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Atomic Absorption, Atomic Emission, and Flame Emission Spectrometry Kenneth W.Jackson* Wadsworth Center for Laboratories and Research, New York State Department of Health, and School of Public Health, State University of New York,Albany, New York 12201-0509
Huancheng Qiao Department of Chemistry, State University of New York,Albany, New York 12222
A. INTRODUCTION
is roving to be very useful in this regard. !'he James L. Waters S osium at the 1991 Pittsburgh Conference honored Sir A E G a l s h , Boris L'vov, Roy Kolrt y o h , and Walter Slavin as ioneers in AAS. Subsequently, each of them wrote a paper fescribing his own pers ective of the history of AAS, which should be essential rea4ng for any student, researcher, or serious practitioner of the technique. Walsh (2) gave a fascinating account of his first experiments on flame atomic absorption spectrometry (FAAS) and the events leadin to acceptance of the technique by the scientific community. &'vov (3) described the birth of ETAAS, which was inspired by Walsh's work, and ex lained how wanin interest in the techni ue was dramaticgy rekindled with t t e introduction of plat!orm technology. Koirtyohann (4) and Slavin (5) each gave his own viewpoint on the developments that eventuall led to the maturity of AAS. Reviews by D a m n (61,De &lan (7),and Toelg (8)compared various atomic spectrometric techni ues. Two text books on atomic spectroscopy have been pub ished (9, 10). Sturgeon (11) presented an overview of the usefulness and future prospects of ETA techniques.
This review covers fundamental aspects of atomic absorption and emission (including fluorescence) spectrometry in electrothermal and flame atomizers. Also covered are related techniques that may not be addressed in the complementary review on emission spectrometry also appearing in this issue. Our aim was to provide continuous coverage of literature published in the two years since the revious review in this series by Holcombe and Hasell (1). $he high uality of the previous review makes Jim Holcombe a "dilficult act to follow", so we tried to lay safe and use a similar structure. However, the reader wd)find mme differences, and these were largely dictated by the changing emphasis of the literature. Perhaps the most notable difference is the omission of the section on 'Specific Methods of Anal is". We believe we have still covered the material that would E v e been in that section, but it has been distributed among the other sections of the review. This review is intended to be critical, and while attempts have been made to include reference to all of the important papers, it is not comprehensive. In particular, gpplication papers are only referenced if they present data that can be used to foster further development or if we believe they will be useful to those pursuin fundamental research. Otherwise, applications are adequatefy covered in the reviews resented in alternate years in this journal. We have coninued the practice of the previous authors in including a chemical abstract number when we have referenced a pa er that may be difficult to obtain. Readers are also referretto the excellent reviews that appear periodically in the Atomic Spectrometry Updates section of the Journal of Analytical Atomic Spectrometry, and the biennial listings of the literature in Atomic Spectrometry. Inevitably, prophets of doom will declare a mature tech!ique such as atomic absorption spectrometr (AAS) to be dying", but studying the literature on electrotiermal atomic abso tion spectrometry (ETAAS) over the past 2 years should disprany such myths. We believe that research activity in this area has never been stronger. An improved understanding of the chemistry and physics of electrothermal atomization (ETA) has been accompanied by improved commerical inqtrumentation (perhaps most notably the new integrated contact graphite furnace), and there is now little doubt that absolute or standardless analysis is a realizable goal. The direct analysis of solids and slurries continues to be stronkly emphasized with progress being made toward understanding the fundamental processes of atomization from solid particles in ETAAS. Laser-excited atomic fluorescence spectrometry (LEAFS) continues to provide the best sensitivity, and the use of low-cost diode lasers is of interest. Activit in this area qould increase dramatically if these light sources L m e more widely available. The most significant new technique to have emerted lately is probably furnace atomization plasma emission spectrometry (FAPES), which has the potential for multielement atomic emission with sensitivity comparable to ETAAS. The quest for higher sensitivity shows no sign of abating, and even ETAAS may lack the sensitivity needed for some environmentalapplications. Therefore, preconcentration techniques are receiving a lot of emphasis and flow injection
C. INSTRUMENTATION AND CALIBRATION Included in this section is generic instrumentation that may be applicable to several relevant techniques. Instrumentation that is specific to a technique will be found in the appropriate section of this review. Instrumentation reviews have appeared in the Atomic Spectrometry Updates section of the Journal of Analytical Atomic Spectrometry (21-23). Also reviewed (24) was simplex optimization of instrumentation in atomic spectrometry. Those interested in instrument design are referred to Voigtman's computer simulation pro ram (251, which allows a system to be modeled on the basis ofelectronic
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1992 American Chemical Society
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B. FUNDAMENTAL DATA A welcome addition to many spectroscopists' software collections will be the database "Spectroscopic Properties of Atoms and Atomic Ions" which is available from the National Institute of Standards and Technology. Those frustrated in their searches for fundamental s ectroscopic data will also be pleased to see the review by beise (12), which lists all current compilations on wavelengths, energy levels and transition probabilities. Scheeline (13)presented a news column on fundamental reference data in Spectrochimica Acta, Part B. Other fundamental data of interest are spectral line intensity expressions (14,151, a calculation of the atomic absorption coefficient for the Se 196-nm line, and oscillator strengths for Ge (16). Devdariani and Zagrebi (17) ublished data to predict the collisional broadening and sEifting of spectral hes. L'vov and Kocharova (18) calculated correction factors to account for the effect of hyperfiie structure on signal amplitude. A potentially useful system for fundamental studies in AAS was a combined thermogravimetric analyzer and AA spectrometer (19), which allowed AA signals and thermogravimetric changes to be monitored simultaneously. Another useful diagnostic technique may be impedance atomic y t r o s c o p y (20), based on laser-induced changes in the ielectric constant of the gases in an atom cell.
ATOMIC ABSORPTION AND EMISSION SPECTROMETRY
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K“th W. Jackson attended Imperial College, London, where he received the degrees of M.Sc. (1970) and Ph.D. (1972). both in analytical chemistry. He then joined the New York State Department of Heatth as a Research Scientist. In 1977 he returned to the Untted Kingdom as a Lecturer and then Senior Lecturer of Analytical Chemistry at SheffleM City Polytechnic. In 1984 he was appointed as Professor of Analytical Chemistry at the University of Saskatchewan, Canada. Finally, he returned to 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 Heatth of the State Universtty of New York at Albany. Professor Jackson’s major research interest is in electrothermal atomic absorption spectrometry, particubrfy fundamental studies of atomization from slurry samples and mechanisms of modification in graphtte fumaces. He has carried out research on ion chromatog raphy for trace metal determinations, and he is also interested in environmental sampling and preconcentration techniques for trace metals.
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Husncheng Oiso is a graduate student in the Department of Chemistry, State University of New York at Albany. He is studying for the Ph.D. degree under the direction of Professor K. W. Jackson, and he hopes he is in his final year. He received his Bachelor’s degree from Hebei University in the People’s Republic of China (1982), and he then taught instrumental analytical chemistry as a Faculty Member in the Chemistry Department of Hebei University until 1986. During 1984-85, he spent time with Professor Tiezheng Lin at the Dalian Instttute of Physical Chemistry of the Chinese Academy , of Science, studying mechanisms of ioniza‘ tion and atomization In inductively-coupled plasmas. Mr. Qiao’s current research is focused on atomization mechanisms from solutions and slurries, and mechanisms of modification, in electrothermal atomic absorption spectrometry. He plans to continue pursuing scientlflc research, preferably in an academic career. 1
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signal and optical component blocks. Diode lasers are an interestin development in radiation sources. Their use was reviewed b 6 ) , and they are discussed in the sections on laser atomic absorption spectrometry (LAAS) and LEAFS. Hadeishi and Le Vay (27) described high-frequency discharge lamps, small enough to fit between the poles of a magnet (for Zeeman-effect AAS) and with stability comparable to hollow cathode lamps. Sample introduction in atomic spectrometry was reviewed (28). A Fabry-Perot interferometer was used in a differential absorption spectrometer (29). Applications of laboratory robotics in AAS were reviewed (30), and an expert system for automated AAS was described (31). There continues to be considerable interest in multielement AA. Simultaneous multielement AA spectrometry with a continuum source and an echelle polychromator has probably been the most successful approach, and it was applied to the analysis of biological materials using both flame and electrothermal atom cells (32). Several papers described the use of diode array detectors with this type of instrumentation. Included are photodiode arrays (33,34). Moulton et al. (34) used a linear photodiode array to provide an improved signal-to-noise ratio through pixel averaging, with a view to building a system that has a series of such detectors. Hsiech et al. (35)mounted a charge injection device (CID) to monitor a 40-nm range in the two-dimensional echelle spectrum. A charge-coupled device (CCD) was used by Schmidt et al. (36). Recent advances of CID and CCD detectors were described (37). Scheeline et al. (38) designed an echelle spectrometer specifically for use with CCD detectors. A book was published on multielement detection systems (39). The use of multiplexed hollow cathode lamps for simultaneous multielement was described in several papers (40-42). A nondispersive ETAA spectrometer (43) used a hollow cathode lamp with three separate cathodes for Fe, Al, and Cu. These cathodes were sequentially pulsed, and a time-divided system allowed the AA signals for each element to be separately detected and recorded. Sakurada et al. (44)used a similar approach with
sequential pulsing of Cu and Ag lamps and a monochromator band-pass wide enough to simultaneously transmit the Ag 328.1-nm and Cu 327.4-nm lines. A Hadamard transform spectrometer was adapted for multielement FAES (45). Busch et al. (46, 47) designed a multiple entrance-slit programmed-scanmonochromator system with fiber-optic light guides for FAES. Another design (48) had a monochromator with a fixed grating position and dispersion achieved through horizontal and vertical displacement of several entrance slits. Calibration. Baxter and Oehman (49) described a multivariate calibration method using multicomponent standard additions combined with partial least-squares modeling and demonstrated its use for reduction of spectral interferences in ETAAS. Hebisch and Henrion (50) compared the leastsquares and least-mean-squares techniques for rejecting outliers in AAS calibration. Black et al. (51) derived an equation to account for stray light in AAS calibration. A cautionary note on the use of standard additions (52)pointed out that, while multiplicative interferences are eliminated by this method, additive interferences are not. Statistical tests of calibration lines were reviewed (53). Thompson and Ramsey (54) described extrapolation to infinite dilution as a method for correcting matrix effects. Detection Limit. Stevenson and Winefordner (55) calculated the variance resulting from errors associated with estimating detection limits. This was done to allow a more meaningful comparison of the detection limits of analytical methods. A review of theoretical calculations of detection limits and error estimation was published (56). Berndt and co-workers (57,58) used signal summation to effectively improve detection limits. O’Haver (59) showed how the conventional wisdom of a narrow source line profile compared with the absorbance profile may not necessarily give the best detection limits. In some cases, better detection limits were predicted when the sources and absorbance line widths were similar. A book entitled Statistics in Spectroscopy was published (60).
D. ELECTROTHERMAL ATOMIZATION Instrumentation and Operation. The benefit of hindsight tells us that complaints of severe interferencesin ETAAS and poor reproducibilitybetween instruments during the 1970s were largely due to the heating characteristics of the Massmann-type furnaces in use at the time; i.e., vaporization of more volatile analytes before the gas temperature in the furnace was high enough to sustain free atoms and lack of spatial and temporal isothermality. The introduction of the L’vov platform in 1978 caused a dramatic improvement in heating characteristics and hence in furnace performance, so that modern ETAAS is relatively interference-free. The side-heated integrated contact furnace, first described by Frech and co-workers in 1986, eliminated the temperature gradient along the length of the tube, leading to a much closer approximation to isothermality. Berglund et al. (61)used this furnace with the added refinement of longitudinal Zeemaneffect background correction. They concluded that there is a potential for its use with a single set of operating conditions for a wide range of elements if a platform and Pd modifier are used. Since the last review in this series, a version of Frech’s furnace with longitudinal Zeeman-effect background correction has been introduced commercially (62). We predict that it will represent another major milestone toward the development of instrumentation that will enable interference-free and, possibly, absolute analysis. Reports of its performance in the analysis of real samples are eagerly awaited. The Massmann furnace, usually with a L’vov platform, remains the most common atomizer for ETAAS. A “fork”shaped design of the platform (63) overcomes problems of conventional L’vov platforms sticking in grooved tubes and thus having irreproducible heating characteristics. The new design rovided consistent results with low characteristic mass ( m J . lchlemmer et al. (64) discussed the effects of various instrumental parameters on sensitivity, detection limit, and dynamic range in ETAAS. As an alternative to the use of a L’vov platform, various graphite tube shapes have previously been described that provide more-nearly isothermal characteristics compared to the Massmann furnace. A disk-ended atomizer (65) is a tube furnace designed to minimize the longitudinal temperature gradient experienced with convenANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
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tional furnaces. At the beginning of the atomization heating step, the ends of the tube are hotter than the center, and then a quite uniform temperature distribution occurs along the entire length. Chakrabarti et al. (66) compared theoretical models of temperature distribution in a graphite furnace with experimental results. Purity of graphite used in ETAS is obviously of aramount importance; Findeisen et al. (67) demonstratelthat element- and bond-dependent limits of purification exist. Systematic errors in Zeeman-effect AAS were observed (68) due to the ac component of radiation emitted by the furnace wall. The problem was eliminated by high-frequency modulation of the hollow cathode lamp. Structured interferences in Zeeman-effect AAS were examined (69) by harmonic analysis of the signals. L’vov et al. (70) calculated the sensitivity loss when Zeeman-effect background correction was used and obtained ood agreement with experimental results. Deuterium-arc bac&round correction was unable to remove the severe background interferences from phosphate decomposition produds in the determination of Se in marine samples unless an uncoated graphite tube with a Ni modifier was used (71).
Improvements in autosampler performance by continuous degassing of the rinse solution were claimed (72). A previously developed impaction electrothermal atomizer was evaluated for the determination of trace metals in aerosols (73),and the effect of particle size on collection efficiency was evaluated (74). An alternative approach to the analysis of aerosols used electrostatic precipitation onto a tungsten electrode, which was the injected into an ETA (75). Performance data were resented (76) for a previous1 described capacitive-discharge keatin circuit. An autoproge-ETAAS instrument was describef (77). For trace and ultratrace metal determinations, ETAAS has many advantages, but fast sam le throu hout is not one of them. A typical atomization cycpe takes a out 2 min/sam le ali uot, com ared with a few seconds for flame and 18, teclniques t at use sample nebulization. Therefore, any means of reducing the time cycle is very welcome. In many cases, a fast temperature program is ossible without degradation of analytical performance. Sivin et al. (78) showed that analysis times could be reduced to about 30 s without loas of precision or accuracy by (i) overlapping the autosampler uptake with the cooldown from the previous atomization cycle, (ii) depositing the sample onto a preheated platform, and (iii) the pyrolysis step. Also, the need for use of modiiers ;!%kced by avoidin the pyrolysis step. Fast rograms were also used for the &termination of Cu, Fe, P I , and Ni in biological materials (79),V in water (801,and As (81). Slurry sampling is also amenable to fast programs (82),h a y been applied to the determination of Pb in soil (83),Mn in bio ogical materials (841, and several metals in coal and fly ash (85). In many laboratories, it is common practice to use a cool-down step after pyrolysis and immediately before the atomization step. Hoenig et al. (86) found that this ractice enhanced sensitivityof refractory and midrefractory efements when peak absorbance measurements were used. Web and co-workers continued their scanning electron microscopic studies of atomizer surfaces, examining total yrolytic graphite tubes (87) and glassy-carbon tubes (88). hicro-Raman analysis was used by Haley et al. (89) to study the surface of a pyrolytic graphite platform. Metal or metal carbide surfaces continue to be popular, especially for carbide-forming elements. For rare-earth elements, the WETA-82 tungsten-tube atomizer gave similar m,values to foil-lined graphite atomizers (90).An improved des furnace (the WETA-90) is transversely heated ( 9 r $ & E to the Frech integrated contact graphite furnace, and was reported to provide spatial and temporal isothennality. it was also reported to enable the determination of carbide-formin elements with amost no memory effects, hi h sensitivity, an! very few matrix effects. Ohta et al. useda platinum-tube atomizer for the determination of Cd (92), citin a major advantage to be the extremely long lifetime of the tu%e (>lo00 firings in air). Metal tube atomizers made of molybdenum have also been described (93,94). A tun sten-coil atomizer (95),used for the determination of As,S%,and Sn, suffered lees from chloride interference than graphite atomizers. This device was also used for the determination of Ba, Ca, and Mg in a tungstate matrix, which is undesirable in a graphite
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atomizer because of carbide formation (96).A tun ten-spiral atomizer (97) was used for determinations f y atomic fluorescence spectrometry (AFS). A separative column atomizer (98) consisted of an alumina tube with a solid stationary phase packed into a sidearm. Sam le vapors passed into the tube via the heated stationary piase prior to AA measurements in the tube, and this led to different retention characteristics for Hg, Cd, and Pb. Lining a graphite tube with metal foil or forming a carbide coating on the tube surface can lead to enhanced sensitivity and reduction of matrix effects, especially during the determination of less volatile elements. Effects of the atomization surface were discussed by Sneddon (99). Sensitivity enhancements were reported for the determination of Ga (100) and In (101) using a tantalum-lined tube, Ba, In, Cu, Ni, Li, and Y using a tun sten-lined tube (102),Li using a tantalum-coated tube (104 Ga using a mol bdenum-treated tube (1041, and Gd using 8’ tantalum boat 606). Reduced interferences were claimed in the determination of T1 using a tungsten-coated tube (106), Pb and Cd in blood and urine using a molybdenum-coated tube (107), and Ru (108) and Tl(109) in a tantalum-coated tube. A method for preparing carbide-coated tubes was described (110). Pyrzynska (111)compared tungsten- and zirconium-coated tubes with uncoated and pyrolytically-coated graphite tubes and concluded that pyrolytically-coated tubes were the most suitable for the determination of Cr. Komarek et al. (112) compared different types of carbon with various metal-lined and metal-treated tube. Radziuk and Welz (113) used model calculations to compare the atomization signals obtained in metal-lined tubes with conventional graphite tubes. Atomization Efficiency and Characteristic Mass. An electrothermal atomizer that provides truly isothermal atomization conditions, both temporally and spatially, would permit a theoretical calculation of m,. This, in turn could allow absolute analysis, Le., without the need for instrument calibration. L’vov (114) pointed out the obstacles that must be overcome to realize this ideal, including the elimination of matrix effects and the availability of improved fundamental data that are used in m, calculations. Recent improvements in the calculation of m values are described in this paper. Advances in atomizer %esignhave allowed very promising progress to be made toward absolute anal sis (115). The stabilized temperature platform furnace (8TPF) concept, which is based on the L’vov platform to+hnique, has shown experimental m data to be quite reproducible between laboratories, and tiis was im roved by using the new “forkimportant component of the sha ed= platform (116). ST8F concept is the use of modifiers to delay analyte atomization until the atomizer has reached a reasonably stable temperature, and this is instrumental in obtaining reproducible m, data (117). The improved temperature control that can be achieved by use of a two-step atomizer allowed Frech and Baxter (118,119) to investi ate the tem erature dependence of experimentally measure8 m, values. veral elements were shown to have temperature-dependent atomization efficiencies, indicating their susceptibility to form stable volatile compounds at lower temperatures. Atomization Mechanisms. L’vov and co-workers published several papers (120-125) in support of the proposed mechanism of reduction of oxides by carbon (ROC). A spirited discussion of the roposed ROC mechanism took place between L’vov and ogers at the first Rio Symposium on Furnace Atomic Absor tion Spectrometry in Sep 1988. At that time Styris and Ho combe agreed to cany out mass spectrometric studies in an attempt to shed further li ht on the mechanism. In subsequently performin this work 6261, they did not find evidence of the proposed kOC reaction pathway, which involves formation of a dicarbide through interaction of Al with graphite. Hence, they were unable to corroborate the ROC theory and suggested that atomization of alumina does not occur by this mechanism. More recently (127), L’vov et al. observed a cloud of soot during the reduction of alumina, and attributed thia to the decomposition of exgaseous carbides participating in the ROC process. The appearance of spikes during the atomization of alumina was said to be associated with the formation of a carbon film on individual analyte articles during the ROC process, and this was studied by endicho and De h-Vollebregt (128). They found that the characteristics of the spikes depended on the distribution of
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sample on the atomizer surface after g and reported that this was consistent with the ROC mo el Kilroe-Smith and Rollin (129) found that pyrolysis temperature is important during the determination of Al, since analytical sensitivity is affected if the carbide species are removed before the atomization step. Mass spectrometry is a powerful tool for investigating mechanisms in ETAAS. Styris and co-workers continue to be very active in this area, with studies on mechanisms of Y (130), Sr (131),and Be, Mg, Ca, Sr, and Ba (132). In most cases (except Be), they concluded that atomization proceeds via dissociative chemisorption of the as-phase oxides on the graphite surface. McAllister showed that as-phase oxides are present during the atomization of As, 8 , and In (133). The atomization of A1 was also studied (134). Laser microprobe mass spectrometry was used by Gucer et al. (135), to study the volatilization and thermal decomposition of Sn salts. Complementary information was also obtained from timeresolved molecular back round absorption spectra by using electron microprobe an8ysis. Kinetic studies of atomization can provide evidence of the nature of the distribution of analyte species on the atomizer surface. L ch et al. (136) reported on apparent first-order release off& and Cu, su gesting desorption as individual atoms. However, fractionaforders of desorption were obtained for Ag and Au, which is consistent with analyte desorption from micrcdropleta, caps, and islands on the graphite surface. The chemical reactivity of the atomizer surface ( yrolyti~ally-coatedgraphite, uncoated graphite, or glassy J o n ) was shown (137) to influence Co atomization. When a tungsten strip atomizer was used (1381, broadened signals for Co and Ni indicated strong interaction with the atomizer surface, possibly resulting in the formation of a solid solution. McNally and Holcombe (139) reported stron interaction of Co and Pd with graphite (to account for their {road absorbance signals), and kmetic data (firsborder release) indicated desorption from the surface as individual atoms. Conversely, In, Mn, and Al showed weaker metal raphite interactions, and desorption from aggre ates on -&, t e graphite surface was indicated. Radziuk ant! Welz (113) showed that the reactivity of anal atoms with aphite surfaces can be described by a mat ematical mo&. Arrhenius plots are often used for kinetic studies of surface reactions, and an improvement of the Smets method was claimed to eliminate the nonlinearity fre uently encountered in these plots (140). The ap roach avoi ed the assumption of first-order kinetics and t f e steady-state approximation appearing in previous models. Normally, Arrhenius lots are only linear if the region close to the beginn’ of the agsorbance peak is used. However, Chakrabarti an Cathum (141) derived equations that allow activation energies to be calculated over a wider range of absorbances (up to the peak maximum). This was applied to an investi ation of Cu atomization (142). Another new method for o%tainingArrhenius plots of better linearity involved simulation of the absorbance signal by a normal distribution function (143). In studying atomization mechanisms from the shape of absorbance profiles, a fast-res onse signal processing system is essential. The influence o time constant and atomic transfer in the gas phase on distortion and the effect on Arrhenius activation energy p ots were studied (1441, and an algorithm was presented to take account of these effects. Fonseca et al. (145)observed firsborder release of Cu from both graphite and tantalum-lined tubes, but a higher activation energy and faster atomization rate were measured for desorption from tantalum. Readsorption of Cu onto both surfacea was evident. In the past, researchers have investigated ,the spatial distribution of gaseous species during atombtiop, in attempta to elucidate atomization mechanisms. Gilmutdinov and coworkers (146,147) used shadow spectral filming to obtain spatial information. In this technique, an image of the furnace interior is obtained by back4 hting with monochromatic radiation at the analytical wavekngth. The images obtained for Tl, In, Ga, and Al indicated that the analyte atomizes from the u per furnace wall, suggesting initial vaporization from the pEtform in a molecular form, followed by atomization of these species on the hotter furnace wall. Thie process was also observed when the characteristic spikes of Al atomization were studied, but in this case ve ra id (explosive) atomization was seen. Huie and Curran748Yused a laser-based vidicon imaging system to observe the spatial and temporal distri-
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bution of Na atoms in the furnace. During the early stage of the atomization period, high atomic concentrations were seen near the furnace wall, especially when a nonpyrolytic gra hite surface was used, indicating strong interaction of Na wit! graphite. Guell and Holcombe modified a previous Monte Carlo model by including consideration of changing gas density as the furnace is heated, and the effects this haa on s ctral line shapes (149). Their modified model was a p p g d to the atomization of Cu. Monte Carlo calculations were used to investigate instrumental detection limits (150). Monte Carlo modeling is generally thought to be limited to those researchers fortunate enough to have enormous computer power available to them. However, an approach has been described that can be executed on a minicomputer (151). Computer simulations were used (152) to investigate graphite furnace AES signals. Other noteworthy papers on atomization include the use of a wavelength modulated echelle spectrometer to measure the effect of gas pressure on collisional line widths (153),a study of the influence of pressure on atomization (154),and the atomization of Ga (155),P (1561,Hg (157), and B (158). Ohlsson and Frech (159) determined concentrations of C and CN under various operating conditions and comparei the values with a thermodynamic equilibrium model. Since the CN concentration was influenced by the presence of ox gen, it was suggested that its measurement can be used to in&& air entrauaent. The dynamic range of ETAAS was extended by using an atom formation model (160). Interferences. Imai and Hayashi (161) observed double-absorbance peaks for Pb in the presence of ascorbic acid when an uncoated gra hite tube was used. It appeared that the pyrolysis of ascorgic acid produced pyrolytic graphitecoated sites on the surface, and this caused the f i s t peak. As a means of investigating interference mechanisms, Majidi et al. (162) showed the feasibility of using molecular absorption, with a laser-induced plasma as the light source, to identify transient molecular species. Katskov et al. (163) used molecular absorption measurements to identify oxides of Ga in an ETA. Multivariate calibration based on a partial leastsquares model was used by Baxter and Frech (164) to correct for spectral interferences when As was determined with continuum source background correction. Interfering species cause an observed change in the absorbance profile, and this was used by Wegaheider et al. (165)to derive empirical models that could be used to predict c es in m, and hence correct for the interference. Fazakas and ugravescu (166)suggested the use of pressurized atomization as a means of reducing some interferences. They attributed the effect to longer residence times leading to a higher vapor temperature, and consequently more efficient dissociation. The spectral interference of Fe on Se atomization was eliminated by using CO as an auxiliary purge gas (167). Its action was to prevent the formation of iron oxide b reducing the partial pressure of oxygen. The resence of gydrogen in the purge gas was shown to reduce gackground absorption from Ca compounds (168). Other papers of interest are investigations of interference mechanism of chloride on Pb and Cr (1691,chloride on Cd and Mn (170), and sulfate on Se (171). Effects of metal chlorides, sulfates, and nitrates on Se atomization ( I 72) and of organic solvents on the sensitivity of several analytes (173) were investigated. Rayson and Johnson (174) studied the nature of preatomization analyte loss durin the pyrolysis stage of furnace atomization. A different linear relationship between the amount lost vs time was derived from various kinetic orders of loss. This allowed the order to be determined ex erimentally by fitting the loas data to the equations. Slavei!ova and Tsalev (175) presented a method for determining the activation energy of analyte loas from the curve of absorbance vs pyrol sis time. The two-step furnace has proved to be extreme& useful for fundamental studies, since it allows accurate and independent control of va orization (cu ) and atomization (tube) temperatures. R e h e l d and Frect, in a cleverely-designed experiment (176), used this atomizer to separate and quantitate the volatile A1 species that are responsible for preatomization loss of Al. Modifiers.There is a tendency to replace the original term “matrix modifier” with “chemical modifier”, presumably because the latter is thought to be a more descriptive term of
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the wa in which these substances work. However, as describezbelow, there is increasing evidence that many of the stabilizing effects may occur through physical interactions. Therefore, at risk of seeming pedantic, we shall simply refer to them as "modifiers". In view of the extensive use of modifiers in ETAAS, a comprehensive bibliography by Tsalev (177),covering the period 1973-1989,and a review (178) of their use during this period and their classification are extremely welcome. Carnrick et al. (179)have also reviewed their use. In spite of the widespread use of modification, there has been comparatively little effort to investigate the mechanisms by which modifiers stabilize anal* to,enable higher pyrolysis temperatures or delay atomization untd the ETA temperature is higher. The most thorough studies have been carried out on Pd as a modifier and have mainly investigated its chemical interactions with analytes. Styris and co-workers (180) used mass s ectrometry to investi ate the stabilization of As by Pd anffound that Pd formef a mixed oxide with As in the condensed phase. In a study of Se atomization (181),they found that a thermally reduced Pd modifier prevented the formation of several molecular species and formed a thermally stable stoichiometric compound with Se. adjan et al. (182) sug ested that the modifyin effect of P (and Pt) at low moc!ifier concentrations may %edue to catalysis of reduction of analyte compounds to the atomic state, whereas at high modifier concentrations an intermetallic compound or alloy is formed. Others have sug ested that an intermetallic species is formed between Pd anfvarious analytes (183), provided that a reducing agent such as citric acid is used. Volynskii et al. (184) found that PdC12 decreased the temperature of reduction of some analyte metal oxides, and they su gested that the resulting free elements then form intermeta& compounds or solid solutions with Pd. It has been indicated (185) that the use of phase di ams of analyte elements with metallic modifiers such as% could be used to indicate their effectivenessin analyte Stabilization. Majidi and Robertson (186) showed that Rutherford back-scattering spectrometry can be extremely useful technique for monitoring surface reactions in ETAAS. They observed temperature-dependent diffusion of the analyte into the graphite surface and also concluded that Pd forms a stoichiometriccompound with Se. Although many researchers claim that Pd is most effective as a modifier when a reducing agent is resent, the combination of Pd Mg is also popular, and Hin& and Jackson (187) systematic y studied the optimum compostion of this mixture. They showed that, in the case of Pb determination, the presence of Mg was essential in order to avoid low analyte recovery. Bozsai et al. (188)found that larger amounts of added Mg improved analyte peak shape. Qiao and Jackson (189),while acknowledging that chemical interactions occur between Pd and many analytes, believe that the stabilizing effect of Pd is physical in nature. They suggested that the analyte becomes embedded in molten Pd and stabilization occurs through the rate-limiting diffusion of the analyte out of the Pd. Sharper absorbance peaks, and in many cases more complete recovery, were obtained when Mg was also present. This was explained by Mg forming an oxide layer on the graphite surface, thus causing more uniform spreading of Pd to produce smaller drop sizes so that diffusion out of the Pd would be faster. Their results are consistent with investigations of metal ion poisoning of Pd catalysts (190). The formation of CdS was said to be responsible for the stabilizing effect of S on the determination of Cd in a molybdenum-tube atomizer (191). Keleman et d. (192) obtained pyrol ais curves for several solid Ba compounds (mol bdate, vanaiate, and molybdovanadate), in order to pre2ct the stabilizing effect of these anions when used as modifiers for the determination of Ba. The stabilization of Ge by Ni and Fe was tho ht to occur through the formation of intermetallic com undsy193). Slaveikova and Tsalev (194)studied several m s e r mixtures containing W, including mixtures of W and Pd, and showed stabilization of several elements. Several mechanisms were proposed, including incorporation of analytes within the crystal lattice of tungsten oxides and formation of thermally stable com unds such as PbW04. The efficiency of the mixed modi!% W/Pd was articularly pronounced for several elements, frequently enatling hi her pyrolysis temperatures compared with the use of Pd$Mg mixtures. A mixture of V/Pd generally enabled maximum
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 12, JUNE 15, 1992
Table I. Applications of Modifiers analyte
matrix
A1 Ba Be Bi
aqueous solution seawater sediment aqueous solution aluminum biological material urine seawater seawater; serum; urine aqueous solution milk infant formula; milk blood; serum gallium arsenide blood fly ash boric acid silicate rocks aqueous solution aaueous -=-----solution - - -
Cd Cr Mn Mo Pb Se Sn T1
modifier
ref
205 206 207 c d ; PC" 208 Ni(N03)z/Cu(N03)2/EDTA 209 S 94 Pd/NH4N03 210 NazWO, 211 Ca(N03)z/MdN03)z 212
CdCIZ(PdC1, ascorbic acid Ca(N03)~ Ni(NO&/EDTA . I -I
W
213 214,215 215 216 217 218 219 220 221 222 106
pyrolysis temperatures that were at least as high as those reported elsewhere for a Pd Mg mixture (195). A Ce/Pd mixture was also effective ( 96). Ohlsson and Frech (159) showed that M (NO4),and (NH,),HPO reduce the partial pressure of CI'!during atomization, wfiich may increase atomization efficiencies of cyanide-forming elements. Ascorbic acid, when used as a modifier for Pb, eliminates interference by chloride and lowers the appearance temperature of Pb. It was proposed by Gilchrist et al. (197) that the gas-phase dissociation equilibrium of analyte oxides is disturbed by the pyrolysis products of ascorbic acid (H and CO). Alkali-metal fluorides are effective modifiers for di determination, and it was su gested that they produce stable hexafluorosilicates (198). mixture of HF and CsF was used for the stabilization of Al(199), and this was thought to be due to the formation of AlF3,which sublimes at 1291 "C. Phosphate is enerally considered to act as a modifier by forming a s d l e pyrophosphate. Hassell et al. (200) used static secondary ion mass spectrometry and determined that, for Cd, these reactions occur on the graphite surface rather than in the gas hase. however, anfthere Such reactions were not observed for was no stabilization of this analyte. Pd/Mg modifier is commonly used for Se determination, but the addition of Ba(N0J2 was found to be beneficial to reduce sulfate interference (171). Modification of As was achieved (201) by its conversion to arsenomolvbdic acid, in order to avoid loss during pyrolysis. When ETAAS is preceded by chelation and solvent extraction to preconcentrate trace metals, it may be useful to have modifiers available that are soluble in organic solvents. Katsura et al. (202) investigated several organo-Pd complexes that were suitable in this regard. An interesting use of a Pd or Pt modifier was to separate temporally the absorbance signals for Ag and Cu, thus allowin their sequential determination in a single firing of the ETA, by using a monochromator with a wide enough bandwidth to pass both the Ag 328.1-nmand Cu 327.4-nmlines from a multielement lamp (203). For ultratrace metal determinations, contamination of modifiers may be a problem. Bulska et al. (204) overcame this by preheating a Pd/Mg modifier to 1100 "C in order to volatilize Cd impurities rior to injection of the sample. Some interestin and unusuaf)applications of modifiers are shown in Table
I
x
2
f
E. FLAME ATOMIC ABSORPTION AND EMISSION The concentric nebulizer and spray chamber in commercial flame spectrometers have remained essentially unchanged over the years, in spite of the low efficiency of sample transportation to the flame. It is interesting to see that attempts to improve the nebulization process continue. Perhapa the most interesting is thermospray sample introduction,which appears to provide substantial improvements in sensitivity compared with conventional nebulizers. Robinson and Choi described
ATOMIC ABSORPTION AND EMISSION SPECTROMETRY
a circular flame atomizer with thermospra sample introduction for the determination of H (223) a n i a thermospray as a reconcentrating device for #AAS (224). Russell and Step ens (225) used a radio-fre uency plasma to generate an aerosol suitable for FAAS. Hylraulic high-pressure atomization was described by Berndt and Schaldach (226). The influence of the hysical roperties of the solvent on the primary aerosol opd! size fistribution, transport efficiency, and sensitivit were examined for pneumatic nebulization (227). The ef&ct of long-chain Surfactants on aerosol properties was examined (228). Published reviews include the properties of aerosols from an analytical spectrometry perspective (229) and recent develo menta in nebulizer systems (230). willis et al. (231) presentelsome criteria for comparing the performance of commercial spray chambers and burners. The also described a new burner with a flared slot to reduce blo&ge when solutions of high salt content are analyzed. A “jaw-type”burner was also described by Gruber and Herbauts (232). Gordon and Cresser (233) incorporated a light-scattering device into a FAAS s ray chamber to monitor changes in aerosol transport, and ence provide an error warnin Back ound correction in FAES can be achieved throug! sampg modulation, which involves periodically introducin the analyte into the flame to produce an ac analytical signa! from the flame background dc s‘ thatcanbedis Wolf et al. (234)eacribed a s tem that used a d u a l - n e b E r system to allow sample modztion by alternately introducing blank and analyte solutions. Hu et al. (235) identified a relationship between the structure of a s ecies and its atomization mechanism in a flame. A methafwas developed (236)for measur’ temporal and s atial temperature distributions in flames.?he spatial disdution of Ti in a N20C2Hzflame was studied (237).The determination of metals in organic solvents is important for monitorinq; wear metals in oils. Watling et al. (238) showed that an i o m t i o n suppressor in aqueous solution was effective when added to the o anic sample stream during nebulization. Metals in organic slvents were determined by using a detergent to form an emulsion with water and directly nebulizing the emulsion into the flame (239). Atomization processes of emulsions were investigated (240). A model was developed (241) to explain the effects of surfactants in FAAS. Pharr et al. (242) investi ated the use of surfactants for sensitivity enhancement. method involving discrete sample nebulization was develo for the analysis of steels (243). Beinrohr (244) compared iscrete and continuous sample introduction and showed that continuous introduction of a diluted sample can still provide lower determinable limits than discrete introduction of the same sample after preconcentration. He also concluded (245) that peak area measurements are referable to eak height measurements for transient sign& geveral interference studies were carried out, including the effects of oxygen-containin compounds on the determination of rare-earth elements in a% M z H 2flame (2461,and alkaliand alkaline-earth-metal s& on transition metals (247). Gilbert et al. (248) developed an automated detection system for interference in flames that involved rapidly moving the burner head up and down. Calibration was performed at two heighta in the flame, and the sample concentration at each height was calculated b comparing the absorbance with the standard curve at that geight. Any significant difference in the two calculated concentrations indicated an interference. It is interesting to see that the conce t of “standardless” or “absolute” analysis is not limited to 8TAAS (see the discussion in that section of this review). Magyar et al. (249) determined the two most important factors needed for calculating theoretical sensitivities, Le., the fraction atomized and the dilution factor arising from the nebulization process.
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B“d
F. LASER-EXCITED ATOMIC FLUORESCENCE A book, Laser Microanalysis ( S O ) , rovides introductory reading on the roperties of lasers a n 8 also a short chapter on LEAFS. TRe ma’or obstacle that has prevented commerical exploitation o i the techni ue is probably the cost and complexity of laser light sources. %herefore, the investigation of simpler and less expensive lasers in Winefordner’s laboratory is of particular interest. Walters et al. (251) described the determination of Rb by flame AFS with excitation by a diode laser at 780.0 nm. Currently, these lasers are of limited
application, since they are only available at operating wavel e n f u greater than about 670 nm. However, &ode lasers have wi espread use in various consumer products, so they are undergoing continual development and this limitation may not persist for long. Resonance line lasers have also been used as a simpler nontunable alternative to dye lasers, and Simeonmon et al. (252) reported sensitivities for the determination of T1 and In that were comparable to those obtained with dye lasers. Some comparisons of different laser-pumped dye laser systems were published (253,254). A ating circuit for use with photomultiplier tubes was designecffor rejecting stray light in laser-excitation techni ues (255). Discussed in this section are sever3 atom sources used for LEAFS, but those seeking to establish new “world record” detection limits (what comes next below attograms?) may be most interested in the extremely powerful combination of a laser and an ETA. Reviews of ETA-LEAFS were published by Sjoestroem (256) and Omenetto (257). The simultaneous measurement of analyte and background radiation is desirable when transient signals are monitored. This was achieved by using optical fibers to split the radiation from the monochromator and direct it simultaneously to two photomultiplier tubes (258) and by using a dual monochromator (259). Wei et aL (260),in comparing different instrumental confiiations, found that nondispersive fluorescence measurements and front-surface illumination of the ETA gave better detection limits than dispersive fluorescence and transverse illumination. Doughert et al. (261) sought to increase the linear dynamic range of 6TA-LEAFS (alread an impressive 4-6 orders of magnitude) and recommendedrfront-sur face illumination to minimize postfilter effects and/or self-absorption. A furnace atomizer allowing continuous sample introduction was described (262). Computer modelin was shown to be a useful tool for optimizing the o tical cofiection efficiency in ETALEAFS, with a view to o!tainin improved detection limits (263). Ap lications of ETA-L&S included determinations of Tl in soktions (264)and solid samples (265),Pb in Antarctic metals in air by direct impaction onto an ETA (267), ice (266), Tl, Mn, and Pb in food slurries (268),and T1 and Pb in solid samples of alloys (269). her-excited molecular fluorescence in graphite furnaces was repoted for the determination of polycyclic aromatic hydrocarbons (270,2711, F as MgF (272, 273), and C1 as InCl (274). A pulsed glow discharge atom cell with synchronous pulsing of the laser source was evaluated for the LEAFS analysis of a ueous solutions (275,276),and solid samples (276). Cath08ic s uttering cells were also used for the determination of Fe in grass (277) and Pb in copper (278). Walton et al. (279) used a flame for the LEAFS determination of organometallic compounds after separation by HPLC. Of possible interest to the analytical chemist is a pa er describing the use of a photodiode array detector for t t e laser-excited molecular fluorescence measurements of OH in flames (280). Sparksource atomization was used (281) for the determination of metals in gas particulates.
G. OTHER TECHNIQUES Furnace Atomization Plasma Emission Spectrometry (FAPES). This technique involves formation of an atmospheric pressure plasma in a graphite furnace. It is undoubtedly one of the most exciting developments to have occurred in recent years, and it should allow simultaneous multielement AES determinations on microsamples with very high sensitivity. The techni ue was first described by Liang and Blades (282), who forme8 a radio-frequency capacitively-coupled plasma between a gra hite furnace tube and an electrode that was mounted coaxidy in the tube. Subsequently, Blades and co-workers described the anal ical characteristics of this source for the determination of (283). A detection limit of
