Anal. Chem. 1000, 62, 169R-184R (474) Aizenber , L. V.; Qoryunova, N. N.; Dvornlkov, A. N. Zh. Anal. K M . 1987, 42, 2871-5; Chem. Abstr. 1988, 108, 87290q (475) Lee, Albert W. M.; Chan, W. H.; Chlu, Connle M. L.;‘ Tang, K. T. Anal. chkn.A . ~_ l a1989. .. 218. 157-80. (476) Iskandw, Madkne’L.; Medlen, H. A. A.; Nashed, S. Mlcrochem. J . 1987, 36, 388-76. (477) Sastry, C. S. P.; Tlpknenl, A. S. R. P.; Swyanaranana. M. V. Indian 1989, 26, 351-3; chem.Abstr. 1989, 111. 100837~. (478) Marlnl, Sergb I d . &V8nde 1988, 17, 423-4; Chem. Abstr. 1989, 110, 73928h. (479) Asabe, Yoshlhlro; Anraku, Masayukl; Takltani, Shoji Iyekuhln Kenkyu 1988, 19, 700-8; Chem. Abstr. 1989, 110, 127812m. (480) Mohamed, Abdel Maboud I. T8bnta 1988, 35, 821-4. (481) Suzukl, Masao; Nlshinaka, Tomoko; Takltani, Shoji Iyekuhln Kenkyu 1987, 18, 749-52 Chem. Abstr. 1988, 108, 1132Od. (482) Onur, Feyyaz; Acar, Nevin Oezl Unlv. EczaCMRC F8k. Dwg. 1988, 5, 187-74; Chem. Abstr. 1989, 110, 219218a. (483) ISsOpoulos, Prodromos E. Analyst (London) 1989, 114, 237-9. (484) Szakacs Pinter, Mar@; Laszlo, Erika; Per1 Mdnar. Ibolya Mgy. Kem. Fdy. 1987, 93, 480-3; Chem. Ab&. 1988, 108, 1056871. (485) Zlvanovlc, Ljnjane; Zlvanov-Stakic, Dobrila Am. arm. 1987,37,2172 2 Chem. Abstr. 1988, 108 137942b. (486) ZbanOvlc, Ljlljane A h . F8fm. 1987, 37,279-83; Chem. Abstr. 1888, 109, 98921t. (487) Kljenska, Danuta R . Cent. Inst. Ochr. R. 1987, 37, 43-8; Chem. Abstr. 1088, 108, 81032k. (488) hntonl, G.; Mura, P.; Pinzauti, S.; Gratteri, P.; La Porta, E. Int. J. phafm. 1989, 50, 75-8. (489) Emara, Kamla M.; El-Kommos, Mlchael F. Alexandrle J . Pharm. Scl. 1989. 3, 19-22; Chem. Abstr. 1989, 111, 71090a. (490) Ock, Chi Wan; Balk, Chae Suen Yekh8k Hoechl 1987, 31, 330-7; Chem. Abstr. 1988, 108, 82228~. (491) Xin, Xun Yeoxue Tongb80 1987, 2 2 , 74-5; Chem. Absf. 1988, 108, 11310a. (492) Qian, Shougen Shipin Yu Fejko Gongye 1987, 48-51; Chem. Abstr. 1988, 108, 201110h. (493) EI-Dln, M. Sharaf; Belai, F.; Hassan, S. Zmtr8bl. pharm., Pharmakolhsr LabOretWhMdkgn. 1988, 127, 133-5; Chem. Abstr. 1988, 109, 798192. (494) ECKommos, Michael E.; Emara, Kamla M. Alexendrie J. phefm. Scl. 1988, 2, 171-8; Chem. Abstr. 1989, 110, 1604844. (495) Chen, Yi YQao Gongye 1987, 18, 327-8; Chem. Abstr. 1988, 108. 11299d. (498) Ichlba, Hldeakl; Morishlta, Muneo; Yajima, Takehiko Chem. Pharm. Bull. 1988, 36, 5009-11.
(497) Nyyssonen. Kristilne; Parviainen, M. T.; Penttila, I. M. J . CUn. Chem. CHn. Blochem. 1988, 26, 219-22. (498) Petrowltz, H. J.; Wagner, M. Fresenk’ Z. Anal. Umm. 1988, 330, 125-8; Chem. Ab&. 1988, 709, 313263. (499) Kuklinskil, A. Ya.; Bogaevskaya, M. T.; Fedotova, L. V.; LarlcMtine, T. V. Nefleperereb. NeMhim. (MOscuw) 1988, 19-20 Chem. Abstr. 1080, 110, 687881. (500) Geblckl, Janusz M.; Gulile, Jennlfer Anal. B/ochem. 1989, 176, 360-4. (501) Slngh. V.; Shukla, S. K.; Mahanwal, J. S.; Ram, Jagdish phamzk, 1989. 44, 229-30. (502) Koleva, M.; Nlnov, S.; Koiev, S. Dokl. Bo/g. Aked. &auk 1988, 41, 39-42; Chem. Abstr. 1989, 110, 107364~. (503) Iovchev, I.; Lazova, 0. F8fm8&&8 SOW 1988, 38, 24-30; Chem. Abstr. 1988, 109, 116148~. (504) Becker, 6.; Krone, I . phann. f m x . 1988, 43, 168-8; Chem. Abstr. 1988. 109, 218078~. (505) Morelii, Basillo; hriani, Marina; Gesmundo, Mariateresa Anal. Lett. 1987, 20. 1429-50. (506) Li, Zhongzhong; Wu. Licun Shenyang Yaoxueyuan Xuebao 1988, 5, 94-6, 148; Chem. Abstr. 1988, 109, 118125t. (507) Gantverg, A. N. G@. Sann. 1988, 36-7; Chem. Abstr. 1988, 109, 188907~. (508) Kuzmicka, L.; Ptuanowska-Tarasiewicz, H.; Tarasiewicz, M. Phamzis 1988, 43. 288-9; Chem. Abstr. 1988, 109, 278761. (509) El-Shabouri. Saiwa R.; Mohamed, Fardous A.; Mohamed, Abdel Moboud I . Talenta 1987. 34, 968-70. (5 IO) Tomaszewski, Leszek; Seraflnska, Barbara; Szaniawska, Marla Dagn . Leb. 1988. 24, 58-67; Chem. Abstr. 1088, 109. 89152r. (511) Fabregas, J. L.; Casassas. E. Drug. Dev. I d . pharm. 1988, 14, 155-63. (512) Yuan, Jimln; CUI, Min Ylyeo O g y e 1987, 18, 497-9; Chem. Abstr. 1988, 108, 27035a. (513) Fllipeva, S. A.; Streiets, L. N.; Petrenko, V. V.; Buryak, V. P. F8fm8tS k 8 (MOSCOW) 1987, 36, 39-42; Chem. AbStr. 1988, 108, 101438~. (514) Veselov, V. Ya.; Savel’ev, Yu. V.; Grekov, A. P. Zh. Anal. Khlm. 1088, 43, 1125-7; Chem. Abstr. 1989, 110, 165286a. (515) Reo, G. Ramana; Avadhanulu, A. 8.; Glrldhar, R. Indkn Drugs 1989, 2 6 , 298-300; Chem. Absf. 1989, 110, 219174h. (516) Saha, Upal J.-Assoc. Off. Anal. Chem. 1989, 72, 242-4. (517) Llang, Derong; Qin, Yongping; Llang, Maozhi; Huang, Ylng; Zeng, Jingze H m l Yke &xu8 Xuebao 1987, 18,256-8 Chem. Abstr. 1988, 108, 87474c. (518) Rizk, M.; Walash, M. I.; ECBrashy, A. Spectrosc. Lett. 1988, 2 1 , 393-409.
Atomic Absorption, Atomic Emission, and Flame Emission Spectrometry James A. Holcombe* and D.Christian Hassell Department of Chemistry, University of Texas at Austin, Austin, Texas 78712
A. INTRODUCTION This biennial review concentrates on fundamental studies in the areas denoted in the title and represents an attempt to continue coverage from the previous review of this same series (AI). A more comprehensive focus on applications can be found in alternate years in this same journal (e.g., ref A2). Complementary reviews in areas of atomic spectrometry in this same journal issue include that on emission (A3) and elemental mass spectrometric (A4) a proaches to analysis. Within the last two years, relevant ‘terature reviews have been published & la Atomic Spectrometry Updates in Journal of Analytical Atomic Spectrometry including a review of atomization and excitation (A5). Others will be discussed in their relevant section. Atomic Spectrometry continues to include their review of the literature on a biannual basis. Other review articles will be found throughout this article in the appropriate sections. In the application section of this review we have attempted to include literature references which are generally available in major libraries in the western hemisphere with a preference given toward articles written in English, unless we felt that the application or a proach was extremely novel. This same filter was not used o!r the studies with a more fundamental bent since this is intended to be the primary focus of this review. In instances where the journal may not be readily
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available in major libraries, we attempted to include the chemical abstract number for the reader’s ease of perusing the general contents. General trends observed since the last review include a continued dominance of this area of the literature with electrothermalatomization (ETA) and an increasing number of studies using this mode of atomization for atomic as well as in ita more traditional role in fluorescence (AF’) atomic absorption spectrometry ( U S ) . One is also left with the im ression that a fundamental understanding of ETA has s t a r t e f t o solidify with an attendant refocusing on the surface chemistry, rather than the gas phase, as a requirement in developing a complete picture of processes res onsible for atomization and interference phenomena. The Lvelopment of some rudimentary understanding of this relatively complex system that makes up ETA has occasionally resulted in a migration of traditional ugas phase spectrosco ists” into other modes of data collection for interpretation of multifaceted and multiphase problem. The flame still serves as a convenient atom generator for purauing other novel spectroscopicprobes, but basic analytical studies of flames and research in the area of flame emission (FE) and flame atomic absorption (FAA) have all but disappeared from the arena of active research compared to other areas. This attenuation in literature citations for FAA and
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FE can be partially attributed to the community’s recognition that techni ues such as inductively coupled plasmas (ICP) are better fllin the niche previously occupied by FAA and FE. Additionaiy , the improvement in competitive pricing of some versions of ICP no lon er presents the large monetary barrier, which previously prec uded adapting this technique in lieu of flame-based methodologies. A continued emphasis on coupling separation techniques with the selectivity of atomic techniques speaks both to the obvious productivity of participating in the trendy implementation of “hyphenated techni ues” and to the emerging interest in obtainin species-speci ic information at the trace and ultratrace leve s. An interest in minimizing sample preparation time as well as providing the potential for improved relative limits of detection has seen a significant growth in slurry sample introduction techniques, with most ap lications in the area of ETA. Improvements in hydride tec niques and preconcentration procedures for ETA also sug est an implied need, or at least the researchers’ desire, for refative limits of detection a t the sub-ppb levels. One must not leave the arena of rognostication without reference to Hieftje’s prediction on t e future decline in AAS as a result of his curve fitting and extrapolating AA literature referencing over the past several years (A6). Thm article serves as a reminder of how interesting the game of prognostication can be .... A recent dramatic increase in the use of mammals (dog blood (A7) and.squirre1 hair (A8))as predictors of environmental contamination and the use of canine serum to leach metals from fly ash (A9) would su gest that one should put one’s cat out at night with justifiab e trepidation just in case some forward looking scientist is lurking in the neighborhood. Certainly literature citations are one barometer of the changing weather but hard1 an accurate predictor of long term trends. Unfortunate y for the prophets, scientific “breakthroughs”, which can create revolutions in any field, are probably better described by the emerging field of nonlinear dynamics (aka Ychaos”)rather than smooth extrapolations of past trends. In short, one must be cautious in forecasting next year’s weather. A visualization often used with chaotic behavior states that the flappin wings of a butterfly in China may cause a hurricane off alifornia. Interesting reading along these lines lies in a recently published book by Roberts entitled Serendipity in Science (AIO) which provides snapshot views of our chaotic forward motion. It is worthwhile closing this philoso hical meandering with the observation of Anatole France w o noted that the future is “a time that is hidden even from those who make it”. Other n e w of interest includes the name change of Progress in Analytical Spectrometry to Spectrochimica Acta R in 1991, while retaining the same general objectives of providing overview articles on spectrometric techniques and methodologies. There appears to have been an increased tendency on the part of ‘ournals to have “special issues” that focus on topical areas. bhile such issues can be useful in their presentation of a reasonable cross section of a particular area or subdisci line, they can occasionally contain papers from a conference wfere the manuscripts may not be quite ready for publication or are overviews of previously published work. Neither of the latter is necessarily “bad”, but it does tend to lower the si nal-to-noise ratio of good research papers in the alrea y burgeoning volumes of literature. Fundamental Spectroscopic Information. As part of a special issue of Spectrochtm. Acta, Part B, Lovett ( A l l ) discusses the importance of fundamental reference data in atomic spectroscopic simulations. With the computational powers available today, the absence of such basic information is often the bottleneck in developing a comprehensive “first principles understanding” of many techniques. Stark broadening measurements for obtain’ fundamental constants using a capillary light source h a v x e e n reported (A12). Theoretical and experimental AA data for Na lines taking into account line rofilea and self-absorption (A13) as well as calculations on atsolute atom number densities in a P b hollow cathode lamp (HCL) (A14) will add to this data bank. The importance of line rofiles with low-pressure vaporization sources for AA has t e e n discussed by Piepmeier (A15). The Voigt parameters of the analytical lines for 19
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elements were determined by using ETA with random error of the determination of leas than 20% (A16). The optogalvanic technique was used to obtain line width information on Ne (A17) and hyperfine structure for Pr (AIS) and 91Zr (A19). Theoretical calculations on the range over which interatomic interactions influence line sha es was noted by Lobb and McCartan (A20). Deavor et al. &21) used a fiber-optic probe to determine Cu number densities in a flame without the need for Abel inversion; densities were calculated from curves of growth.
B. INSTRUMENTATION AND CALIBRATION A review of instrumentation in atomic spectroscop appeared ( B I ) ,as well as a new journal column by Broeeaert (B2). A very comprehensive review of electrodelessdischarge lamps should be of general interest (B3). A fiber-optic-based rapid scanning spectrometer was used to study low-pressure premixed flames at a scan rate of 100 scans s (B4).A diode array covering 2.5 nm was used with ETA- S and presented a spectrum every 0.33 ms (B5). A similar system with a continuum source and flame was also reported by the same research group (B6). The retroreflective array was recently applied to ETA-AAS to demonstrate a means of reducing detection limits by a factor of 2 (B7). As a key proponent of charge coupled and charge injection devices for detectors in atomic spectrometry, Denton and his group (B8) discuss the applications of these detectors for emission spectrometry and the future in analytical spectroscopy. Image-dissector systems with an echelle s ectrometer and continuum source AA also have appearef (B9, B10). Single mode diode lasers have been used with ETA-AAS for the determination of Rb and Ba with background correction capabilities and an extended dynamic range (B11).Tomographic imaging appears to be a novel mode of looking at spectroscopic sources and has recently been presented (B12). Fourier-transform FAA with a continuum source reported limits of detection about a factor of 10 poorer than continuum source AA with a multiplier phototube detector (B13). Results in the UV region from a Fe hollow cathode lamp (HCL) from a commercially available Fourier transform spectrometric (FTS) system were reported by Thorne and Learner (B14). While the potential of the Los Alamos FTS system was reat this writing ported at the start of this review period (B15), the system is no longer operational and lans for this to serve as a national resource appear to have een thwarted. A new design for a boosted-discharge HCL was used with FAA and several advan es were reported in comparison with the commercial HCL’s ( 16). A demountable multielement cathode was used in a Grimm-type glow discharge for simultaneous multielement AA measurements with encouraging results (B17). Kureichik et al. (B18)report on a short duration pulsed HCL, which appears to be similar in concept to that originally reported by Smith and Hieftje (B19). Calibration and Software. In one of the few flame emission papers detected in this biennium, multivariate calibration was tested as a means of minimizin interferences (B20). A factorial design scheme also was usefin comection with FAA analysis of Ti in glass ceramics (B21). A set of coefficients for analyte-interference pairs has been suggested as a means of computationally correctin for interferences in FAA (B22) and FORTRAN and BAS18 software packages have been published for analysis of computer-generated AA data (B23). The use of a sliding average with a time interval of ca. half the width of the absorbance signal in ETA-AAS was reported to give optimum precision with the lowest detection limits (B24). Several ”robust methods” based on the median are discussed as a strategy to deal with outliers in routine analysis (B25). Harnly (B26) considers several characteristics of the ETA signal in an attempt to isolate the parameter(s) that may be useful as a di nostic of an interference problem for ultimate utility in ver%ation of analytical accuracy. Finally, a closed-loop feedback-controlled data collection system for ensemble averagine with wavelength modulation as a means of improving the signal to noise ratio (S{N) in AA has been reported (B27). t has been an accepted fact that ETA results will be better obtained from an instrument with an adequate response time. Atnashev et al. (B28) have used a Gaussian approximation for the signal and determined that an optimal signal-to-noise ratio can be realized if the response time is 2.075 times the
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ATOMIC ABSORPTION SPECTROMETRY James A. Holcombe received his Bachelor's Degree st Colorado College (1970) and his PI1.D. at the UnkersW 01 Michigan (1974). Since that time. he has been a member 01 the facuW in the Chemistry Departmem at The unlversny of Texas at Austin. During 1984-1985. he Spent a year as -am Dhecta tor C h e m ical Analysis at the National Science Foundation. Professor Hoieombe is an Associate Ediior lor Applied Specnoscopy and is On the a w q board for Spechmhimica Acta B. Rcqess in AnsMiwI Speclrorcopy and me CBmdian Jovnsl of Spechoscopy. Pmtessar HOICOmbe'S research intereStS include hace metal analysis and the study 01 fundamental processes occurring during atomization and excitation. Much 01 his current work cent e r ~around graphhe turnace atomizationand the chemical interactions taking piace on the ~urfaceand in the gas phese. Time and spatially resolved spectroscopy. mass spectrometry. high-temperature static-SiMS and computer simulatims are ail empbyed in unraveling this complex system. Anom er area 01 Current research inwives the use ot biological organisms tor very unratrace metal preconcentrationand unratrace metal speciation.
D. Chrlnlan Haasell is a fourthyear doctarai Canddate wnClng under the auspices of Professor J. A. Hoicombe. He recelved a B.S. degree from Brioham Youno UniversW (1986) and has work& tar Hercules Aero; pace and Alplne West LaboTatoTies. His reSearch has Included studies 01 low-pressure vaporization in ETA-AAS and is cumentiy tocused upon the application ot high-temperalure static secondw ion w s s spectrometry (SIMS) to elucidate graphite surface reaction mechanisms in electrothermal atomizers. Other research interests include high resolution chromatography. chemometrics. / and minimization 01 CollisionaI phenomena Involving the research director's catamaran. He plans to pursue an academic career. He is a recipient 01 the 1990 Kirkbright Bursary
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standard deviation of the Gaussian ap roximation. On the other end, Fazakas (B29) has discussejwhich heating rates are 'optimal" with instruments exhibiting a poor response time from the detection system. L'vov and co-workers ( 8 3 0 , 8 3 1 )continue to discuss the feasibility and agreement between theory and practice for the concept of "absolute analysis" by ETA-AAS. However, Frech et al. (832)suggest that this approach may require a time and spatially isothermal furnace if agreement between theory and experiment is to be realized. In a similar vein, a means of arriving at absolute number densities of atoms in a graphite cuvette has been proposed (833). The ability to bypass calibration curves and to translate an absorbance signal into a concentration or mass of analyte by using only a few keystrokes on a calculator is certainly one of the ideal characteristics of "the perfect analytical technique". However, the absence of reported use of this approach sug ests that there is not sufficient faith in the concept that anafysts are willing to abandon the preparation of standards. Interestingly, abandonin the old practices of calibration curves and standard ajditions for this absolute method probably should be employed today for many analyses, even if an accuracy of f30% were all that could be expected from the approach. The missing ingredient which stymies this new direction (which will produce a substantial savings in analytical time and expense) is education: education of the regulatory agencies, the plant engineers, and the analysts. The requirement for 5% accuracy at the ppb level is often times absurd. For example, the "cutoff limits" for toxicity levels and impurity tolerances likely have uncertainties far greater than 30%. (Can one translate the "test tank guppy kill" into human toxicity levels with an accuracy that should require the analyst to differentiate between 1.3 and 1.0 ppb of Cd? It's doubtful.) It is time again for a biennial visitation of "peak height vs peak area" in ETA. Herber and Pieters (834) suggest that the height often gives more precise results but not significantly better than that obtained by using peak areas. Berndt and Sopczak (835)suggest peak heights may often improve S/N
and expound on the utility of the general appearance of the peak in recognizing errors. A factor of 2 improvement in the amount of analyte was realized if peak areas are used with ETA Zeeman systems (836). It may he instructive to note that using peak areas attempts to compensate for changes in the rate or mechanism of atom generation. If the generation is the same from sample-to-sample-to-standard, then heights should be similarly precise. Several algorithms and software have been developed for calibration procedures with AA. Software that fits standards (3-10) and considers errors (837)as well as one that involves a Baule-Mitscherlich transfer function (838) for ETA have been published. A procedure for linearizing AA working curve involving a power expansion has also been discussed (839). A personal computer was used to select the optimal integration times for ETA-AAS signals (840) and an Apple IIe was adapted to a Pye-Unicam for data collection and reduction (841). An approximate-inverse Savitzky-Golay convolution was applied to near-IR spectra with success and may find utility on some digitized atomic signals as well (842). Simple summation of repetitive firings for ETA to increase sensitivity suggests a sledge hammer approach to lowering detection limits (843). Attempts to provide assurances on the accuracy of the final signal magnitude were reported by Michaelis through peak shape monitoring ( 8 4 4 ) , while Harnly ( 8 4 5 ) reports on a first effort to carry such an effort to its logical evaluation procedure via a Fourier transform of the signal and subsequent evaluation of the phase and frequency components as a signature for analytical samples. Gardner and Gunn (846) suggest conditions favorable for the use of standard additions in lieu of a calibration curve approach for quantitation. They also note that a reduction in precision over a narrower concentration range was obtained by using standard additions. Andersen also reported comparable precision by the two approaches (847). The use of two signals in FAA along with continuous variation of the concentration of the sample solution assists in constructing calibration graphs and accounts for apparent deviations in Beer's law (848). In another orocedure. a combination of standard additions method and successive dilution methods was reported to circumvent some problems from nonspecified interferences in AA analyses (849). Factorial designs were used in the determination of Ph by ETA (850).Four interfering metals and the influence of a matrix modifier were used as test cases. Sample Introduction. A brief overview of thermospray introduction ( M I ) ,the use of this approach with flames ( B W , and a novel approach capable of 60-fold enhancement over more conventional thermospray systems have all been presented (853). Injection methods for characterizing Ni alloys (R54),microsampling using solution drops on a rotatin plate (360 samples/h) ( 8 5 9 , and discrete nebulization appficable to high salt samples have all been suggested (856). A 5+400 bar feed pressure on a liquid sample in a sample introduction loop was used by Berndt (857) to generate an aerosol for introduction into flames or plasmas. Comments were also presented on recirculating nebulizers in AAS with regard to stability and ease of sample changing (858). Kantor (859) reviews the mode of thermal dispersion methods for introducing dry aerosols into secondary excitation or atomization sources. The article may be worth considering in light of the efforts extended toward coupling similar devices to ICP systems. Finally, a direct comparison of gaseous introduction of metal-containing compounds versus aerosolswas made and the increased sensitivity of the vapor introduction noted (860). ~~
C. ELECTROTHERMAL ATOMIZATION Temperature Studies. Common sense has once again triumphed over slo ans, thanks to the experimental verification by Welz a n f colleagues ( C l ) that a gas-phase temperature gradient does exist within the furnace during sample vaporization of material from a platform. In a set of nicely designed temperature measurements using coherent antiStokes Raman spectroscopy (CARS), they have shown time and spatial temperature gas phase temperature profilm within an operating furnace with platform, an article that is well worth reading before one considers the assumption of "isothermal" or "constant temperature" when discussing the furnace atomizer. It is only unfortunate that the experimental ANALYTICAL CHEMISTRY. VOL. 62. NO. 12, JUNE 15. 1990
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design does not ermit a large number of laboratories to routmely obtain t is information for their particular system. However, Rademeyer and Human (C2) have used this new information in their temperature modeling program to permit better predictions of the thermal profile for other designs and heating programs. Resistive and conductive heating of the platform have also been studied by Shuttler and Delves (C3) to show that radiative heating is not the sole source of ener transfer for platforms inserted into grooved slots within t e furnace. Other temperature studies include the use of the conventional two-line method for wall atomization (C4),contoured tubes (C5),and W atomizers (C6),for samples released from a graphite “probe” inserted into the furnace (C7), and for a comparison study of several different furnace designs (C8). The effects of various furnace operational parameters on vapor temperatures (C9) as well as a model for determining the surface temperature of a W strip atomizer (CIO)have been reported. Mechanisms. Increasingly powerful (and expensive) techniques are being used to focus on elucidating processes occurring within the conventional electrothermal atomizer in addition to continual interpretation of the.absorbance signal. Welz and co-workers (CII)present a very mteresting pictoral survey of ETA usin scanning electron microscopy. (If you be of interest.) are tired of words a n i equations,this article By use of a pulsed dye laser, a novel use of time-gated resonance schlieren spectrometry focused on the spatial distribution of Na within the atomizer (C12). Armed with a tunable dye laser and a video detector, Carman and Curran (CI3)also looked at Na distribution using absorption measurements over the entire furnace cross-sectionalarea. In general these two independent sets of results were in agreement, although the former approach required larger sample amounts to realize a suitable SIN. Direct observation of interfering molecular bands using a high-resolutionoptical system with time resolution represents a useful study of this recurring spectral problem (C14). This work is far superior to previous D2 absorption studies where the major attribute is convenience of measurement. Another direct measurement of a key parameter needed to unravel the within the furnace was performed com lex chemistry oc by &urgeon and F a l k m who made direct measurements of CO in the furnace using CO bands from a specially designed CO-flushed HCL. Interesting discussions of sources for generating and removing free oxygen can be found in this paper. Considering the ability of metals on graphite to catalyze the C O2reaction, speculation on the role of Ta- and La-treated furnaces in facilitating atom release by reducing the gas-phase oxygen content has been discussed (CIS). Oxygen-containi acids have been implicated in the increase in Cz radicals a3subsequent metal carbide formation and a resulting loss in sensitivity for four test elements (C17). Katskov and Kopeikin (C18, C19) used a quasi-equilibrium model to discuss o en concentrations and ih general impact on atomization mec anisms. Pb was one of the f i t elements reported to show abnormal appearance temperature shifts with oxygen in the gas hase. and, Recentl ,thiswas looked at with mass spectrometry unlike dTA at 1 atm, no shift was found with an oxygenated surface in vacuo (C20). The authors then focused on the potential im ortance of COCO2in retarding free Pb release. Ascorbic acix modifier lowers the appearance temperature of Pb; the evolution of H and CO in a as- hase reaction with Pbo was sug ested as %e mechanisms 8ilchriit et al. (C21). Wend1 and hueller-Vogt ((222) used -ray diffraction and molecular absorption to study the atomization of Pb in a more general sense. Phos hates and chromates were found to stabilize Pb through t t e formation of the respective salts. A dual cavity platform isolated the Pb/chloride interference to the surface and showed the absence of interference with phosphate/m esium nitrate modifier (C23). Chlorides and the impact o f x graphite surface conditions were evaluated in another study (C24). A vaporization dependent on surface coverage at low analyte concentrationssuggested first-order release for Cu (CW). This was again c o n f i i e d with vacuum vaporization ex riments, which also proposed desorption from dispersed, a g r b e d Cu atoms on the graphite (C26). Interestingly, this study also showed distinct differences in atomization behavior for oxide
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powders, metal powders, and solution Sam les, and further dismissed the likelihood of Cu2formation. contrast, Wang et al. suggested vaporization from Cu(,, from X-ray photoelectron spectroscopy studies (C27). By use of 51Cr radiotracers, thermal pretreatment temperatures were evaluated with various matrices and uncoated and coated tubes (C28). W coatings with suitable modifiers assisted in further stabilizin the Cr. The authors also noted no significant retention of r via carbide formation. In contrast, another study suggests that carbides assist in stabilizing the Cr a ainst loss throu h the vaporization of the chloride species (829),while a thirf suggests that the carbide formation with organic solvents depresses the signal (C30). Perhaps the final word on Cr has not yet emerged ... although the votes keep coming in. Radiotracers and simple abrasions experimenta were also reported in mapping Cr on a graphite surface (C31). The semimetals have always presented analytical difficulties, yet only two papers address the fundamentals of vaporization. The first considers the inter retation of activation energies from the absorbance profile (832)while the second uses MS results with thermodynamics and kinetic data to evaluate a similar system (C33). Comparing the results of these independent studies, like much of ETA research, is difficult because the same questions are not asked and there appears to be enough variables that are different so as to make correlations between authors difficult. More practically, a new method of “tantalum carbide coating” on the platform roduced fewer interference problems with se, but provide! no performance increase for Sn or Rh (1234). Other elements for which basic atomization information was sought include studies of Ba (C35),Mn (C36),V (C37),and phosphorus (C38). An interesting study on the process by which externally generated hydrides are adsorbed on the furnace was also presented by Sturgeon et al. (C39). Other articles on hydride/ETA approaches can be found in the section on hydrides. Finally, L’vov and co-workers (C40) report on the detection of metal carbide absorption. The note that, with the exception of the spectral location of A l 2 4 band, the spectra for dicarbides of Ga, In, Ti, and T1 have never been reported previously. Shekiro et al. (C41) used a diode array to monitor chloride interferences as a function of time. Such an array detector with adequate spectral resolution would be very useful in evaluating the bands reported by L’vov et al. In spite of the widespread use of Pd as a modifier, the literature has not been overrun with attempts at ex laining how or why it works. A paper by Rettberg and Beacf (C42) does make a start by looking at peak shapes and the influence of the mass added. Ta and graphite platforms were interchanged in a graphite tube to study release rates and wall readsorption (C43). The authors propose “carbon shells” as an integral part of the release mechanism. Other authors discussed diffusive loss from the furnace and calculated diffusion coefficients (C44). They assumed negligible loss from the dosing hole, although another article (C45)supports earlier experiments and models that suggest the importance of the dosing hole. The relationship between the generation function and the observed si nal has been discussed by Gil’mutdinov and Salakhov (C467 and a means of extracting the generation function proposed. Much of the effort in deducing mechanisms revolves around the interpretation of Arrhenius-type plots from the absorbance profile. (In many cases, “over interpretation” is often used, according to these authors.) An explanation on the curvature in these plots has been explored recently (C47, C#), and the erroneous use of this type of lot to deduce the presence of a gas-phase reaction has been c early pointed out (C49). Organic solvents were suggested to produce active carbon, which sponsored the reduction and early release of Sn, although the effect disap ed with increased char times (C50). Reports have been p u E h e d on the impact of anions, group I and 11metal salts and acids on several metals in biological samples (C51),as well as on the stepwise evaluation on the Cu signal of components in a seawater matrix (C52). Mechanisms have been proposed for the interference of perchloric acid on Li and Ge [C53] and bromides for Ag,Bi, Cd, Sn, and T1 (C54). Models. What appears to be a very exacting approach to signal dependence on furnace length has been reported by
E
P
ATOMIC ABSORPTION SPECTROMETRY
Goell and Holcombe (C55)using Monte Carlo simulations for Cu and Ag. The same authors also presented a preliminary report on o timization of furnace design using this simulation technique or Cu (C56).This modeling approach appears to offer the least number of assum tions of techniques used to date. (CAUTION: We may betiased on this point.) Welz and colleagues (C57,C58)continue to show encouraging results using a simpler (albeit, not "simple") model. Their model was tested a ainst various furnace compositions and configura"sim le" mathematical model based on kinetic tions. processes for reyease has also been proposed and shows ood agreement with the experimental data presented ((397. Background Correction. Slavin and Carnrick review the development of modern background correction techniques (C60),and the leveling or roll-over common to the SmithHieftje system has been explained b . Larkins (C61), who points to the general importance of the L e wmgs on the shape of calibration curves. Another article considers several atomic spectral interferences which appear with the continuum source background correction approach (032). Potential errors in correcting with Zeeman-based systems aa a result of absorption by several elemental lines and metal oxide and bromide bands have been reported (C63). A longitudinal ac Zeeman with transverse heated tubes produced a workin s stem capable of extended workin range using a three-fie18 &eman system (C64). Finally O'haver et al. ((265) report on the use of analog-to-di ital range switching to improve the background correction a ilities of continuum sources. Furnace Design and Operation. Total pyrolytic graphite (TPG) tubes showed similar general performance characteristics to that of pyrol ic paphite coated tubes except that shager (due to eat differences) were apparent with TP tu s (C66).U n c o a a , pyrolytic coated, continuously in situ coated, and Mo coated tubes were tested for lifetime using V (C67). Information on lifetimes as well as interference effects and detection limits is given. At the birth site in Sweden of both the two-step furnace and the integrated contact cuvette (ICC), the parents evaluated their children's performance (C68).While the two-step furnace provided a good design for fundamental studies, the ease of usin the ICC made it preferable for routine analysis &ere spatiti uniform temperatures were an advan e never afforded by t i e more conventional Massmann-type urnace. A similar study was done with the simultaneous multielement analysis by continuum source AA (alias SIMAAC) system (C69). De Loos-Vollebregt and De Galan ((270)consider the analytical advantages of several heating modes (transverse vs longitudinal) and different carbon-based tube materials. Three in-house designs for furnaces combined with a double pass optical s stem were compared with more conventional systems (C7If The authors report a loss in sensitivity but an im rovement in throughput with the new design. (FAST, SENATIVE, CHEAP Pick any two!) Different platform were also evaluated during this time nod (C72). The is now commercially available a n g new design for front entry along with a discussion of design considerations and some erformance characteristics has appeared in the literature P ~ 7 3 ) . Resurrecting the possible advantages of high pressure atomization, Fazakas et al. (C74476)have become proponents of the advanta es of elevated pressures for improved sensitivities. Hasselfand co-workers (C77) have explored the other end (Le., low pressures) as a means of assistin vaporization of refractories as well as suggesting the possitilit of identifying different chemical forms of an element in a soEd sample inserted directly into the furnace. Through the control of the atomizer temperature to control the release rate of the monitored species, Pelieva and Martynenko (C78)have presented a model to ex lain the operation of this s stem, which can be used for t e concentrations ranging $om traces to minors. Speakin of temperature, a com lete description of the circuitry n e d e d for a capacitive d&e heating system has been presented (C79). This may remove the "engineering barrier" that existed previously which discouraged many from dabbling with this rapid heating mode. Three papers appeared on the tential utility of eliminating the char cycle and using a higptemperature dry to increase throughput in ETA (CSGC82). The use of methane (C83) and Freon (C84)admixed with the sheath gas has once agm appeared in the literature as a possible modifier to assist
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vaporization as well as extend tube lifetimes. Test procedures using Cu and Cr (C85)as well as Ag (C86)have been suggested as a means of ensuring optimal performance of the ETA-AAS system. Metal atomizers (viz.W tubes) have been addressed almost exclusively by Ohta and co-workers with studies on the metal release mechanism in the presence of ammonium thiocyanate (C87, C88),the impact of hydrogen on Fe, Co, and Ni (C89), the influence of Ar mixed with either hydrogen or He on the AA signal (CW),and the enhanced sensitivity from admixin hydro en with Kr in place of Ar (C91). Finally, Berndt an! Schalfach (C92)used the W filament from a projection bulb in place of a more conventional furnace geometry and reported comparable sensitivities to GFAAS for more than a dozen elements.
D. FLAME ATOMIC ABSORPTION, ATOMIC FLUORESCENCE, OTHERS Flame AA. Organic solvents with conventional nebulizers
( 0 1 ) and admixed as an organic/aqueous solution using discrete nebulization ( 0 2 ) with FAA showed general increases in sensitivity. Organic complexing a ents were reported to produce some interferences with FAA [ut could be minimized with a more reducing flame ( 0 3 ) . In comparison with line sources, a continuum source with wavelength modulation provided better rejection of the Ca emission when monitoring Ca AA in flames ( 0 4 ) . Optimization of the continuum source FAA system (aka SIMAAC) for the analysis of rocks was also reported (05).Anion interferences in Cr determinations (D6),dealing with perchlorate and evaluation of a accumulations in FAA burners (07) N,O/H, flame for clinical samples (08)finalize the less application-oriented papers in flame AA. Atomic Fluorescence. This techni ue will have few spectroscopic peers for r e a c h y the 1owestLits of detection, and a recent paper using a HC as the atom source illustrates this point with a 1.8 ag lunit of detection for Pb ( 0 9 ) . A review listing tables of detection limits and experimental conditions for atomic fluorescence is worth perusin even if one has only a passive interest in AJ? (DIO). Two ad&ional overviews and perspectives ( 0 1 1 , 0 1 2 )along with more lengthy and general overviews of the technique ( 0 1 3 , 0 1 4 ) have also been presented. Electrothermal vaporization of one form or another has extended both the absolute sensitivity and the applicability to microsamples. How far is this combination from the "intrinsic limits of detection"? Answers are in an article by Omenetto, Smith, and Winefordner ( 0 1 5 ) . Graphite cups ( 0 1 6 ) , tubes ( 0 1 7-021), tubes with Zeeman background correction (022,023),systems capable of working at pressures from vacuum to 1atm ( 0 2 4 ) ,a W spiral ( 0 2 5 ) ,and a capacitively heated W spiral ( 0 2 6 ) have all been successfully demonstrated. Glow discharges (027) and metastable nitrogen ) been used as atom plasmas with ETA introduction (028have sources, although the former presented poorer detection limits than those observed by using emission spectroscopy with the same source. While dye lasers remain the dominant excitation source, laser diodes have been employed in the determination of Rb (029). A modular AF system with boosted HCL's with air/CzHz and air/H flames produced results similar to the commercial ICP-AFk system ( 0 3 0 ) . The use of a modified filter fluorometer for the determination of H by AF at less than 10 parts per trillion has also appeared f 0 3 1 ) . To end on a chillin note, subpg/g levels of Pb have been successfully determine! for Antarctic ice samples ( 0 3 2 ) . Coherent Forward Scattering and Laser-Enhanced Ionization. Coherent forward scattering (CFS) for atomic magnetooptical rotation spectroscopy (AMORS) continues to be the object of several studies with considerable effort still involved in the refinement of components inherent to the technique. A n excellent review of the technique and its future prospects were recent1 presented by Hermann, a steadfast researcher in the field 6 3 3 ) . The same group also discussed the use of an optical multichannel analyzer for simultaneous determinations ( 0 3 4 ) as well as reporting on employing a magnet from a commercial AA spectrometer along with the continuum source to realize detection limits in the p b range for many of the test elements (0%). Phase-sensitive Stection with source modulation has helped reduce the background from furnace emission for CFS ( 0 3 6 ) ,and the theory and ANALYTICAL CHEMISTRY, VOL. 62, NO. 12. JUNE 15, 1990
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ATOMIC ABSORPTION SPECTROMETRY
Table I. Hydride Generation element
Sb As
matrix PVC biological ref materials nickel-based alloy geological marine organisms hydrofluoric acid
Bi In
Pb Te Sn As, Se
Sb, Bi As. Sb. Se. Sn
seawater aqueous solutions biological samples aqueous solutions food, biological samples aqueous solutions marine samples biological samples seawater river water coal agricultural, environmental
natural waters aqueous solutions
comment
comparison of inorganic acids preconcentration in quartz tube cooled with liquid N2 continuous flow system with on-line matrix removal separation by xanthate extraction photooxidation of arsenicals to arsenate by UV radiation As(II1) and total As determined for As(V) speciation membrane gas-liquid separator for flow injection
automated speciation acid digestion simplifies sample prepn, improves detection limit comparison of flame and quartz tube atomization aqueous slurry technique
Fe(II1) minimizes interference from Ni(I1) and Cu(I1) in situ concentration of stannane on a heated tube silica gel cleanup speciation and determination of tin and organotin cmpds semiautomation using a gas collection device AAS and AFS compared, continuous flow system elimination of interelement interferents thermal decomposition and accumulation in an ETA generation and atomization under low pressure
experimental implementation of a Wollaston prism analyzer with beam splitter has been discussed (037). Detection limits have been lowered by using an on-axis multisystem (038). Gated o eration of a photomultiplier tube combined with a pulsed row discharge system further assisted the separation of the &S signal from emission generated in the atom source, which was a glow discharge ( 0 3 9 ) . A resonance monochromator in a Voigt configuration alon with an Ar gas sputtering discharge has also been reportecf ( 0 4 0 ) . High-frequency modulation was used in another study for the pur ose of reducing background interference from the incan1escent atomizer and permitted the achievement of shot noise limited conditions with the minimum of technical difficulty ( 0 4 1 ) . Stephens ( 0 4 2 ) expounds on the inherent ability of CFS to permit identification of spectral interferences, which is not the case for most other atomic spectral techniques. Laser enhanced ionization (LEI) provides an alternate means of signal detection to conventional photodetectors. Interestingly, the majority of the work in this area over the last two years has been conducted in the USSR, with the exception of the Michigan State group which develo ed a means of modeling the noise in laser-induced ionization b43). A means of enhancing sensitivities, using In and P b as exam les, includes use of multistep hotoionization in the C2&/air flame with the widest possigle laser beam width to ensure complete ionization and maximum area coverage within the source ( 0 4 4 ) . In another study the best detection limits were achieved by using nanosecond lasers with multistep excitation and ionization (045).The method for minimizing shot noise in LEI ( 0 4 6 ) as well as a differential method of signal measurement ( 0 4 7 , 0 4 8 )have been reported to improve the technique. Two papers expound on the use of this technique within a T-furnace (049,050) as well as with vaporization from a gra hite cup ( 0 5 1 ) . As exam les of potential applications, stu&es discussing the state-seyective opportunities available with this technique ( 0 5 2 )and the analysis of calcium with detection limits of 0.02 ppb in aluminum (053) were resented. Oder Techniques. Furnace atomization using nonthermal excitation s ectrometry (FANES) and its molecular analogue (MON#S) has been applied to the determination of rare-earth elements by Dittrich et al. (054). Multivariate o timization of several elements using the FANES sptem has arso been discussed (055).A review of FANES and furnaces as atomizers was recently published by Falk (056). A chan e in the FANES electrode geometry b making the graphite tu& the anode rather than cathode has L e n introduced by Ballou and co-workers (057). They re rt that for Cd and Cu, limits of detection are competitive wiEFANES. Two papers dealing with ETA-AES have appeared and discuss the experimental and theoretical intensities (058) as well as the otential of time-de endent calibration raphs to improve t i e dynamic range OPETA-AES(059). T%e cou ling of an Ar/H2 microwave plasma to ETA was evaluatefas a possible analytical source (060). Finally, the introduction of solution samples t74R
ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
ref E43 E44 E45 E46 E47 E48 E49 E50 E51 E52 E53 E54 E55 E56 E57 E58 E59 E60 E61 E62
onto the surface sputtered by the jet-enhanced sputterin system has been initiated by Chakrabarti et al. (061,0627 along with discussions on the mechanism of atomization by sputtering as well as the system’s analytical performance. To round out this potpourri of “other selected techniques” ... by taking advantage of the nonlinear characteristics of lasers, Winker and Wright (D63)utilized the nonlinear mixing of two tunable lasers in a conventional flame to produce an output that is enhanced as a result of coincidence with a resonance line of one of the elements introduced into the flame. Nolan et al (064) continue with their four-wave mixing technique with low pressure sampling to conduct isotopic analysis of Li mixtures. An article dealing with the detection of Pb in ambient air samples using “spectral-phase effects” has been presented in the Russian literature (065,0 6 6 ) . A source in the optical literature that may be of interest is a broad band (ca. 70 nm) amplified spontaneous emission source with an output of several millijoules and nanosecond pulse durations (067).
E. VAPOR-PHASE SAMPLE INTRODUCTION This section encom asses hydride generation atomic absorption spectrometry &GAAS), cold vapor atomic absorption spectrometry (CVAAS), and several other techniques in which a volatile species is generated prior to introduction into the absorption cell or atomization source. HGAAS continues to receive the most attention of these techniques: notable, specific HGAAS studies are outlined in Table I. Dedina ( E l ) reviewed methods of hydride generation, interferences, and mathematical descriptions of generation and atomization,and with co-workers (E2)presented a mechanism for Se hydride atomization. A novel FIA system emplo ed polymer-bound tetrahydroborate for arsine eneration ($3). Arsines were collected on a cold trap and vofatilized sequentially into a quartz furnace AA spectrometer (E4). Differential hydride generation was used for speciation of Sb (E5). A GC column was used to generate the hydride and separate different species for determination of inorganic and organometallic Sb, As, and Sn (E6). Many interference studies focused on hydride generation determination of As (E7-E9), Sn (ElO),Sb ( E l l ) ,Se (E12, E13), As and Sb (E14),As and Se (E15),and As, Sb, Se, and Sn (E16). A persulfate-nitric acid oxidant mixture was utilized to study interferences in HGAAS determination of P b (El 7 ) . An interesting variation to standard HGAAS involves the adsorption of the hydride onto the inside of a graphite tube, followed by conventional GFAAS determination of the deposited analyte. Such a system permitted sensitive P b determinations in food (E18). Hydride trapping and graphite furnace atomization were improved when the furnace was coated with Pd, as demonstrated for determination of As,Sb, and Se (E19), Bi, Ge, and Te ( n o ) ,and Ge, As, Bi, Hg, Sb, Se, Sn, and Te (E21). The mechanism of sequestration of volatile element hydrides by Pt group metals was described by Sturgeon and co-workers (E22).
ATOMIC ABSORPTION SPECTROMETRY
Table 11. Direct Solids and Slurry Analysie (ETA-AAS Unless Otherwise Indicated) comment
element
matrix
Pb, Cu, Mn Se Cd, Pb Cd, Ni Pb, Zn, Mn
biological bovine tooth powder coal, fly ash coal carbonate rocks high-purity Ga glass semiconductor Si
CU
B< Cd, Hg, Pb, T1 Pb Sb, Pb, Mn
coal
Be Se Pb Ca
food geological pine needles
Na, K, Ca, Sr, Fe, Zn Cu, Cr, Fe, Pb Mn, Ca, Mg, K Cu, Zn, Fe, Mn Cd, Cu, Fe, Mn, Pb, Zn
plant tissue sewage sludge
Solids Zeeman BC, powdered samples, inner miniature cup Zeeman BC, cup-in-tube technique, Pd modifier subsampling strategy outlined calibration considerations, Cr and V analysis problems powdered samples powdered samples, mixed with graphite powder calibration with electrolytically spiked samples Zeeman BC aqueous standards, problems with Sic formation Slurries platform atomization, magnetic stirring in autosampler Ni(N03) matrix modifier Pd matrix modifier, magnetic stirring in autosampler flame AAS, powders treated with acids pulsed nebulization into flame and arc-flame sources results comparable to solution methods flame AAS, sample prepn vulnerable to contaminants ashed samples flame AAS, partial microwave digestion with nitric acid
Daniels and Wigfield (E231 investigated the effect of CVAAS experimental parameters, speciation of sulfhydrylbound H and acidic and alkaline reduction. NaBH, was shown t o t e a better reducing a ent than SnC12when usin collection on Au (EN). Ngim and co-workers (E%) compare! reaction mixing times for microdetermination of total Hg in undigested biolo ‘cal samples. Unusually large blank CVAAS values were exp ained along with precautions (E26). A sinle-stage impaction system was coupled with a CVAAS system !or determination of airborne Hg a t ng/m3 levels (E27). A liquid chelating exchanger was used to extract Hg from wastewater samples (E28). CVAAS digestion procedures were compared for soil samples (E29). Ri ‘n (E30) generated volatile fluoride, bis(trifluoroethyl), and xthiocarbamate anal e species, which were then separated by GC and atomize by gas phase ETA; this was then a plied to multielement analysis using a continuum source (131). In situ preconcentration in a heated aphite furnace was used to determine P b in blood by ethygtion (E32) and Ni in marine samples by carbonyl generation (E33). Alkyllead compounds were speciated in FAAS with vapor-phase sample introduction (E34). Volatile metal acet lacetonates were studied in Cr, Zn,and Mn determinations (i35);matrix effects for this chelating agents were considered for Cr in steel (E36). Tyson and colleagues (E37) generated volatile Cu trifluoroacetylacetonate for effective matrix isolation. Volatilization of As as the trichloride facilitated sample introduction and matrix isolation with a flame-in-tube atomizer (E38). A low-temperature hydride furnace was modified for the AAS determination of metals with low appearance temperatures (e.z.. Bi. Cd. Pb. T1. Zn) (E39). . X.graphite furnace was used for vapor formation with subsequent introduction into a flame to form a new hyphenated combination. One such “ETA-FAAS” system volatilized Rh from a solid or li uid pol mer following the ash stage, thus reducing the ba& rouni signal obtained with I n another system Si fluoride conventional ETA-AAS (E40). was evolved in a modified furnace and subsequently transported into a flame with Ar carrier gas (E41).In a reverse configuration, Hg was determined by CV generation and then collected on a Au-coated graphite tube for subsequent atomization (E42). F. ANALYSIS OF SOLIDS AND SLURRIES Major objectives in the development of all analytical techniques are to minimize both the sam le preparation time and the probability of contaminating t t e sample when ultratrace levels are being sought. Direct analysis of solids and slurry samples achieves these goals to the extent that several instrument manufacturers are now offering instruments or attachments dedicated to solid or slurry sample introduction. This burgeoning field is also being explored for emission and mass spectrometric analysis, so the interested reader is di-
P
T
ref
F21 F22 F23 F24 F25 F26 F27 F28 F29 F30 F31 F32 F33 F34 F35 F36 F37 F38
rected to relevant reviews in those areas. Table I1 details specific applications of solid and slurry analyses. Baxter (F1)evaluated the generalized standards addition method for analysis of solid samples. Kurfiirst (8’2)reviewed instrumental requirements, analytical erformance, and characteristicsfor solid sam le analysis by &FAAS. Easer (F3) reviewed solid sampling A& methods in industrial chemical analysis. Brown et al. (F4)described a new solid sampling tube and platform for GFAAS. Vaporization kinetics and a second surface atomizer were used for the determination of P b in metal samples (F5).Certain problems associated with solid geological matrices were eliminated by sample dilution with graphite powder (F6). Synthetic reference materials for solids analysis by ETA-AAS were shown to require homogeneous distribution of the analyte (F7). A general review on cathodic sputtering has recently been presented by Hannaford and Walsh (F8)which covers the last 30 years in the authors’ laboratory on this mode of sample introduction. Within the last 5 years the use of gas jets to enhance the sputtering efficiency for final utilization in measurement of the generated atoms by atomic absorption has sponsored a renewed interest in the sputtering rocess; fundamental studies with an emphasis on Cu sampgs have recently been conducted (F9). Gough and co-workers (F10) studied factors influencing sensitivity and reproducibility for several sputtering atomizers for AAS. Spectral characteristics of glow discharge sources as atomization cells and the influence of various parameters which are at the experimenter’s discretion have also been published (F11).AAS was used to observe sputtering phenomena with alloy samples in a hollow cathode glow discharge (F12). The general sputterin technique has also been applied to the analysis of nonconckucting powder samples by mixing the powder with a Cu host matrix (F13). Miller-Ihli (F14) investigated slurry sample pre aration for simultaneous multielement GFAAS. Hoenig ant! colle (F15)devised an automated sequential multielement GF% technique. An automated slurry introduction system was used for GFAAS analysis of river sediment (F16). Haswell et al. (F17)determined As in solid samples by slurrying samples in HCl prior to hydride generation. Electrothermal atomization of P b from soil slurries was investigated for effects of matrix components (8’18)and atomization characteristics with various matrix modifiers (F19).Holcombe and Majidi (F20) discussed the impact of errors in pi etting, particle size, and number of particles in sampling o slurries. G. SAMPLE PREPARATION Haines (GI) reviewed various sample preparation techniques for AA analysis. Eller et al. (G2) covered various aspects of sample preparation for many sample types amenable to ZAAS and XRF analysis. Several papers compared digestion procedures for specific matrices: atmospheric
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ATOMIC ABSORPTION SPECTROMETRY
articulates (G3),ceramics (G4),food (G5),and coal (G6). Fhree types of closed cells were com ared for acid-peroxide digestion with microwave heating (87). An electric spark discharge was used to generate and suspend colloidal particles from metal sam les (G8). Laser ashing, followed by acid digestion and leacking, was found to be a rapid and reliable technique for plant sam les (G9). Acid vapor-phase pressure decomposition was use for the determination of several elements in biological materials (GIO). Al detection limits were improved and biological sample preparation time greatly reduced with a clean room and microwave di estion techniques (G11). Various approaches to fusion with litkum borate were investigated for geological samples (G12). Modifications in nanoliter handlin techniques improved the GFAAS microanal sis of Na ancfK in biological fluids (G13). Trace GFAAS ietermination of several contaminant metals in GaAs samples was simplified with selective evaporation of the matrix with bromine vapors (G14).
B
H. SEPARATION AND ISOLATION APPROACHES Flow Injection Analysis (FIA). T son and co-workers
FIK,
(HI)addressed several aspects of including on-line preconcentration and matrix isolation, calibration, dispersion, and FIA versions of the standard additions method (H2),and network FI manifolds for sample dilution and calibration (H3). Fang and Welz (H4)investi ated optimization of experimental parameters, including nebdizer uptake rate, carrier flow rate, air/water compensation, and flow spoiler; application of FIA to the analysis of samples with high dissolved solids content (H5);and described a high-efficiency low sample consumption on-line ion-exchange preconcentration system for flow injecdeveloped a model based on the tion FAAS (H6). Bezur (H7) mass balance of the flame atomizer to evaluate impulse nebulization and FIA. The utility of FIA for reconcentration was demonstrated and In (H12). for P b (238,H9), Cu, a n a P b (HIO), V (HII), Donnan dialysis was shown to be an effective method for analyte preconcentration (HI3). Stable-compound interferences were removed for Zn analysis (H14). Attiyat compared A 16 solvents both as sample solvents and carriers (H15). nebulizer interface allowed modification of an existing instrument for flow injection (H16). A monosegmented continuous-flow s stem was used for FAAS sample introduction and comparedlto conventional flow injection (HI7). Flow injection studies were not limited to flames. The unique design considerationsof FIA interfaced with ETA-AAS were evaluated (H18).An automated wet digestion, flow injection hydride generation and quartz atomizer system was used to determine Se in biological materials (H19). Bank and co-workers devised a novel flow injection thermospray sample deposition system, reducing cycle time while maintaining conventional ETA-AAS sensitivity (H20,H21). FIA was also interfaced with HGAAS (2322) and CVAAS (H23, H24). Separations. The term se aration encompasses (1) speciation, i.e, separation and &termination of individual ionic or organometallicspecies, and (2)separation of an analyte from the matrix for the purpose of either preconcentration or elimination of an interferin constituent (i.e., matrix isolation). Many sey t i o n p r o J w e a utilize similar techniques; for example, co umn elution chromatography is used for speciation of Sn and organotin compounds (H25), Cd preconcentration (H26), and water matrix isolation (H27). In many cases these techniques accomplish more than one separation, such as the use of modified resins for both se aration and preconcentration of Pb, U, and Cu in waters (k28). The previous review (AI) contained many papers in which liquid extraction was used for speciation or matrix isolation, but it now seems that more attention is being focused on chromatographic-based separations. Among the myriad papers is a book entitled Environmental Analysis Using Chromatography Interfaced with Atomic Spectrometry (H29). Speciation. The reactivit and toxicity of an element often is a function of the chemicdform or species of that element. This indicates the need to determine more than just the total concentration of a given analyte. The book Metal Speciation: adroitly covers many Theory, Analysis and Application (H30) aspects of this broad area and would be especially useful to the method developer. 176R
ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
Gardiner (H31)reviewed chemical speciation in biology and medicine and the role of atomic spectrometric techniques. Harriot and colleagues (H32)compared atomization methods, cell design, and stationary phases for metal speciation with gas chromatograph -AAS. Nygren (H33,H34) evaluated a hi h resolution Gdr- AAS system for alkyllead speciation. G8-AAS was also used for speciation of ionic alkyllead in and aerosols (H36),determination of Se in biowater (H35) logical materials (H37,H38), organomercurials in fish (H39) and air (H40), and butyltin compounds in oysters (H41). Irgolic and Stockton (H42)reviewed GFAAS and ICP-OES as element-specific detectors for liquid chromatograph (IC and HPLC) in the determination of As compounds. Egdon and co-workers introduced an nebulizerless interface for directly coupled HPLC-FAAS (H43)and compared mobile and stationary phases for As speciation (H44). Cu and M were speciated by using HPLC and thermospray nebulizer %AAS (H45). HPLC-FAAS was also used for speciation and determination of Cr in waters (H46),organotin com ounds in water (H47), and Fe complexes (H48).Both {AAS and ETA-AAS were employed as detectors for HPLC separation of Ca, M , Zn, Fe, Cu, Pb, Cd, and Sn species in urine (H49). HPLC-AGAAS was used for urinary As speciation (H50). HPLC-ETA-AAS was used for characterization of various and Cd trace-metal compounds in geological samples (H51) in crab meat (H52). A thermospray-microatomizer interface was used for determination of ionic Pb compounds by HPLC-quartz tube AAS (H53). An ion exchange chromatography-AAS method was used for studying interferences in determination of As(II1) and As(V) s ecies (H54) and for automated on-line trace enrichment (855). Miwa and co-workers (H56) evaluated cationexchange and chelating resins for speciation of Cu in waters. Chelating resins were also used for separation of A from Cu matrices (H57), Ba from seawater (H58), and Cu, P%,Cd, Zn, Ni, and Co in waters (H59). Size-exclusionchromatography was used to study A1 speciation in soils (H60) and serum Al/citrate complexes (H61). Rounding out the chromatography family, aqueous butyltin com ounds were separated by paper chromatography prior to EJA-AAS analysis (H62). Subramanian (H63) utilized a selective extraction procedure for Cr(II1) and Cr(V1) determination without requiring oxidation of Cr(II1) to Cr(V1). Selective extraction was used to separate tributyltin from mono- and dibutyltin in environmental samples (H64). Electrochemical separation was used Fe(I1) and for s ciation of Hg compounds by CVAAS (H65). F e ( I 6 cyano complexes were speciated by FIA with FAAS and electrochemical detectors in series (H66). Preconcentration. Precipitation continues to be used in a number of studies for trace analyte preconcentration. A method was developed for tetrahydroborate reductive precipitation of 16 elements in seawater (H67). Cd, Cu, Mn, Pb, and Zn was preconcentrated in water and seawater by recipitation with 8-quinolinol (H68, H69) and a dimethyl$ oxime/Ni/ 1-(2-pyridyloxo)-2-naphthol complex (2370). dru in silicate rocks was preconcentrated with rubeanic acid continuous precipitation (H71). Ueda and co-workers utilized HaOH for precipitation of Be (H72),Ge (H73), Ga and In (H74),and Sn (H75). Knapp and co-workers (H76) present an overview of automation in element preconcentration with chelating ion-exchange media. A polymer-supported macrocycle was employed to preconcentrate Cu and Zn in seawater (H77). Pyrocatechol violet-loaded exchange resin was used for preconA column flow system centration of Zn and Cd in water (H78). utilizing silica-immobilized8-hydroxyquinoline was used for the preconcentration of several transition metals in seawater (H79). T1 was preconcentrated from water for ETA-AAS analysis by oxidation and retention as tetrachlorotha!late(III) on an anion exchange column (H80). Water-soluble pol supported on a membrane filter concentrated trace metals in water (H81). Thiocarbamate compounds were used for extraction preconcentration of Sb, Cd, and Zn (H82) and Cd, Co, Cu, Fe, Ni, and P b (H83);supported on cellulose collectors for several heavy metals (H84),and used with an Fe collector for many transition metals with h recovery from soils and sediments (H85) and Fe matrices ( 86). Multielement preconcentration of several trace metals was achieved by solvent extraction with an alkylated oxine derivative (H87).Activated carbon was
E :
%
ATOMIC ABSORPTION SPECTROMETRY
Table 111. Specific Analyses (ETA-AASUnless Otherwise Indicated) element
matrix
comment
ref
A1
biological samples blood serum blood, serum dialysis fluids geological samples whole blood geological samples unspecified marine mollusks environmental samples urine plant matter biological materials plant matter urine unspecified Na serum plants, soil urine water plasma, urine plant matter biological samples alloys, envirnl samples serum, blood coal air particulate Cu-based alloys A1 alloys biological samples water sol compounds DNA unspecified serum aq solns blood river sediment biological samples various water waters
requirements for reference materials protein binding studies mech studies of K2CrzO7matrix modifier matrix interference studies FAAS, extraction procedures, W strip atomizer effect of ashing temp, matrix modifiers extraction, modifier procedures problems with analysis, use of Th-treated tubes FAAS, selective extraction from Ca matrix evaluation of Mg(NO& modifier, Th-treated platform mech of ammonium molybdophosphate modifier use of Zr-coated tubes and noncoated graphite tubes Mo tube atomizer, interference and S modifier studies FAAS, optimization of atom trapping FAAS, on-line preconcentration FAAS, HClO, interference mech, use of SrC1, modifier FAAS, selective extraction in nonaqueous solvent interference studies, comparison of cuvettes FAAS, interference studies, factors influencing sensitivity Zeeman vs continuum BC with omission of ash stage speciation of Cr(II1) and Cr(V1) comparison of matrix modifiers FAAS, discrete nebulization of a CCll extract cysteine an effective matrix modifier FAAS, includes comparison with colorimetric procedure TaC-coated graphite tube, “,NOB matrix modifier use of ascorbic acid to minimize Fe interference direct electrostatic capture in graphite tube FAAS, quartz tube atom traps use of La matrix modifier review of procedures and instrumental parameters Zr-treated tubes, Zr injected prior to sample introduction quantitation of Pt-DNA binding study of interferents and use of carbide-coated tubes ZBC, Ag + Cu + Mg matrix modifier, oxygen ashing anion interferences, comparison of matrix modifiers refinement in use of Pd-ascorbic acid matrix modifier FAAS, effect of ultrasonication on extraction Pt modifier increases ashing temp from 400 to 1300 O C comparison of Pd and HzS04matrix modifier picric acid matrix modifier toluene extraction, tubes treated with Na tungstate sample injection into preheated graphite tube-
i41 142
Sb As Ba Be Cd Ca cs Cr
co
cu
Au
Hf Li Pb Mg Mn P Pt
Ru Se Sr Te T1 Sn V
urine ~~~. ~
used as a collector for separation and preconcentration of while dithizone supported Mo(VI) in a ueous solutions (H88), on activatejcarbon was also effective for preconcentration Satake and coof trace metals from acidic solutions (H89). workers investigated solid chelating compounds supported on and Ni (H91) naphthalene for-preconcentration of Cu (H90) in various matrices. compared liquid-liquid exMetasti and co-workers (H92) traction, svnthetic resins, and Dreciwtation with NaBHl for preconcentration of several me‘tals. -Cd was preconcentrited from aqueous solutions for ETA by using a pure strain of metal-bmding ae which had previously been used for waste cleanu (H93, 94). Cd was also reconcentrated with an acid-soyuble membrane filter (H95pand by electrochemical de ition on graphite electrodes followed by cup atomization X(H9f3). Matrix Isolation. Liquid extraction re& by far the most common method of separating the analyte from an interfering matrix, although many of the aforementioned preconcentration methods also accomplished some degree of matrix isoreviewed extraction in lation. Pili enko and Samchuk (H97) ETA and F!hS analysis of natural samples. Levesque and compared eight extractants for FAAS analysis Mathur (H98) of Cu, Fe, Mn, and Zn in soils. Complexation was employed to extract Pd in catalysts with 4,6-diamin0-3,5-dicyano-W1-thiopyran-2-thione in perchlorate buffer (HB), W in alloys and environmental samples with a hydroxamic acid (HIOO), and Co in water with 2-nitroso-1-naphthol (H101). Several transition metals were isolated with monohalogenobenzoylCu and P b were determined simultanehydrazones (H102). ously by two-channel GFAAS following extraction with zephiramine (H103). Pd was extracted from interfering &group
%
143 144 i45 146
147 148
149 150 151
152 153 154
155 156 157 158 159
160 161
162 163
164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 ~~
~
metals with a sulfonated azo dye (H104). Fe was separated from interfering environmental sample constituents with a specific hydroxamic acid (H105).Zr was isolated from soil samples with a @-diketoliquid chelatin exchanger (H106). Inorganic Sn and organotin compoun s were successfully separated from seawater interferents for GFAAS analysis (H107).Se was extracted from eological samples with 4nitro-0-phenylenediamine (H108f
d
I. SPECIFIC METHODS OF ANALYSIS The criteria for inclusion in this section were that a specific method either incorporate aspects which might be useful for general method development or provide a source for further fundamental mechanistic studies. Since the line between “fundamental”and “specific analysis” is fuzzy, the appropriate areas in sections B-D of this article should also be consulted. As stated, it is beyond the scope of this review to note every publication on analytical ap lications which featured a spectroscopic topic covered in &is review, yet the need exists for more effective dissemination and retrieval of specific methods and procedures. This is dramatically illustrated by an observation that the present controversy over the role of A1 in Alzheimer’s disease is due to difficulties in analytical procedures and lack of strict adherence to methodologies (11). The growing availability of computer database searching is revolutionizing information exchange: in the time that the word “blood” can be found in one years’s index of Chemical Abstracts, one could have accessed CAS On-line and found almost every paper ever published on determination of P b in blood. Absorption. Of particular note to those analyzing biological materials is the recent publication of Methods in
ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
177R
ATOMIC ABSORPTION SPECTROMETRY
Table IV. Indirect AAS Analysis species
matrix
analyte
ref
fluoride chlorine iodine chlorine, bromine sulfur sulfate sulfonamides thiosulfate phosphorus phosphate sulfide, orthophosphate phosphate, arsenate, arsenite tungstate vanadate anionic detergents cationic surfactants cysteine malathion mancozeb MSMA pesticide lidocaine, tetracaine, procaine mustard gas caffeine desferrioxamine, ferrioxamine, aluminoxamine
drinking water, seawater chloroacetic acids seaweed, table salt organic compounds Mo-FeS cluster aqueous solutions pharmaceutical preparations photographic waste effluent water steel aqueous solutions water rat tissues neutral media water frozen squid wool agricultural formulations air fiiter media commercial formulations pharmaceutical preparations body fluids analgesics biological fluids
AlF Cr Hg Ag Cr Pb Cu, Ag Pb co Bi Zn, Mg Mo Fe cu cu cu cu Pd Mn As
I84 I85 I86 I87 I88 I89 I90 I91 I92 I93 I94 I95 I96 I97 I98 I99 I100 1101,I102 I103 I104 I105 I106 I107 I108
co
Au Mo Fe, A1
Table V. Comparison of Analytical Techniques element
methods compared
comment
ref
Pt Se Tb Sn Ti Cd,Cr, Fe, Mn
FAAS, UV-vis spectrophotometry FAAS, UV-vis spectrophotometry ETA-AAS, DPASV FAAS, FAES CVAAS, kinetic thermometry CVAAS, GC GFAAS, diff pulse polarography GFAAS, HGAAS, fluorometry GFAAS, fluorometry GFAAS, ICP-MS FAAS, UV-vis spectrophotometry GFAAS, DCP
study of various hydroxamic acid extractanta UV-vis more sensitive, both improved with new extractant results comparable for tooth samples lower detn limit, lesa accuracy with AES for serum analysis CVAAS quicker and more convenient, but more expensive resulta comparable for biological and envirommental samples direct determination in urine resulta comparable in biological fluids lower detn limit with fluorometry ICP-MS more sensitive for biological and geological samples UV-vis more sensitive, both improved with new extractant GFAAS has better sensitivity, but slower throughput
Jll 512 513 514 515 516 517 518 J19 520 521 522
Ha In Pb Li Hg
Enzymology (Metallobiochemistry,Part A), edited by Riodan and Vallee (12) in which 42 renowned contributors provide excellent overviews of sample reparation, anal ical techniques, and guidelines for ana&sis of specific e ements. A number of reviews ap eared that addressed specific matrices: and biotic (16) analyses by AAS; blood (13,141,clinic~(Z5), biological materials by GFAAS (17,181;environmental (19) and marine (110) samples by various techniques. Several particular1 problematic analyses merit mention: im urities at the ppb revel on Si wafers by M S (111);rare-earth e ements by FAAS (112) and ETA-AAS (113). McKinney (114) studied the effect of various procedural steps on precision and bias in GFAAS of environmental samples. Schmitt (115)reviewed the influence of various preanalytical factors on AAS determinations in biological samples. Metal oxide cloths were investi ated as possible disposable charring/atomization surfaces or biological matrices (116). Several papers studied interferents and matrix modifiers. Boric acid was found to interfere with the 248.33-nmFe line (117). Various organic and inorganic matrix componentswere studied for their effect on Cr atomization in GFAAS (118). Chloride interference in Tl determination was diminished with pyrolysis of the Pd modifier prior to sample introduction and the use of hydrogen purge gas (119). X-ray photoelectron s ectroscopy was used to investigate the mechanism of cboride interference and ammonium salt matrix modification in Sn atomization (120). Determination of several transition elements was im roved with used of reduced Pd (121) and Pd(NO&/Mg(NBs) (122-ZH)matrix modifiers Amines were found to enhance A\ determination in FAAS due to the formation of CN, NH, and CH reducin radicals (125).Longchain quaternary ammonium salts eeminated interelement
?
P
f
178R
ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
interferences of Fe, Au, Mo, Sn, and Sb in FAAS (126). Organophosphorus acid interferences with 18 elements were studied extensively for FAAS (127).Flame polarized Zeeman AAS with discrete nebulization was applied to determination of Cd, Pb, and Cu in foods (128). Kitagawa and Kikuchi (129) cleverly utilized laser Fraunhofer diffraction to correct hysical interference due to v 'ng droplet size in a FAAS netulizer. Table I11 outlines X i t i o n a l element-specific studies that were deemed worthy of citation. Application of relevant techniques to indirect determinations of various species are given in Table IV. Fluorescence. Laser excited atomic fluorescence spectrometry (LEAFS) with ETA was a plied to analysis with several matrices, including methods opreducing matrix effects (130); Tl in biological matrices was determined with a 6-fg limit of detection (131). Low-temperature GC with cold-vapor AF detection was used to determine volatile Hg species at the picogram level (132). Hydride generation nondispersive AFS was ap lied to the determination of Te (1331,Se (Z34),and Sn (138. A NdYAG pumped dye laser was evaluated for use in Ni determination in sediment using nonresonance flame AFS (136). Other Techniques. GFAES was applied to analysis of solid biological sam lea (137). Several elements were determined in body flu& by discrete nebulization with atomic emission s ectrometry (138). FANES and MONES were applied to 8 e determination of BgTc (139). CFS was employed for both simultaneous and sequential multielement analysis of water with electrothermal and flame atomization (140).
J. TECHNIQUE COMPARISONS Epstein (J1)reviewed methods used to calculate, report, and compare detection limits in AAS, LEAFS, and plasma
ATOMIC ABSORPTION SPECTROMETRY
emission spectroscopy. AAS and DCP were compared b 12 criteria for qualitative and uantitative elemental an ysis (52). Limit of detection, reyiability, and economy were reviewed for atomic absorption, atomic emission, laser, X-ray, mass, and nonatomic spectrometries (53).AAS, AES, and mass spectrometry were reviewed for direct determination of nonmetals in solution (54).Proton-induced X-ra emission (PIXE) and GFAAS were compared for many egmenta in rainwater analysis; results were comparable, although PIXE can simultaneously determine 26 element8 (55). Arsenic speciation was evaluated by comparing methods of both se aration (HPLC and GC) and detection (FAAS, FAFS, and I&-OES) (56).Many techniques were reviewed for analysis of environmental samples (Jn,minerals and refractories (58), clinical and biological materials, foods, and beverages (59), and determination of lanthanides in geological samples (510). Other element-specific comparisons are given in Table V.
J
ACKNOWLEDGMENT The authors acknowledge Chemical Abstracts Service for providing CA Selects to aid in the literature search used in the preparation of this work. LITERATURE CITED A. INTRowcTlON (Al) Holcombe, J. A.; Bass, D. A. Anal. Chem. 1988, 60, 226R-252R. (A2) Application Reviews. Anal. Chem. 1989, 67. (A3) Keliher, P. N.; Ibrahim, H.; brth, D. J. Anal. Chem. 1990. 6 2 , 164R. (A4) Koppenaal, D. W. Anal. Chem. 1990. 62. 303R. (A5) Sharp, 8. L.; Barnett, N. W.; Burrklge, J. C.; Littlejohn, D.; Tyson, J. F. J . Anal. At. Specfrom. 1988, 3 , 133R-7313. (A6) HiOftje, G. M. J . Anal. At. Spectrom. 1989, 4 , 117-22. (A7) Kucera, E. Envkon. Monlt. Assess. 1988. 70, 51-7. (AB) Pratt, C. R. J. Pa. Aced. Sci. 1988, 6 2 , 3-5. (A9) Hank, W. R.; Sliberman, D. Envhn. Scl. Technol. 1988. 2 2 , 109-12. (A10) Roberts, R. M. -: Accldental~mverfesIn Sclence; (Wiley: New York, 1989).
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