (644) Wang, J.; Marshall, W. D. Anal. Chem. 1994, 66, 3900-7. (645) Karim, M. R. O.; Karadaghi, T. M.; Najib, F. M. Mikrochim. Acta 1994, 116, 57-62. (646) Issa, Y. M.; Ibrahim, H.; Shoukry, A. F.; Mohamed, S. K. Mikrochim. Acta 1995, 118, 257-63. (647) El Ries, M. A. Anal. Lett. 1995, 28, 1629-39. (648) Esmadi, F. T.; Attiyat, A. S. Anal. Sci. 1994, 10, 687-90. (649) Esmadi, F. T.; Khasawneh, I. M.; Kharoaf, M. A.; Attiyat, A. S. J. Flow Injection Anal. 1995, 12, 35-53. (650) Attia, F. M. A. Egypt. J. Anal. Chem. 1994, 3, 173-9. (651) Eisman, M.; Gallego, M.; Valcarcel, M. J. Anal. At. Spectrom. 1993, 8, 1117-20. (652) Esmadi, F. T.; Kharoaf, M.; Attiyat, A. S. J. Flow Injection Anal. 1993, 10, 33-47. (653) Jimenez de Blas, O.; Pereda de Paz, J. L.; Hernandez Mendez, J. Quim. Anal. (Barcelona) 1992, 11, 173-84. (654) Nerin, C.; Cacho, J.; Garnica, A. J. Pharm. Biomed. Anal. 1993, 11, 411-4. (655) Ayad, M. M.; Saleh, H. I.; El-Adl, S. M.; El-Kousy, N.; Taha, E. A. Zagazig J. Pharm. Sci. 1994, 3, 32-9. (656) Bermejo-Barrera, P.; Moreda-Pineiro, A.; Aboal-Somoza, M.; Moreda-Pineiro, J.; Bermejo-Barrera, A. J. Anal. At. Spectrom. 1994, 9, 483-7. (657) Kim, I. Y.; Nakamura, H.; Yamaya, K.; Yoshida, M. Anal. Sci. 1995, 11, 363-8. (658) Bermejo-Barrera, P.; Aboal-Somoza, M.; Moreda-Pineiro, A.; Bermejo-Barrera, A. J. Anal. At. Spectrom. 1995, 10, 227-32. (659) El-Ries, M. A.; Abou Attia, F. M.; Abdel-Gawad, F. M.; Abu ElWafa, S. M. J. Pharm. Biomed. Anal. 1994, 12, 1209-13.

256R

Analytical Chemistry, Vol. 68, No. 12, June 15, 1996

(660) El Ries, M. A. Anal. Lett. 1994, 27, 1517-31. (661) Khuhawar, M. Y.; Chanar, A. H.; Qazi, I. A. J. Chem. Soc. Pak. 1994, 16, 194-5. (662) High, K. A.; Blais, J.-S.; Methven, B. A. J.; McLaren, J. W. Analyst 1995, 120, 629-34. (663) Kabzinski, A. K. M.; Takagi, T. Biomed. Chromatogr. 1995, 9, 123-9. (664) Hifney, Z. M.; Amer, S. M.; El Sherif, Z.; Amer, M. M. Egypt. J. Pharm. Sci. 1994, 35, 325-43. (665) Stuzka, V.; Soucek, J. Collect. Czech. Chem. Commun. 1994, 59, 2227-34. (666) Lahuerta, Z. L.; Martinez, C., J. Anal. Chim. Acta 1995, 300, 143-8. (667) Alaa-Eldin, F.; Kamau, G. N. Int. J. BioChemiPhysics 1994, 3, 47-50. (668) Yebra, M. C.; Gallego, M.; Valcarcel, M. Anal. Chim. Acta 1995, 308, 275-80. (669) Chattaraj, S.; Das, A. K. Indian J. Chem. Sect. A: Inorg. 1993, 32A, 1009-11. (670) Chattaraj, S.; Das, A. K. Indian J. Chem. Technol. 1994, 1, 98102. (671) Parashar, D. C.; Singh, M.; Singh, N.; Sarkar, A. K. Indian J. Chem. Sect. A: Inorg. 1994, 33A, 86-7. (672) du Toit, M. C.; du Preeez, C. C. Commun. Soil Sci. Plant Anal. 1995, 26, 69-83. (673) Yebra, M. C.; Gallego, M.; Valcarcel, M. Anal. Chim. Acta 1995, 308, 357-63. (674) Ghazy, S. E.; Kabil, M. A.; El-Defrawy, M. M. Asian J. Chem. 1994, 6, 1025-9.

A1960012L