Atomic absorption, atomic fluorescence, and flame emission

DOI: 10.1021/ac00163a017. Publication Date: June 1988. ACS Legacy Archive. Cite this:Anal. Chem. ... Julian F. Tyson. Analytica Chimica Acta 1990 234,...
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Anal. Chem. 1908, 60,226 R-252 R N2. CEMENT AND CONCRETE

(N2.1) Bayles, James, Gouda, George R., Nlsperos, Arturo, Eds.; 1986; 343 pp (Eng). Chem. Abstr. 1987, 106, 72014n. (N2.2) Campbell, Donald H. 1988. 128 pp (Eng). Chem. Abstr. 1987, 107, 82828r. (N2.3) Battagin, Arnaldo Forti Ceramlca (Sa0 Paulo) 1988, 32(196), 85-92 (Port). Chem. Abstr. 1988, 105, 85279d. (N2.4) Jefferson, D. J. Proc. Int. Conf. Cem. Mlcrosc. 1988, Bth, 73, 83 (Eng). Chem. Abstr. 1987, 106, 89248q. (N2.5) Oberste-Padtberg, R.; Clooth, G. Proc. Int. Conf. Cem. Mlcrosc. 1988, Bth, 161-173 (Eng). Chem. Abstr. 1987, 106, 89252m. (N2.6) Shukuzawa, Jorge Klyoshl; Zampleri, Valdir Aparecido. Ceramica (Sa0 Paulo) 1988, 32(200), 215-218 (Port). Chem. Abstr. 1987, 106, 8734v. (N2.7) Fuetlng. Manfred. Silhtaftechnlk 1988, 37(9), 310-315 (Ger). Chem. Abstr. 1988, 105, 231452b. (N2.8) Chromy, S. Zem.-Kak-Glps, Ed. 8 1988, 39(12), 693-695 (Ger). Chem. Abstr. 1987, 106, 107002m. (N2.9) Thomas, Craig 0 Thomas, Robert C.; Hover, Kenneth C. J . Environ. Eng. 1987, 113(1), 16-31 (Eng). Chem. Abstr. 1987, 106, 107012q. (N2.10) Reeves, Nancy K.; Bailey, D. E. SPE Prod. Eng. 1088, 1(3), 174-178 (Eng). Chem. Abstr. 1987, 106, 106998d. (N2.11) Tlttlebaum, Marty E.; Eaton, Harvlll C.; Cartledge, Frank K.; Walsh, Marie 6.; Roy, Amitava ASTM Spec. Tech. Publ. 1986, 933, 308-318 (Eng). Chem. Abstr. 1987, 107, 45648m. (N2.12) Weber, Mlroslav; Zavrel, Stepan; Bojanovsky, Mlroslav; Chromy. Stanlslav Cem., Vapno, Azbestocem ., Sadra 1987, (l), 12-14 (Czech). Chem. Abstr. 1987, 107, 63722k. 02. GLASSES, CERAMICS, ABRASIVES

(02.1) Apostol, D. Mater. Chem. Phys. 1988, 15(3-4), 233-254 (Eng). Chem. Abstr. 1988, 105, 160959j. (02.2) Lascar, G. Verres REfract. 1988, 40(2), 178-182 (Fr). Chem. Abstr. 1986, 105, 47568t. (02.3) Bohnhoff-Hlavacek Microscope 1988, 34(4), 319-328. (02.4) Nongalllard, B.; Logette, P.; Rouvaen, J. M.;Saisse, H.; Fevrler, H. NDT Int. 1988, 19(2), 77-82 (Eng). Chem. Abstr. 1988, 105, 47555m. (02.5) Schreiner, Manfred; Ettmayer, Peter; Wruss. Werner; Simon, Marla. Radex Rundsch. 1988, (2-3), 159-168 (Eng). Chem. Abstr. 1988, 105, 47557p. (02.6) Roth, Don J.; Kllma, Stanley J.; Kiser, James Douglas; Baakllni, George Y. Mater. Eval. 1988, 44(6), 762-769 (Eng). Chem. Abstr. 1986, 104, 229192~. (02.7) Klima, S. J.; Baaklln, G. Y.; Roth, D. J. NASA Tech. Memo. 1988, 11 pp (Eng). Chem. Abstr. 1987, 106, 54801~. (02.8) Roth, Don J.; Generazb, Edward R.; Baakllni, George Y. NASA Tech. Memo. 1988, 23 pp (Eng). Chem. Abstr. 1987, 106, 54602~. (02.9) Kamllll, Diana C.; Steinberg, Arthur Archaeobglcal Geology; Rapp, George Robert, Jr.; Glfford, John A., Eds.: Yale University Press: New Haven, CT, 1985; pp 313-330. 02. SEMICONDUCTORSAND ELECTRONICS

(Q2.1) (Q2.2) (42.3) (02.4)

Footner R . Mlcrosc. SOC. 1987, 22(4), 13. Nyman R. Mlcrosc. Soc.1987, 22(4), 15. Stevens R . Mlcmsc. SOC. 1987, 22(4), 15. Anon. Met. frog. 1988, 130 (No. l), 80.

R2. CRIMINALISTICS

(R2.1) Petraco Microscope 1988, 34(4), 341-345. (R2.2) Petraco Microscope 1987, 35(1), 83-92. (R2.3) DeForest; Shankles; Sacher; Petraco Mlcroscope 1987, 35(3). 249-259. (R2.4) DeForest; Ryan; Petraco Microscope 1987, 35(3), 261-265. (R2.5) Strange, A. Microscope 1987, 35(3), 337-338. (R2.6) McCrone, W. C. Microscope 1987, 35(3), 338. (R2.7) Springer, E.; Zelchner, A. J . Forensic Scl. 1987, 32(1), 248-253. (R2.8) Locke, J. Microscope 1987, 35(2), 151-157. (R2.9) Sklrlus, S. Mlcroscope 1988, 34(1), 26-27. (R2.10) Barna, C. E.; Stoeffler, S. F. J. Forenslc S c l . 1987, 32(3). 761-787. (R2.11) Hopen, Thomas J. Microscope 1986, 34(3), 273-274.

(R2.12) McGinnis, M. D.; Thornton, J. I.; Esplnoza, E. N. J . Forensic Sci. 1987, 32(1), 242-244. (R2.13) Palenik, Sklp Mcycbped& of Po!vmr Science and Englnmrlng, 7(2), 279-289. (R2.14) Palenik, Sklp Fwensic Science hndbodc; Saverstein, R., Ed.; Prentice-Hall: Englewocd Cliffs, NJ, 1988; pp 162-208. (R2.15) Petraco, N. J. Forenslc Scl. 1987, 32(3), 765-777. (R2.16) Laing, D. K.; Hartshorne, A. W.; Cook, R.; Robinson, G. J. Forensic Sci. 1987, 32(2), 364-369. (R2.17) McCrone, W. C. Forensic Science, 2nd ed.; American Chemical Society: Washington, DC, 1988. (R2.18) Bremen J . Forensic Sci. 1988, 31(2), 455-463. 52. FOOD AND FEED

(S2.1) Lewis, C. R . Mlcrosc. Soc.1987, 22(4), 29. (S2.2) Flint R. Mlcrosc. Soclety 1987, 22(4), 29. (S2.3) Flook R. Microsc. SOC. 1987, 22(4), 29. T2. BIOLOGY AND MEDICINE

(T2.1) Fox Microscopy 1988, 34(3), 245. (T2.2) Brabander; Nuydens; Geuens; Geerts; Moeremans; Mey R. Microsc. SOC. 1987, 22(4),-25. (T2.3) Pettipher R. Microscoplcal SOC. 1987, 22(4), 27. (T2.4) Hess; Huang; Weber; Seegmlller; McArthur; Meyer R. Mlcrosc. SOC. 1987. 22141. ,. 40. (T2.5) Recent Advances in Electron and L/ght Optical Imaging In Blobgy and Medicine; Somlyo, A,, Ed., 1986; 472 pp. (T2.6) LeBeau Microscope 1986, 34(3), 253. (T2.7) Dunn; Brown R . Mlcrosc. Soc. 1987, 22(4), 23. (T2.8) Brlggs; Daft R. Mlcrosc. SOC. 1987. 22(4), 22. (T2.9) Tolivia, D.; Tollvla, J. J . Mlcrosc. 1987, 148(1), 113-117. (T2.10) Kroese, A. 6.; van Netten, S. M. J . Mlcrosc. 1987, 145(3), 309-317. (T2.11) Park, C. M.; Reid, P. E.; Walker, D. C.; MacPherson, B. R. J . Mlcrosc. 1987, 145(1), 115-120. (T2.12) Standardization and Quantitatlon of Diagnostlc Stalnlng ln Cytology; Boon, M. E., Kok, L. P., Eds.; 1986. (T2.13114) Yaegashi, H.; Takahashl, T.; Kawaskl, M. J . Mlcrosc. 1987, 146(1), 55-65.

. ~~.

X2. PHASE AND STRUCTURE ANALYSIS

(X2.1) Diller, K. R.; Aggarwal, S. J. J. Microsc. 1987, 146(2), 209-219. A3. ASBESTOS ANALYSIS

(A3.1) McCrone, W. C. (re: Asbestos Particle Atlas) no other info. (A3.2) Millette, J.; Krewer Microscope 1987, 35(3), 31 1-318. (A3.3) Shenton-Taylor; Ogden Microscope 1988, 34(3), 161-172. (A3.4) Ogden; Thomson; Ellwood Mlcroscope 1988, 34(3), 173-179. (A3.5) Spelght Microscope 1988, 34(2), 93-108. (A3.6) Armstrong Microscope 1987, 35(3), 267-271. (A3.7) Jones, William G.; Thorne, Peter S.; Clere, Jerry L. Appl. Ind. Hyg. 1988, 1(4), 191-195 (Eng). Chem. Abstr. 1987, 106, 22494~. (A3.8) Delly, John Gustav Microscope 1988, 34(4), 331-9 (Eng). Chem. Abstr. 1987, 106, 95045n. (A3.9) Nikolova, St. I.2 . Qesamk, Hyg. Ihre Gwnzge6. 1988, 32(2), 93-95 (Ger). Chem. Abstr. 1986. 104, 220236n. (A3.10) Marconl, A.; Macclone, M.; Rossi, L. Med. L a v . 1988, 77(5), 496-510 (Ital). Chem. Abstr. 1987, 106, 178975s. (A3.11) Steel, E.; Hartman, A.; Hembree, G.; Sheridan, P.; Small, J. J. Trace Microprobe Tech. 1988, 4(3), 147-161 (Eng). Chern. Abstr. 1988, 105, 237514~. (A3.12) Peck, Alan S.; Serockl, John J.; Dicker, LOISC. Am. Ind. Hyg. Assoc. J. 1988, 47(4), A-230, A-232, A-234 (Eng). Chem. Abstr. 1988, 104, 212276k. (A3.13) Woltowltz, H. J.; Manke, J.; Brueckel, B.; Roedelsperger, K. Zentralbl. Arbeitsmed., Arbeltsschutz. Prophyl. Ergon. 1988, 36(12), 354-364 (Ger). Chem. Abstr. 1987, 106, 107268a. (A3.14) Kelfer, M. J.; Buchan, R. M.; Keefe, T. J.; Blehm, E. D. Environ. Res. 1987, 43(1), 31-38 (Eng). Chem. Abstr. 1987. 107, 5381411. (A3.15) Mlrtlc, B. Kem. Ind. 1987, 36(8), 337-341 (Slovenian). Chem. Abstr. 1987, 107, 160540g. (A3.16) Delly, J. G. Am. Lab. (Fairfield, Conn.) 1987, 54-61.

Atomic Absorption, Atomic Fluorescence, and Flame Emission Spectrometry James A. Holcombe* and Dean A. Bass Department of Chemistry, University of Texas at Austin, Austin, Texas 78712

“You don’t know much and that’s a fact.” While this ori inates from Alice in Wonderland, it adequately describes t f e feeling at the end of a biennium when 2000+ articles are 226 R

0003-2700/88/0360-226R$06.50/0

perused before compiling this review. During the last two years articles have appeared which solved problems that experience and history suggest never were a problem, while other 0 1988 American Chemical

Society

ATOMIC ABSORPTION SPECTROMETRY

b a n A. Bass Is cunentty wwking as an ETA-AAS BppilCatbnS SClttntlSl f a ” a c t ’ Instruments in Danbw. CT. He has bach~lot01 am degree.rrom me universn 01 Northern Iowa (1981). a m s l e r of art degree from me U n i v e r s ~ 01 Texas at Auf tin (1986). and recently received his dmta ale from The Universw Of Texas at Austl (1988). While at the Universb 01 Texas. h received ROfeSsionaiDevelopmem Award (2). Robert A. Welch Fellowships (2).an the Giibwl A. Ayres Award tor outstandin

~ a r s ’current s interests include Computer modeiing of furnace syste&s and also h k l n g at problems assoc$led wim specitic analyses in varied matrices.

articles present solutions to problems that were thought to be insoluble. new ideas, explanations, and instrumentation push a t the frontiers while last year’s sparks are still trying to initiate a flame. The area of electrothermal atomization (ETA), alias graphite furnace AA (GFAA), continues to attract the majority of the research efforts in the areas of fundamental studies and new methodologies for sample analysis. As will be seen, increasingly sophisticated probes are being focused on the technique to better understand its operation and more elaborate mathematical models are b e i employed to eventually place the scientists in a predictive mode regarding the behavior and performance of the system. Many of the modern instrumentation modifications that have occurred within the last five years have still not found their way into a large number of laboratories and, as a result, many of the application-oriented articles outline chemical modifications that are procedural protocols which appear to circumvent the problems encountered in older furnace designs. This would include such aspects as less than optimal heating rates, nonreproducible temperature ramps, and slow electronics. However, routine determinations at the part-per-billion level are becoming commonplace with modern furnace technologies, and new analytical frontiers are being pursued in the areas of extreme ultratrace analyses (sub-pph), isotopic analyses, and the direct introduction of solids or slurries into the furnace. Flame atomic absorption (FAA) and flame emission (FE) remain aa workhorses in the analytical laboratory due to their simplicity of operation, modest cost, and reasonable limits of detection. This sowce is much better understood with regard to the basic flame chemistry due to the years of research in combustion science, and many of the analytical applications of the technique likewise appear to have received sufficient attention to cause basic researchers to move on to other areas

where a greater need for fundamental knowledge is obvious. Atomic fluorescence (AF) has retained a foothold in fundamental investigations. As has long been acknowledged, it represents one of the methods of ultimate sensitivity in optical atomic spectrometry (although a recent article presents data for the detection of a single trapped ion by absorption (AI)). Recent advances include the successful utilization of fluorescence within graphite furnaces where the advantages of increased residence times are combined with the high detectability afforded by laser-excited atomic fluorescence spectrometry (LEAFS). The argument continues whether this approach will make it into the analytical laboratory for routine analyses of complex samples. However, there seems to be little debate regarding its utility for the extremely low levels that are unavailable by any other analytical atomic technique. Laser-enhanced ionization (LEI) still receives research attention and shows promise in the areas of isotopic analysis as well as ultratrace determination for several metals. Coherent forward scattering (CFS) appears to have once again emerged on the scene after many years of lying dormant. In spite of the supposed utility of the technique using continuum sources, much of the effort within the last two years has focused on the use of tunable lasers. This review covers the fundamentals in the field of atomic absorption, atomic fluorescence, and flame emission spectrometry since the last review (AZ) and provides selected coverage of applications. The companion reviews entitled “Emission Spectrometry” (A3) and ‘Atomic Mass Spectrometry” (A4) in this issue should also be consulted for complementary coverage of related topics. The selection criteria for the applications cited in this review reflect the authors’ opinion regarding journal availability, appropriate central focus of the article, and novelty of the approach. For example, an article on the inorganic content of amnionic fluid in goose eggs (A5) employs flame photometry, but the authors felt that both journal availability and the central focus (Le., goose eggs) did not warrant its inclusion. Analyses by sample types are reviewed in this journal on odd years and should be of interest to readers searching for methodologies and analytical approaches (A6). Table I presents a look at the literature cited in this review and the demographics associated with the various journals. The table is not representative of the literature as a whole with regard to all of the work conducted in the areas discussed in this article but rather represents the selections used hy these authors as noted above.

A. BOOKS AND REVIEWS A second edition of Welz’s Atomic Absorption Spectrometry is on the market and provides a broad overview of the discipline with a comfortable mix of theory and practice (An. A book on ETA-AAS analysis of Pb and Cd in freshwaters has also appeared (As). Other review articles dealing with specific types of analyses or samples will generally be covered in their respective sections in the text. General literature reviews that may be consulted for additional information include the atomic spectrometry updates which are published in J. Anal. At. Spectrom. They include topical areas of atomization and excitation (A9, AIO), instrumentation (All, A12), chemicals, iron, steel, and non-ferrous metals (A13, A14), minerals and refractories (A15, A16), environmental analysis (A17, A1.9). and clinical materials, foods, and beverages (A19, AZO). At. Spectrosc. also provides a general literature review in the area of atomic spectroscopy every six months, and B o g . Anal. Spectr. specializes in broad overview articles. A good review of the past 25 years by leaders in the field of atomic spectrometry provides interesting reading (AZI). A pair of articles by De Galan (A22, A23) considers some future directions and accomplishments for various atomic spectrometric techniques. With the availability of a number of analytical atomic spectrometric options, the dilemma of “How do we choose the optimal method?” is addressed by Slavin (A%). Similar articles dealing with modem analytical atomic techniques in the laboratory with comparisons have been presented (A25-AZ7). Modern plastics used for trace analysis have also been revisited (A28). Reviews on the fundamentals of AA (A29, A30) and advances in and use of GFAAS (A31) also have been presented. A review of instrumentation dealing with the AA data logging and data processing software (A32) has appeared. The use ANALYTICAL CHEMISTRY.

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ATOMIC ABSORPTION SPECTROMETRY

Table I. Distribution of Citations’ sections

a Only

journal

A

B

C

D

E

F

G

total

Anal. Chem. Anal. Chim. Acta Anal. Lett. Anal. Sci. Analyst Appl. Spectrosc. At. Spectrosc. Fresenius’ Z. Anal. Chem. J . Anal. At. Spectrom. Microchem. J . Prog. Anal. Spectrosc. Spectrochim. Acta, B Talanta Zh. Anal. Khim.

5 3 0 0 1 0

5 5

21 18 1

10

4 12 0 4 3 0 0

0 2 7 0 0 6 3 0

9 14 8 27 28 0 2 46 5 5

6 13 4 4 10 0 7 10 6 1

3 0 0 0 4 0 3 3 3 1 0 0

70 91 13 19 71 35 65 145

1

10 1 0 0 0 0

20 37 7 12 36 8 45 93 45 13 4 16 27 5

total

32

43

185

72

56

368

17

0

0 1 1

1

113

5 5

1 1 10 0 2 6

0 0

111

16 10 76 40 11

756

those journals with more than nine citations are included.

of hollow cathode lamps (HCL) in atomic spectrometry (A33); thermospray (A34),aerosol (A35),and general sample introduction approaches (A36);and background correction techniques (A37, A B ) have all been reviewed within the past two years. The probe continues to receive attention from the StrathClyde group which was led by one of the more productive workers in the area of graphite furnace, Professor John M. Ottaway, until his untimely death in 1986. A review of probe and insertion desi s has been reported by this group (A39). Platform and p r o E studies have been covered by Wu et al. (A40). An excellent review by Sturgeon of his perspective on the current standing of interpretin Arrhenius-type plots in deriving activation energies as we 1 as a discussion on the absorbance signal and secondary techniques for the determination of the oxygen partial pressure within the furnace has appeared (A41). Frech et al. (A42) have published an interesting article on their diagnosis of various interferences and on their techniques and interpretation of data for ETA-

f

AAS.

Very extensive reviews have also been published on atomic fluorescence (A43) and laser microanalysis (A44). Anomalous saturation curves in laser-induced fluorescence were also covered by one of the giants in the field, Alkamade (A45). The proliferation of nomenclature in the area constantly needs to be brought under control. This represents one of IUPAC’s useful functions. Recent recommendations for quantities and units in clinical chemistry which include nebulizers, flame properties, flame emission, and absorption spectrometry have recently been published (A46). The long needed guidelines for the field of electrothermal atomization are currently being devised and are anxiously awaited by the community. One is never sure whether one should “heat”, “ash”, “char”, or “thermally pretreat” the sample.

B. INSTRUMENTATION AND COMPONENTS Use of the retroreflective array proposed by Morris (B1) has been applied to FAA (B2). This clever approach provided

reductions in the refractive noise in the determination of Mn with a slight increase (10-50%) in sensitivity for noisy flames. Resolution of a linear photodiode array has been enhanced to roduce wavelength accuracies which are better than the indhdual diode array element (B3). An image dissector with an echelle spectrometer was successfully coupled to a continuum source for AAS by Masters et al. (B4). The plenary lecture of Denton (B5) discusses the use of intelligent systems and information management with charge coupled detectors (CCD) and charge injection devices (CID) as well as future possibilities for laboratory automation and multitasking. An analytical laboratory information management system, which is applicable to instrument control for a number of techniques including spectrometers, balances, automatic titrators, etc., has been described by Blair and co-workers (B6). The use of a computer for controlled slew/scan flame AF has been reported (19 elements, two wavelengths each in less than 15 min, and quantitative results 228R

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ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

(* 5%) at 5 min per element) (B7).Computer-controlled dilution systems for estimating dilution factors for techniques such as flow injection analysis (FIA) (B8)and FAA (B9)have been presented. A novel calibration approach is presented for the latter. Software to permit the use of calibration curves with more than three standards or with multiple standard additions has been proposed as an add-on package to the 3600 data station of the Perkin-Elmer Zeeman AAS (B10).Improved high-speed data acquisition techniques have been B12). A FORTRAN IV package with algordescribed (B11, ithms for smoothing and improved background correction based on second derivative information of the background signal has also been published (B13). A hardware system is discussed by Bass et al. (B14) for computer control of the supply voltage to heat the graphite furnace atomizer with a precision of *0.4% or better. With the availability of stripchart recorder pens that operate for extended periods of time, nebulizers now seem to be the bottleneck because of their potential for clogging and nonreproducible efficiencies with complex samples. The glass frits nebulizer introduced by Apple several years ago was considered for use in flame AA by Niemczyk and Espinosa (B15). At low flow rates, delivery efficiencies were nearly loo%, although the signal attenuation was significant compared to that for conventional nebulizers. Sample turnaround time of approximately 10-15 s may be a factor in some instances although the use of the fine or medium pore frits significantly reduced the noise and may be worth a slight sacrifice in analytical speed. The importance of the liquid transport characteristics through the nebulizer for AA was discussed, and a pump was hooked to a commerical pneumatic nebulizer for better control of sample delivery rates (B16).By control of the aspiration rate for water samples into an FAA system, an overall 100-fold enhancement in the absorbance signal has been claimed (B17).A modulated sample introduction device and selective line modulation were used in flame emission spectrometry (FES) and provided improvements if source background was a significant contributor to the overall signal-to-noise ratio for the system (BIB, B19). The pulsed sample introduction system was used by the same group to cleverly modulate the emission line from an ICP by passing the emitted radiation through a flame into which the analyte was modulated (B20). Improvements in ICP/AFS using a cyclone chamber have been presented (B21). A nebulizer interface for HPLC-FAA has been developed which gives 3-5 times better characteristic concentrations and detection limits for Mn, simplifies the optimization of flame stoichiometry, and results in a peak width increase of only ca. 1 s (B22). The use of the commercially available unit for aerosol deposition into an ETA has been reported with simultaneous multielement atomic absorption with a continuum source (SIMAAC) (B23). Wennrich and co-workers (B24)attempted to continuously introduce the nebulized sample into an already heated graphite furnace and found that sensitivities lie between that obtained for discrete aerosol sample introduction with ETA and conventional sample introduction with FAA.

ATOMIC ABSORPTION SPECTROMETRY

A CuO/Cu denuder has been used for aerosol S analysis by detecting evolved SOz using FES (B25). Discrete nebulization attempting to use small sample volumes received some attention within the past two years. Discrete sample amounts (20-100 pL) were injected into a carrier stream which was fed to a nebulizer (B26). The approach is very similar to flow injection analysis and, like FIA, is reported to reduce the physical and chemical interferences that might normally originate. The carrier solution contained ZOO0 ppm La to minimize suppression of Al and phosphoric acid in the determination of Mg and Ca in one example. The effect of organic solvents on dmcrete nebulization systems also has been reported (B27). The authors found that with a 50-pL sample a 50-50 (v/v) mixture of water/ethanol or water/ acetone solutions gave areas of approximately twice that achieved by using aqueous solutions while signals of 0.8 and 1.06 times the aqueous counterpart were recorded using peak hei hts. The reduced uptake rate may explain the behavior ancf is consistent with a recent study of Liu and Zhang ( B B ) who showed that decreasing the uptake rate and volume increased the nebulization efficiency in FAA. Nebulization efficiencies, in general, were studied by using FE in a short paper which demonstrated that ionic redistribution depends on the nature of concentration of the element as well as solvent and solution composition, gas phase temperature, nebulizer design, and the conditions under which the nebulizer is operated (B29). Continuing with their excelleat studies looking a t ionic redistribution as a source of enhancements and depressions caused by ionic mobility within the particles, Skogerboe and Butcher (B30) used a dual nebulization system to study K in the presence of Cs, viewed traditionally as an ionization suppressor. While the effect of Cs as an ionization suppressor was operative with both dual nebulizers as well as a single solution with single nebulizer, the impact of the Cs was greatest when both species were present in the same solution droplets. The same group also discusses ionic redistribution and the Ca/P interference (B31) as well as presenting three back-to-back articles covering different aspects of aerosol generation (B32-B34). With a droplet generator, morphological characteristics of solid particles collected at different locations in an air/C2H2 flame have also been presented (B35). In a more direct fashion, Skogerboe and co-workers (B36) interfaced a cascade impactor directly in-line with an AA system to allow the measurement of signal magnitude as a function of mass of analyte associated with various droplet sizes. This is in contrast to other methods used previously where a filter within the impactor was used to collect the anal* and then analyzed after a fixed collection time. The authors claim this new approach prevents possible size measurement biases since impaction is not a requirement for the analysis of a given sized droplet. The interrelationship between the location of the impaction bead (B27, B38) as well as the spray chamber character (B39) have been discussed. Ni carbonyl generation for determination of Ni by AA has been reported by Alary et al. (B40). Transport efficiency was blamed for the depressive effects of F-on B determinations by FE (B41). In the absence of F- they claim that most of B volatilizes in the spray chamber. Laser ablation of a solid sample with subsequent introduction onto a 300 K graphite furnace wall was reported (B42) along with studies detailing electrostatic accumulation of material directly into furnaces (B43,B44). While the majority of the work dealing with ETA introduction of material into an ICP is covered elsewhere in this issue (A3,A4), an article by Kumamaru et al. (B45) describes the conversion of a commercially available graphite furnace atomizer to a sample introduction system for an ICP with rapid conversion back to the ETA-AAS mode of operation. Instrumentation and procedures for dealing directly with solid sample analysis in ETA-AAS are covered in the section on solid sample analysis in this review. The application of Zeeman correction to LEAFS with the excimer pumped dye laser beam passing through the m poles and parallel to the magnetic field was reported%::! The detection limits for Co were doubled by using the Zeeman system with no change in the linear dynamic range. Modulation of the degree of polarization has been successfullyemployed in minimizing background and nonspecific emission from a furnace used for atomization with CFS (B47, B48).

The absence of any additional work in the last two years on novel excitation sources for either fluorescence or absorption suggeats either that current technologies are adequate or that other roblems are more paramount. Microwave excited electrJeless discharge lamps (EDL’s) were compared with HCL’s in a commercially available Zeeman AA system (B49),and a new Zeeman system using Ta-lined tubes, rapid heating, and a boxcar integrator was described (B50). An electrically heated quartz atomization cell or hydride generation AA has been described (B51) as well as a new slot burner which handles both premixed and diffusion flames and generates increased atom densities (B52). A flashback-resistant burner for combustion diagnostics and analytical spectrometry has also been developed at the University of Florida (B53). This is an extremely versatile burner insofar as a wide variety of gas mixtures and fuel/oxidant ratios can be used in the study of combustion processes and their impact on flame temperatures in atomic spectrometry.

C. FUNDAMENTAL STUDIES 1. Procedural and Calibration Studies. Methods for

estimating atom densities within commonly used sources have been presented for use in emission, absorption, and fluorescence curves of growth for flames or plasmas to provide a reasonable estimate of the atom or ion densities ( C l ) . The same group has also devised a computer program which calculates the curves of growth for atomic fluorescence with unique capabilities of working with excitation sources whose line widths are not si ificantly different from the absorption line width in typical &es qnd plasmas. The approach allows the estimation of number densities to be easily determined within an order of magnitude even if collection optics and atomic line width parameters are not well-known, and *lo% if these values are known (C2). In a slightly different vein, LEAFS was used to evaluate the absolute spectral sensitivity of a spectrometer/detection system (C3). The approach was applied to He and Ne transitions. A Russian article appears which reemphasizes the impact of signal distortion in ETA-AAS (C4). A Fabret-Perot interferometer was used to study the collisional shifts in the resonance absorption line for several metals when working in the temperature range of 1500-2200 K (C5). Discussions have also been presented on the interference caused by quenching which may produce line broadening due to a reduction in the excited-state lifetime (C6). These data combined with those of Lovett (C7) may allow for improved refinements in the concept of “absolute analysis“ that has been pursued for ETA-AAS techniques. Manning and Slavin have looked at the potential utility of resonance line selection for Zeeman AAS for A1 (C8) and Cu (C9). The articles evaluate the wavelengths based on limits of detection as well as linearity of standard addition techniques. The advantagesand abuses of standard addition techniques have been discussed and are worthwhile reading (C10, C11). Comparisonsof standard additions and the use of calibration curves were presented (C12) along with subsequent comments on the issue (C13). Six commercially available c w e fitting algorithms for FAA were compared; and all provided errors well below 5% with the exceptions of a simple parabolic fit, linear interpolation between data points,and manual methods, all of which showed significantly poorer performance (C14). Baker (C15)expounds on the difficulty in interlaboratory data correlations due to the variety of curve-fitting routines present on commercial instruments and suggests an improvement would be realized if a single algorithm were used for every instrument. Bayunov and L’vov (CIS) have pro osed a corrected absorbance, defined as A / ( 1 a A + bAB), where A is the measured absorbance, to correct for the nonlinearity observed in ETA-AAS. It would seem that similar atomization mechanisms would be required for successful implementation of this approach. In another article, considerations of measurement and coneentration errors are applied with nonlinear regression functions to compute calibration curves (CI 7). Kindevater and OHaver (C18)have developed a calibration scheme by using continuum source AA and the wavelength-dependenttransmission profile to construct a calibration curve to cover several orders of magnitude and provide a greater degree of linearity than was previously achieved. The method relies on ensemble-averaging of the transmission profile of the analytical line and fitting

+

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it to a computer model, taking into account the line shape, instrumental resolution, background intensity, as well as stray light. Koscielniak and Parczewski (C19)have published another in their series of empirical models and transformations used to approximate the shape of the analytical working curve in the presence of interferences in FAA; and other researchers have developed software for calibration procedures in AA (C20). Improvementsin the efficiency of factorial experiments to model interferences in AA (C21)and an error recognition routine for ETA-AAS (C22)have been reported. Smith et al. (C23) discuss in detail a new optimization algorithm (Optiplex). The technique combines the iterative nature of a simplex optimization with global aspects of response surface fitting and provides a check for the location of a global, rather than a local, maximum. Limits of detection (LOD) continue to be of interest in all the techniques covered by this review. In a simplified fashion, the value obtained provides some measuring stick for the utility (or futility) of the parameter being altered by the researcher in an effort to point to improved directions for design or operational protocol changes. There are a large number of parameters that contribute to the value of these detection limits, not the least of which is the operator. One view on the operational parameters necessary to optimize the limits of detection with the Zeeman AA system for ETA has recently been presented (C24). A discussion of "detection limits in real samples" has also been presented in a brief paper where seawater, urine, and ZnSOl were considered (C25). Since ETA-AAS has a signal which is mass dependent, it follows that increasing the volume of a solution sample should fortunately increase the relative sensitivity of the technique. This has been taken into account by ensemble-averaging of a number of separate firings of the atomizer ( C 2 6 4 2 8 ) . Maximum likelihood estimators have been proposed by Chung (C29) in the estimation of quantitative results that normally lie below the LOD for the technique. Repetitive optical scanning in the derivative mode was used with FE in a N20/C2H2flame to significantly lower the limits of detection (C30). The dynamic range is another parameter that is deemed significant in A4 since the sister methods of emission and AF often boast of 5-6 orders of magnitude in linearity. Using a three-field ac magnet with Zeeman ETA-AAS system, the group at Delft found they could increase the dynamic working range by approximately a factor of 10 (C32). A similar extension was achieved by using ETA-AAS half-widths in constructing the calibration curve (C32). 2. Electrothermal Atomization. After another two years carbon-based materials remain the dominant substrate for manufacturing of electrothermal atomizers. A very nice overview covering materials used in these furnaces has been presented by Huettner and Busche (C33). This article begins to provide insights into the complex chemistry occurring on the graphite surface as well as furnish criteria for selection of different types of carbon-based materials that might be used in the manufacturing of electrothermal atomizers. Gunther and Findeisen (C34) also report on the requirements of the carbon materials for use in the graphite furnace. They conclude that a combination of pyrolytically coated tubes and either pyrolytic graphite or glassy carbon platforms seem to be the best compromise. Much of the research in the following section deals with efforts to better understand the chemistry occurring on the surface prior to and during the vaporization/atomization process. It is the hope of these researchers that once a fundamental understanding of the basic chemistry has been established, an intelligent and efficient means of screening prototype furnaces can be employed. A set of intriguing studies by Ortner et al. presents some interesting views of graphite tubes and platforms via scanning electron microscopy (SEM). They report the microscopic view of unused and corroded tubes as a consequence of various lifetime experiments and note the deposition of carbon in very thin layers as well as nodules in the hot central region of the furnace (C35). The formation of the graphite nodules was particularly interesting in the f i s t article and is dealt with in greater detail in a second article (C36). Finally, they look at the SEMs for platforms made from total pyrolytic graphite used within uncoated or coated tubes (C37). They note that the sensitivity 230R

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for the carbide-forming elemenb is strongly dependent on the quality of the tube coating which strongly implicates secondary gas surface interactions rather than primary generation as a possible limiting step in the vaporization of these refractory compounds. A few years ago, glassy carbon was thought to be the new panacea as a substrate material in ETA. This assumption was based on the relative inertness the glassy carbon shows toward such processes as acid oxidation in the normal laboratory environment. Welz and Schlemmer (C38)explore the use of glassy carbon tubes and report the improved performance for the volatile elements (approximately a factor of 2) but a degradation in the signal for the less volatile elements (approximately a factor of 5 ) . The authors conclude that the glassy carbon does not appear to be superior to, and may be inferior to, pyrolytic graphite. The same authors report similar results in another article (C39). Memory effects were significantly worse for Ba determinations in uncoated tubes than in either pyrolytic graphite, electrographite and pyrolytically coated electrographite tubes (C40),and general periodic trends in sensitivity for some of these surfaces have been outlined (C41). Laboratory-installed coatings on the commercial furnaces continue to be reported. Norval (C42)found that she could machine her own furnaces or modify existing furnaces with the insertion of a small indentation where the sample would be located and greatly improve the signal magnitude and tube lifetime if the tubes were coated. The apparatus used to provide the coatings is outlined. Liquid Ti, Mo, and W chlorides were introduced under reduced pressure to fill the voids in the graphite and then heated under a prescribed thermal program to provide metal carbide coating, which was confirmed by gravimetry and X-ray diffraction (C43). Oxides of these metals were also noted on the surface and SEMs of the Ti-treated cuvettes showed small particles of discrete carbide phases. They report that carbide-forming elements produced enhanced sensitivity with the treated tubes and postulate that the main function of the metal carbide is to "seal the graphite". In a study of Ta-, Mo-, and W-coated graphite tubes, it was found that earlier vaporization of Cu and Fe was observed with the coated furnaces (C44). Another study of Zr- and W-coated tubes showed improved sensitivity for A1 (C45)while Th-treated platforms (a la Brueggemeyer and Fricke) for the determination of A1 and Pd were commented on by Carnrick and Slavin (C46). The performance of standard and thoriated platforms was also compared (C47). W-tube atomizers have been discussed by Ohta (C48), and vaporization of lanthanides from Ta surfaces and Ta coatings have been studied (C49). Improvements for some elements and no effect for others were noted. Another article suggests that the atomization from the Ta-treated tubes and platforms provide the most sensitive results for the determination of Si (C50). Tube "rejuvenation"and W-carbide coatings also were discussed in light of Cu and Cr determinations (C51). The variability in the pyrolytic graphite coatings from one tube to another has been discussed in a pair of articles (C52, C53). Signal integration and frequent use of standard solutions for recalibration have been suggested as measures to minimize the problems. There appear to be some advantages, in some cases, with surface modifications: either via the standard soaking-andheating routines (which provide an overall metal oxide/metal carbide coating) or via the use of modifiers (which may accomplish the same task during the normal course of furnace operation). However, it generally appears that the normal pyrolytic surface provides the best general surface for analysis for the broadest range elements. 3. Temperatures. The heating characteristics of the furnace are extremely critical to its analytical performance as well as to the integrity of studies conducted to understand basic release mechanisms and interference effects associated with ETA-AAS. Falk and Glismann ((254) have measured tube temperatures as a function of time and axial position while Human and Rademeyer (C55) have provided calculations for the tube temperature for various shapes of graphite tubes and consider the matrix effects for elemenb in six different matrices. A limited degree of improvement was found for the contoured furnace and the platform. The same authors have also reviewed previous efforts for calculating and measuring temperature distributions of wall and gas phase

ATOMIC ABSORPTION SPECTROMETRY

within conventional (C56) and contoured tubes ( 1 3 7 ) . Chakrabarti and his group have developed models for heat transfer for both the gas phase and furnace wall temperature as a function of time and have considered the possible expulsion of the analyte due to the thermal expansion of the gas during the heating cycle (C58). Their data suggest the temperature of the gas phase is slightly lower than that of the wall and that thermal expulsion of the analyte is only significant a t high heating rates with volatile species. Using a combination of modeling and experimental studies, the same group has considered temperature distributions and analytical performance of the probe and platform techniques (C59). A mathematical model for platform heating (C60), W-ribbon furnace heating (C61), W-ETA heating (C62), and gas and surface temperatures for a capacitative discharge furnace (C63, C64) have all been presented. Pelieda and Uzunbadzhakov considered different means of stabilizing temperature by using the integrated radiation from the internal wall in combinationwith sample evaporation from a platform (C65). The same authors also recommended use of a platform with two-step heating mode which involves rapid heating to a hi h temperature, past the final “hold” temperature, followef by a “hold” a t the final temperature (C66). Finally, an expression has been presented that describes the relationship between heating rates and peak height absorbance under nonisothermal and isothermal furnace conditions (C67). 4. Furnace Designs. The quest continues for the optimal furnace design with the apparent ob’ective of longitudinal isothermalityboth during the ramp and hold mode. The most exciting innovationthat has occurred involves the introduction of integrated contact cuvettes (IC-cuvette) which involves single-piece construction with side contacts dong the entire length of the furnace (C68). Less signal tailing and fewer memory effects were observed. In the area of ETA sample introduction approaches, developments in probe designs have been presented (C69), and vapor movement within the tube-in-tube design, originally roposed by Sperling, has been discussed (C70). An insertable Eybrid of this design and the probe was proposed by Carroll et al. (C71) to permit use of larger volumes than easily accommodated by the probe. Lastly, a platform “insertion tool” for HGA-500 ETA’S has been presented (C72). Rettberg and Holcombe ((273) have published a performance evaluation that uses the second surface atomizer (‘‘the plug”) displaying excellent results for a number of metals in a number of matrices. Katskov et al. (C74) employed a metallic collector-ballast consisting of a metallic wire cluster used to assist atomization within a raphite furnace. They report similar advantages to those o served with platforms, ballasts, and metallic atomizers. Improved lifetimes for totally pyrolytic graphite (TPG) cuvettes have been reported (C75, C76). Brown and Lee (C77) compared peak shapes for nine elements and three different tube types and show, as would be expected,that higher heating rates with TPG required shorter atomization times and showed increase sensitivity for the volatile and medium volatile elements. A W-coil atomizer, overcoated by chemical vapor deposition of methane admixed with Ar, showed improved sensitivity for several elements (C78). Atomization mechanisms were discussed. Tungsten tubes with a movable cover for opening and closing the sample injection port show LOD improvements of a t least an order of ma nitude over previously developed ETA devices (C79). Finaby, Fazakas (C80) reported an improvement in peak height and area sensitivity for Be when a pressurized atomization cell was used. They claim the broadening of the spectra line because of the elevated pressure is more than compensated for by the reduction in diffusion rates. 5. Background Correction. As an important integral part of successful analyses with ETA-AAS, considerable effort over the past 10 years has been devoted to studies aimed at understanding the source of background as well as instrument techniques for circumventing the problem. Broad band sources such as Dzbackground correction will work in most instances while correction closer to the analytical wavelength (e.g., Zeeman or Smith-Hieftje-type systems) is more often required when molecular fine structure occurs. A survey of interferences observed in ETA-AAS using continuum back-

fl

ground correction (C81) and comparisons of Zeeman and continuum source (C82)as well as Smith-Hieftje and continuum source (C83) have appeared. Interferences which would arise as a consequence of using a Zeeman system have appeared in the literature (034, C85). Wibetoe and Langmyhr published parts 2 and 3 (C86, C87) on spectral interferences and overcompensation in the inverse Zeeman-corrected AA system. For example, looking at the effects of Co, Mn, and Ni on 30 elements and 53 lines, they outline selected wavelength and analyte/matrix combinations that should be circumvented (C86). In part 3 they present atomic line tables in which they present pairs of elements which have contiguous lines that are liable to suffer from spectral interferences when using a Zeeman system. Of the 18 cases, only four occurred at recommended wavelengths for analysis: interferences from V and Cs on Eu; Pb on Pd; and Eu on Pt. De Loos-Vollebregt and De Galan (C88) discuss the effects of stray light in backgroind correction in a Zeeman system using pulsed hollow cathode lamps. The good news is that the background was useful in differentiating humic and ligninsulfonic acids using ETA (C89), and as an alternative to thermogravimetric techniques (C90). Photochemical reactions to form particles were also used for the determination of hydrides by light scattering (C91). The absorption of vibrationally excited oxygen at the Se 196-nm line has been discussed and the effective absorption cross sections were determined by using ETA and a Se EDL (C92). The well-known interference of Fe in the determination of Se when using Dzbackground correction has been discussed (C93), and the advantages of Zeeman systems for clinical samples have been presented for Cr in urine and Au in blood as examples (C94). Macdonald et al. (C95) have attributed the spectral interference of Mg(NO& in the 270-300 nm region absorbance in the wings of the 285-nm Mg line. Magnesium nitrate, a common matrix modifier, presents the most serious difficulty for Ga, Pb, Mn, and Sn. While wavelength considerations are critical for accurate background correction, the timing sequence and the interpolation between measurements have been shown to be important for accurately reflecting the background signal at the instant in time that the analyte absorbance measurement has been made. Holcombe and Harnly (C96) expand on their earlier work but use nonlinear curve fitting routines with three or four background values to provide improved correction accuracy. The article compares accuracy of several different approaches. The authors have also proposed a means of using the results of the error curve as a diagnostic tool and as a means of additionally improving the integrity of the analytical signal (C97). The need for rapid background correction was also alluded to in an article which emphasized rapid pulsing of the lamps for improved accuracy in the correction process ((298). Finally, a computer-controlled system was discussed for wavelength modulation using a continuum source for atomic spectrometry (C99). The modulation approach can also be used for emission techniques or steady-state atomization modes such as FAA. 6. Basic Release Mechanisms. The possibility exisb for absolute analysis with ETA-AAS techniques because of a number of characteristics unique to these systems. Using integrated absorbance signals, L’vov et al. (ClOO) compared calculated values for the characteristic mass with those obtained experimentally for 40 elements. Using platform atomization 30 of the elements showed the characteristic mass that was calculated to be, on the average, 90% of that determined experimentally with an RSD of 125%. Baxter and Frech ( C l O l ) argue that the longitudinal temperature gradients in the furnace are the ultimate limiting factor for absolute analysis. By use of the new IC cuvettes, experiments were conducted for 21 elements and good agreement was realized if provisions were made to limit the lifetime of free atoms beyond the end of the cuvette. By use of a simple two-step kinetic model for vaporization and loss, ”atomization efficiencies” (defined as fraction of the analyte present as an atomic vapor at the peak of the absorbance signal) between 30 and 40% were calculated for six elements ((2102). The discrepancy observed for several of the metals when compared with work done by others was attributed to the much higher heating rates used in these particular experiments. Sturgeon and Arlow (C103) compared Arrhenius plots from vacuum vaporization with activation energies obtained from ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

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an ETA system operating at atmospheric pressure. They concluded that the leading edge of the signals showed no significant differences. For the elements Pb, Bi, and Au they also conclude that the Arrhenius data support the vaporization model which assumes the analyte to be present on the graphite surface in the form of small particles. This is consistent with a more detailed study presented by McNally and Holcombe (C104) which used signal profile shape, activation energies, and spatially resolved data as a means of diagnosing the existence of microdroplets versus dispersed atoms in the graphite surface immediately preceding atomization. The test elements used in this study were Cu and Au. They concluded that Au is released from microdroplets or hemispheres on the surface while Cu is dispersed as submonolayer amounts across the graphite surface. A theoretical model also was proposed for the transient characteristics of the absorbance pulse produced by atomization on the graphite platform using Pb as the test element (C105). Porous graphite and glassy carbon surfaces were used in another model in the evaluation of the ratelimiting process for the release (C106). Using a first-order assumption, Nakamura (C107) employed Arrhenius-type expressions to model the release of Ag, Cu, and Fe from a W strip. In a very interesting study (the authors stated humbly), Monte Carlo techniques were employed to model the vaporization of Cu (C108). The technique permits detailed considerations of the geometry of the furnace including the impact of the dosing hole as well as heatin characteristics and chemical interactions. The data showef good agreement with experimental profiles. Gil’mutdinov (C109)makes note of the fact that the generation function is distorted by the finite residence time of the atoms within the furnace which causes a broadening and a shifting later in time as a result. The readsorption of atoms onto the wall on W and graphite surfaces for a number of elements and the subsequent influence on profile half-widths also has been presented (C110). The graphite furnace has been used by L’vov and Nikolaev (CI11) in calculating diffusion coefficients for 60 elements. A pair of articles has been presented by L’vov (C112, C113) which outline his theory regarding the incomplete atom formation in the graphite furnace due to either monoxides or carbides in the gas phase. In this article he stresses his belief in a nonequilibrium excess of gaseous carbon and uses this to provide an explanation for many of the observations seen within the furnace. The Umea group has also presented studies that use CO as well as nitrate salts to consider gas phase reactions for several metals and the consequence of oxide formation and decomposition in these systems (C114). By use of a less complete set of thermodynamic data, an equilibrium model has also been presented to explain the appearance temperature and peak shape when oxygen was introduced into a furnace (C115). The observed shift in some of the absorbance signals has been attributed to gas phase equilibrium and its perturbation by the presence of oxygen. Using computer models, the same group concluded that free oxygen depended on initial concentrationsand furnace heating rates (C116). Pulse shifts were also discussed. By use of time and spatially resolved AA data and ”the first shot back effect”, the effect of oxygen on the late shift of Pb due to chemisorbed oxygen is argued as being inconsistent with a mechanism involving gaseous oxygen (C117). Ascorbic acid or refractory metals have been suggested as a means of binding the excess oxygen (C118). Sensitivity for the determination of Sn was improved by decreasing the free oxygen content in the gas phase. Another study involving the carbon-oxygen reactions using a Massmann-type atomizer coupled to a gas chromatograph suggested that heterogeneous equilibria are not obtained at temperatures less than 2600 K (C119). The cleverly devised system employs a means of detecting CO and COz using methanation of COz and CO and FID. In a similar vein, gas impurities and their effect on atomization mechanisms have been discussed by Wang and Lin (C120) and molecular formation discussed by Doerffel et al. (C121). While the previous articles were cited as general explanations of overall reactions that may be occurring in the furnace, it is obvious from the larger number of manuscripts dealing with specific elements, specific matrices, or specific analytical samples that much of the chemistry occurring within the atomizer needs to be dealt with individually rather than with “broad brush strokes”. 232R

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The interest in a particular analyte or matrix modifier makes it difficult to discuss one aspect isolated from the others, since many of the studies are directed at the impact of the matrix on a particular analyte or vice versa. For example, a mixed Cu-Fe matrix was recommended for water and wastewater analysis (C122). Zeeman background correction was required. In another study, the impact on the Se signal by acids, salts, and organic reagents was considered (C123). More fundamental studies of Se involved the thermodynamic calculations of Cedergren and co-workers (C124) looking at both platform techniques as well as constant temperature furnaces. They note, for example, that Ni is significant in stabilizing Se(1V) and Se(V1) in the presence of several species but the stabilizing effect is lost in the presence of an organic matrix, Le., glucose and NaCl. Se2, HzSe, and SeS were some of the stable species found from their calculations. By use of radioactive tracers, it was reported that Se lost during the dry cycle in the presence of H could be retained if an activated graphite surface was used (C125). The use of Ni as a modifier was looked at in yet another study by these researchers (C126). They drew, in part, from the very interesting mass spectral (MS) results of Styris (C127) as well as radioactive measurements and thermodynamics in discussing the role of Ni in stabilizing Se. Styris’ paper and that of Droessler and Holcombe ((2128)(who also conducted MS studies on the Ni/Se system) are worth reading and furnish interesting, but sometimes conflicting, conclusions. While discwing MS data used in understandin ETA processes, the papers of Styris and W i e l d on Be (a297 and Al (C130)must be noted as examples of interesting research. It is difficult to discuss Se without reference to the increasingly widespread use of Pd as a modifier in place of Ni. While it appears to be ideal for stabilizing Se, it has been applied to a wide variety of elements with very positive results allowing thermal pretreatments 3OO-60O0 higher than might n o d y be expeded in the absence of a modifier. Schlemmer and Welz (C131) suggest that Pd combined with Mg nitrates provides a broad-based modification system useful for a number of elements. Voth-Beach and Shrader (C132) also discuss the advantage of Pd as a modifier but, in contrast to the use of Mg nitrate, suggest that the reduction of the Pd on the graphite surface is critical for its effective utilization in delaying atomization of the metals. Chemical reducing agents were tested, and SEM’s were used to support the supposition that the physical form of the Pd was a major contributor to its effectiveness. X-ray photoelectron spectroscopy was used to look at the mechanism for the retention of P b and Bi by the Pd modifier (C133). Binding energies indicate the fctrmation of Pb-Pd and Bi-Pd. In another article researchers have pyrolyzed albumin to form a graphite or carbon black overlayer on the surface of a pyrolytically coated tube which gave good Se retention and higher sensitivities when used with Pd (C134). Electron microprobe analysis revealed a Se:Pd ratio of approximately 1 with no apparent detection of free Se. This approach was used in the analysis of freeze-dried human serum. Mo, due to its refractory character, has been the focus of several studies. Chakrabarti et al. (C135, C136) consider, in detail, the mechanism of Mo vaporization using X-ray diffraction and SEM. Using relatively large quantities of analyte, they found oxides and carbides. The articles also offer an explanation for the loss of Mo between 1200 and 1800 K. Wend1 ((2137) studied the chemical transformations for Mo and Ge (and V, Cr, Cu, Ag, and Au in lesser detail) using X-ray diffraction, SEM, and molecular absorption. A similar study focused on Ge has also been presented (C138). Ge is the focus of another study where the loss of Ge as the oxide is cited as the cause for the relatively low sensitivity in the absence of any modifiers (C139). Prevention of the reduction of GeOz to GeO was successful by reducing the activity of the graphite by either chemisorption of oxygen on the surface, using oxidizing acids such as nitric or perchloric in the sample solution, or using Ta-treated graphite furnaces. As will be seen in the section dealing with applications, the interest in reliable techni ues for the detection of trace amounts of Al has reached a new ?ugh point over the last two years. The Al-0-C system was studied by using equilibrium calculations and the transfer of A1 oxide to the gas phase has been suggested (C140). P b and the role of phosphate modifiers were investigated by MS by Bass and Holcombe (C141). Surface-boundspecies

ATOMIC ABSORPTION SPECTROMETRY

are suggested and MS COZ data are used to pinpoint reduction rocesses. Double peaks, commonly observed for Pb, have een attributed to two different P b oxide forms in the gas phase, one being the isolated PbO with the other being a polymeric form ( n = 4 ) (C142). They explain the absorbance signal as being due to the gas phase decomposition of Pb oxide instead of carbon reduction in the condensed phase. This is inconsistent with vacuum MS data where Pb is detected from surface reduction. Another set of authors suggest the MgCl interference in the determination of Pb is a result of solid phase reactions (C143). The addition of (NH,) (EDTA) minimizes this problem, probably due to the release of chloride as NH4C1. The determination of P received attention from the group at Bodenseewerk in a series of three articles. They found that the larger number of active sites provided more favorable conditions for P determinations (C144). These active sites were more prevalent on uncoated tubes as well as on tubes which had been activated by the addition of oxygen. Several modifiers were studied and Ca was su gested as the best modifier (C145). La, Y, and Ni permitte! the same pyrolysis temperatures but did not produce the same short and long term signal stability. Mechanisms involved in the reactions were discussed and the use of a dual cavity platform was used as a diagnostic tool in this study. S E M s of the surface when the La modifier was em loyed in the determination of P have also been presented (&46). Some very interesting micrographs were presented showing the obvious pitting and corrosion in the graphite surface and suggesting the formation of intercalation compounds between La and the graphite. Interestingly, they found no indication of the La carbide on the surface. They were also to discern the corrosive patterns caused by the La from those which were produced by the presence of P on the graphite. Wang and Lin (C147)considered the variation that might occur in the appearance temperature in the study of Cu using pyrolytic and Ta-coated tubes. A French group also considered Cu vaporization in an artificial seawater matrix and suggested the use of HN03 or HzS04as a modifier to minimize the MgClz interference (C148). Cr in urine was once again cited as a problematic analysis (C149). These authors suggest atomization at temperatures less than 2400 K to reduce the background interference, which they suggest is caused by the emission from Cr together with K and Na. Other surface modifiers including ethanol, 12-KI, Ca(I1) and Mg(I1) as surface treatment agents were coupled with methane-im regnated Ar sheath gas to enhance the Cr behavior via the &activation of surface sites through carbide formation and molecular intercalations (C150). They also noted that, as other people have seen, the platforms are not advantageous for the determination of Cr in many instances. Cr along with Mn and Ni were studied with res ect to the interferences of Mo and Rh and a variety of acids (8151). The depressive effect caused by H3P04 was attributed to the formation of heteropoly anion lattice which traps the analyte. The presence of ligands was used to minimize the impact. In a study of Bi, Pb, Cu, and Cd, the use of thiourea is discussed and activation energies using the absorption profile are presented (C152). The authors suggest that complexes with thiourea are formed on the surface, converted to the sulfide, and vaporized directly from the sulfide, which minimizes other compounds from forming. The basic interference of CuC1, and NaCl with the platform for a number of elements was investigated (C153). These experimental results were combined with the ex erimenta from constant temperature atomization as well as ual-cavity platforms and thermal dynamic calculations to postulate on the mechanisms for the interference. Estimates are also presented for the partial pressure of Clz within the atomizer and were found to be largest when a CuC1, matrix was employed rather than a NaCl matrix. Other halide interferences were discussed by Tsunoda et al. (C154). Various inorganic matrices were evaluated in the determination of Cd (C155). Activation energies were compared with literature data and results presented for several acid and salt combinations. The use of a dual-cavity platform to examine the influence of Ni chloride in the determination of Sb has been conducted (C156). The authors suggest that gas/ solid interactions are prevalent and the interferences are eliminated when nitric acid is used as a modifier in the sample.

E

P

The authors suggest the presence of gas phase equilibrium under the conditions studied. We have long felt that the use of ases as matrix modifiers may prove very beneficial in ETA& in place of the more conventionally used modifiers which are added to solution. Some of the big advantages of gases, of course, are their high degree of purity and general freedom from trace metal contamination as well as the relatively easy means by which they can be introduced into the furnace and the speed with which they can be purged from the furnace. Each year a spattering of articles deals with the use of added gases to the furnace to facilitate a particular analysis or as a means of modifying the furnace environmentfor a more fundamental investigation. An article which explores the use of alternate gases in a more general sense has recently been published by Schlemmer et al. ((357) who discusses briefly the use of oxygen to pretreat samples in organic matrices (i.e., convert pyrolysis to a combustion process) as well as stabilizin many analyte species. Methane addition is also used to en\ance atomization efficiency for some of the refractory elementa in glassy carbon tubes and to a lesser extent in pyrolytically coated tubes. They also mention the use of Freon to reduce memory effects of the more refractory elements. The use of chlorine for the same purpose was also discussed by Matusek and Powell (C158). They suggest the possible introduction immediately after the peak absorption has occurred or during the purge cycle with applications demonstrated for V and Cr. Hydrogen has proven effective in minimizing sulfate interferences on Pb (C159)and in generally assisting its volatility (C160). The latter paper suggests the utility of Hz with real samples may lie in the removal of chloride as HC1. While this would obviously involve the reaction with some other species in the system in order to form HC1, details were not discussed. Hz (5% in Ar) also was used with graphite furnaces lined with Ta foil in the determination ofrare earths (C161). The tube lining was extended to more than 400 firings with improved precision and sensitivity. 7. Flames, Atomic Fluorescence, Flame Emission. People working with flame techniques should be aware of some of the potential hazards which have been highlighted recently in an article by Everson (C162)who discussed the potentially serious explosions with the use of acetylene tanks which are overfilled or small leaks that may exist at junctions. While a considerable amount of analytical work continues in the area of FAA and, to a lesser extent, FE, a majority of the more fundamental research appears to have centered on AF or other excitation techniques discussed in the next section. This fact is possibly due to the success of past efforts extended in understanding flames and the interferences, and developing a means of circumventing the problems. Although, sample introduction and nebulization studies discussed in other sections should be of obvious interest. A large amount of basic research on flames and combustion chemistry, in general, has occurred but is beyond the scope of this review. Such articles as use of two-photon excitation for the measurement of hydroxyl radicals (C163) and absorption, two-wavelength fluorescence and two-wavelength ionization to study inhomogeneities in flames (C164)certainly will be of interest to people working in the area of developing fundamental understanding of analytical flames. “Local electrical fields in flames” have been probed by using Rydberg states of Li and LEI (C165). A fundamental study looking at the ionization of Ba in a Ca matrix with a NzO CzHzflame has been presented by Takada and Satho (C166/. Using the Saha equation, they found the ionization of Ba in Ca was about one-third of the value expected theoretically. L’vov et al. (C167)have extended some of their basic theories in the area of ETA to the area of air/CzHz.flames. They postulate that the decrease in analytical sensitivity for several metals in a reducing flame is related to formation of thermally stable carbides. They support this postulate by considering equilibrium reactions. Using a technique which had previously been employed in studying ETA processes, Kantor (C168) used his ETA system in-line with a flame to study the basic effect of Al and La on the determination of Ca. Mechanisms responsible for the matrix effects caused by Al, Mg, Ni, and Cr in the determination of Y and La have been recently reported for NzO/CzHzflame (C169) as well as some general observations re arding the relative atomization efficiencies in an air/CzHzdame (C170).The latter considers variations ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

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ATOMIC ABSORPTION SPECTROMETRY

in the degree of atomization as a function of flame temperature. Revisiting the use of surfactants (more specifically crown ether surfactants) and enhancements for flame spectrometry, Ward et al. (C171)attribute the enhancements of the Ca signal to the presence of a surfactant but claim that the slight decrease in the size of the aerosol produced in the nebulizing chamber is not primarily responsible for the effect, but exact responsibility has not been assigned. The formation of heteropolyanions and the entrapment of metals in the structure in cool flames is cited as the source of interferences on the absorption signal for Mn, Cr, and Ni (C172). Stable metal oxyphosphorus compounds are postulated as a source of FAA interferences caused by organophosphorus acids for several metals, and evidence for the formation of stable Mo carbides has been presented for N20/C2H2flame (C173).In the absence of releasing agents such as La or Sr, As(V) showed a higher degree of interference than As(II1) in the Ca determination using N20/C2H2FAA (C174).Mechanisms were presented based on volatility of the respective As-Ca compounds. The mechanism of releasing action of common reagents of Mg in silicates has a peared (C175)and an economic, as well as improved, methozfor use of air/CzHz flames proposed (C176).FON gas (a mixture of propane and butane) was used as a diluent for the C2H2with a net savings in fuel consumption and an increase in the absorption sensitivity for the metals tested. A means of enhancing the signal by analyte preconcentration directly in the flame or by increasing the residence time in the optical path proceeds. A slotted quartz tube set over a burner was used for four elements in environmental and clinical samples (C177). The “atom-trap“ involves the insertion of a cool area (often a water-cooled silica tube) within the flame to effectively preconcentrate the analyte before impulse release by shutting off the coolant flow. The approach was used in the determination of Pb and Cd (C178).Detection limits of 0.8 and 0.1 ng/mL, respectively, were observed ... which ain’t bad for a flame (although a 2-min collection period was involved). After the solution extraction of Cu from a water sample, Cu was also determined successfully by using the atom-trap technique (C179).Additional studies using this approach have also been reported for Cd (C180) and Pb (C181).

FE has received sparse attention in the development areas of atomic spectrometry over the last couple of years, although several application papers are reviewed in later sections. Rezaaiyaan et al. (CI82)covered once again the use of additives to solutions to help enhance the FE signal. Looking at the Ca signal, hydrozine and nigrosin solution additives were both evaluated as vaporization aids. Enhancements were thought to be due to enhanced vaporization rather than more rapid aerosol desolvation. A new background correction technique involving modulation of the HPO signal from a fluctuating magnetic field was used to lower LOD’s in FE by a factor of 5 as well as correct the background during the determination of P (C183). AF continues to receive a large degree of attention. The driving force for this technique lies partially in the extreme sensitivity available compared with other atomic spectroscopic techniques as well as in the potential use of the method as a diagnostic tool of other excitation/atomization sources. Wu and Michel (C184)considered the interferences associated with continuum source A F and discuss the impact of the band-pass as well as the potential use of a double monochromator in AF. Comparisons with EDL’s are made. Correction and elimination of scattering in flame AF (C185) as well as the use of Zeeman splitting of both the light source as well as the atom generator have been discussed (CI86). A Cu-vapor pumped dye laser was employed both for A F as well as laser-enhanced ionization (LEI) spectrometry (C187). Methods are outlined showing how a significant reduction in high-frequency interferences associated with this type of pump laser system can be minimized. Use of stimulated Raman scattering in Hz at liquid nitrogen temperatures was evaluated as a tunable, sharp-line UV laser source for elements such as As, Se, Te, and Zn (C188). A Nd:YAG pumped dye laser served as the primary radiation source, and data were compared with those obtained in the ICP using HCL excited AF. Because of the excellent atomization characteristics associated with the ICP, a number of papers have appeared which 234R

ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

discuss the use of AF with an ICP atomization source. ICPAF, some of the instrumentation, and its freedom from many interferences present in ICP-AES have been presented (C189). Improved detection limits for HCL-excitation have been given (C190). Dual-plasmas and “atomizer, source, inductively coupled plasmas, in atomic fluorescence spectrometry”(ASIA) have been discussed (C191), including the production of two ICP’s from one free running high-frequencygenerator (C192). The extended sleeve ICP torch has been studied and the power and frequency dependencies detailed (C193,C194). Atom production with ETA combined with AF has a potential of providing the maximum sensitivity due to the enhanced residue time afforded by ETA and the high detectabilities available from laser excited atomic fluorescence. A tube furnace was compared with a cup atomizer and found to be far superior especially when a platform was employed (C195).In another study, a variety of furnace tube designs were evaluated by Michel’s group (C196). LOD’s for the tubes compared favorably with the best graphite cup data in the literature and were 2-500 times better than ETA-AAS. The use of platforms also was evaluated for several metals by this group (C197).Goforth and Winefordner compared a number of cup designs and surfaces (C198) and also evaluated the performance for tubes (C199).In the determination of Pb by AF, furnace vs cup atomizer (C200)and SIT vidicon vs photomultiplier tube (C201)were evaluated. A W coil was also tested for atom formation with encouraging results (C202). Vacuum conditions were used for the AF determination of Co and Sn (C203).They report that the vacuum atomization decreases matrix interferences as a result of “collisionless expansion”. It is not clear that the absence of high-temperature gas-phase collisions would unilaterally assist in the elimination of matrix effects, especially if the analyte is originally volatilized from the surface as a molecular species. AF can obviously be employed with any source that is capable of generating atoms. This was attempted in the case of direct current plasmas (DCP) where background scatter appeared to be a limiting factor (C204,(7205).Solutions were also analyzed by using glow discharge va orization and a nonresonance transition in studies of In (8206). AA has been used to study atoms and ions in an ICP (C207); neutral and ionic species of Ca and Sr were mapped by AF spatial profiles (C209)and interstudies of an ICP (C208); ferences by easily ionized elements (C210)were obtained by AF and emission studies of an ICP; and enhancement effects in DCP’s were probed by AF (C211).In another study AF was combined with Raman and emission studies to look at the channel characteristics as well as analyte dynamics in the high-voltage spark (C212).“DIP” spectroscopy with atomic and ionic fluorescencehas been proposed as a diagnostic tool for flames and plasmas (C213). 8. LEI, Coherent Forward Scattering, and Miscellaneous Techniques. Not covered in this section are techniques associated with MS detection such as resonance ionization spectrometry (RIS) and resonance ionization mass spectrometry (RIMS). Man of the studies associated with these techniques are covereIin the review by Koppenaal in this issue (A4). Recent advances in laser-enhanced ionization (LEI) were presented by Turk (C214)and three-dimensional atomic spectra in flames using LEI have also emerged from the same group (C215).A fundamental study has appeared that looks at the impact on the LEI signal as the excitation wavelength was gradually raised (C216). The authors found that the ionization signal has a smooth behavior when passing from bound states into a continuum state. Several articles focused on Sr and basic processes occurring in air/C2H2flames have emerged since the last review (C217-C219). Theoretical analytical considerationsfor puke sources have also been presented by Omenetto et al. (C220)using both single-step and double-step techniques. The elimination of spectral interferences using two-step excitation LEI has also been discussed for flame systems (C221).The authors note that the impact of Na on the LEI signal for Mg is diminished by a factor of 70 when two-ste excitation is used, and an additional factor of 20 is obtainelby optimizing the laser light intensity. Single versus stepwise excitation was studied for Zn and several transitions were evaluated for suitabilitv in LEI (C222). Over the last two years a handful of applications have

ATOMIC ABSORPTION SPECTROMETRY

emerged including the investigation of the multielement capabilities for LEI in flames on aqueous solutions (C223,C224). GaAs semiconductormaterials were analyzed for six elements by using a two-color LEI experiment with flames (C225). Use of ETA has also been popular with LEI (C226, C227). Obviously the point of interest in these articles is the configuration for the electrodes. An atmospheric pressure microarc plasma also was investigated as an atom reservoir for LEI ((2228). Atomic magneto optic resonance spectrometry (AMORS) and CFS have recently been discussed in a general article by Hirokawa (C229),and the potential advantage of the technique for multielement analysis using a continuum source was discussed. The sensitivities obtained by using a laser-probed Voigt-effect system were ca. 200 times better than those for AA for Na (C230). An interesting article by Davis et al. (C231) considers a transverse magnetic field around either a flame or a graphite furnace. They make several interesting points in a theoretical evaluation of the technique. Noise characteristics as a Voigt-effect CFS system also emer ed from this group (C232). Response linearization by small ctanges in the analyzer offset angle and a comparison to the noise characteristia for two systems was discussed. In another article out of the orange juice state, Davis and Winefordner (C233) discuss CFS using a continuum source in flame and furnace. By use of an HCL and a longitudinal magnetic field, four elements were determined in simple aqueous solutions and LOD's were found comparable to those obtained in Zeeman AAS for Cd, Cu, Pb, and Ag (C234). Several elements were also determined in metal samples after dissolution in nitric acid using CFS techniques with a graphite furnace (C235). Y and other rare-earth metals were atomized in a cuvette and also determined by CFS using a continuum source (C236). Degenerate four-wave mixing (DFWM, that's not the Dallas-Fort Worth Metroplex)originally presented by Ramsey has appeared again in the literature and continues to show interesting possibilities for the "one atom per swimmiig pool" regime. Estimated limits of detection for Na of 0.1 ppt have been made (C237). In another article an LOD of 5 ppb Na was reported using a CW laser with an air C H2 flame and phase conjugation by resonant DFWM ( C k 8 f . Another interesting twist is the work conducted in concentration-modulated absorption spectrometry (COMAS) which again exhibits the capabilities of detection at the superultratrace levels (C239). Time-dependent variations in the gain (C240),the effect of a finite spectral line width (C241),and the use of CW vs pulsed lasers (C242) have been presented. Furnace atomization nonthermal excitation (FANES) has seen some activity although the majority of the efforts still originate in the GDR. Falk has presented general coverage on this topic (C243). Characteristics of the commercial instrument along with H, line profile measurements and information on the background spectra have also been discussed (C244). Results for the analysis of Cd in blood on this sgstem have been reported as being better than those obtained for ETA-AAS, although the ETA data used for comparison represents atomization from a tube wall from a relatively old paper (C245). Solid plant material has been analyzed directly by using a new tube design (C246). A comparison of ETAAAS, LEAFS, and FANES (C247) as well as an overview of FANES and molecular absorption and emission data for fluorides and other halogenated compounds (C248) have appeared. Graphite furnace atomic emission has received little attention the last period with the exception of an article from Lundberg et al. (C249)where they look at detection limits for 16 elements using wavelength modulated background correction. A technique entitled saturation spectroscopy was used for optically thick atomic vapors to extract precise line width information (C250). Whitten et al. (C251) have used a lowpressure interface in conjunction with an air/CzH2flame to minimize collision broadening (50-MHz resolution for Na) in a modified form of saturation spectroscopy. Finally, a distantly related but very interesting topic has been continued by Kujirai, Davis, and Winefordner (C252) which involves emission and fluorescence techniques using secondary wavelength modulation. This particular article deals with the analysis of a Cu alloy in an Ar-separated air/CzHzflame. The results for the determination of six elements in the alloy are reported.

D. HYDRIDE AND COLD VAPOR TECHNIQUES 1. Hydride Generation. Hydride generation constitutes an important means of volatilizing several metals. Much work has been done studying chemical process associated with the hydride techniques. Agterdenbos et al. (01)and Parisis and Heyndrickx ( 0 2 )studied the improvement in sensitivity and reproducibility with specific quantities of oxygen, and enhancements using other oxidizing agents were also investigated ( 0 3 , D 4 ) . The effect of temperature on the generation and decomposition of As, Sb, and Bi was investigated by Fujita and Takada ( 0 5 ) . Welz and Schubert-Jacobs ( 0 6 )looked at general atomization mechanisms in hydride-generation AAS. Other fundamental processes were investigated for As ( 0 7 DlO), Se (011-013), and Ge ( 0 1 4 ) . Interferences were also investigated by several authors. Dittrich and Mandry ( 0 1 5 ) suggested the formation of diatomic molecules between the analyte and matrix as a major source of interference in ETA-AAS and recommended using tube temperatures greater than 2000 "C. Hershey and Keliier ( 0 1 6 ) investigated possible interelemental interferences. Other interference effects investigated include hydride forming elements on As, Sb, Se, Sn ( 0 1 7); HF on As and Sb ( 0 1 8 ) ; Cu, Co, and Ni on As ( D l 9 ) ;Fe and Ni on Se ( 0 2 0 ) ,Fe and Cu on Se ( 0 2 1 ) ;and divalent cations on Se ( 0 2 2 ) . General instrumental parameters have been presented (023, OH), and instrumental modifications that attempt to improve hydride generation have appeared in the literature. Ikrenyi (025)described a recirculating hydride technique which was more sensitive and reproduciblethan open-ended gas systems. Hassett and Hassett ( 0 2 6 ) described a modified generator. Other miniature generators were also described ( 0 2 7 , 0 2 8 ) . A membrane gas-liquid separator was introduced for flow injection hydride generation ( 0 2 9 ) . The use of various tubes as atomization cells was also investigated (030-032). Dittrich et al. ( 0 3 3 , 0 3 4 )reported LOD's of 0.1-0.5 ng, which were up to 1000 times better than with quartz tube atomizers using a graphite paper atomizer. General improvements for As determinations were attained by using iodide trapping soluBravko . (039)suggested using an Sb-coated tions (035-038) surface for As and Se determinations. Brown et al. ( 0 4 0 ) described an electrically heated silica tube and AF. Nakahara and Wasa ( 0 4 1 ) also used hydride generation AF for the determination of Sn. Sturgeon et al. (042-045) looked at As, Se, and Sn in marine samples, and Welz et al. (046-048) looked at Se in biological samples. Other applications of hydride generation techniques can be found in Table 11. 2. Cold Vapor Generation. Cold vapor atomic absorption spectroscopy (CVAAS) is used almost exclusively in the determination of Hg. Adeloju and Mann ( 0 7 9 ) studied the effects of "OB, HC1 and HzS04and found NO, to be major interferents. Iodide interferences were reduced by use of alkaline Sn(II) solutions (080). The role of filter material and additives in the thermal volatilization of Hg was examined by Rozanska and Domanska ( 0 8 1 ) in CVAAS. Ahmed and Stoeppler ( 0 8 2 , 0 8 3 )looked at storage and stability of Hg and methyl-Hg compounds. The interference effect of Se in aqueous samples was also discussed (084). Other applications of CVAAS include the analysis of water ( 0 8 5 ,0 8 6 ) , aquatic food chain (D87),biological tissues (088),plant samples (Dag), mushrooms ( 0 9 0 ) ,medicinal pills (0911,and geological materials ( 0 9 2 ) . E. HYPHENATED ATOMIC SPECTRAL TECHNIQUES AND SPECIATION On-line sample preparation schemes usually involve a separation technique prior to detection. While this is useful in separating the analyte from the matrix, it is more often used as a means of speciation. Flow injection analysis (FIA) seems particularly adaptable to FAA, and a special section for the numerous FIA-FAA papers has been made. 1. Hyphenated Techniques. Maketon et al. ( E l ) studied the direct and indirect chromatographic detection of several samples by AA. Rapaomanikis et al. (E2) looked at a purge and trap technique for P b speciation using chromatography with AA detection. Galante and Hieftje (E3) further characterized "replacement ion chromatography", while Aihara and Tanaka (E4) used a similar approach with FAA and a Ca-exchanged Chelex-100resin. Tabini and Kratochvi (E5) AIUALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1966

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ATOMIC ABSORPTION SPECTROMETRY

Table 11. Hydride Generation matrix

element

As

medicine

Bi

geological

Ge

geological

Pb Sb

gasoline water marine organisms PVC biological

Se

cu

bovine liver plants

Sn

coal water environmental human tissue seawater, water canned foods, A1 alloys wine, food food

As, Sb

sewage atmosphere geological

As, Se

coal Cu, Ni powders

As, Sb, Se Sb, Bi, Sn

protein pollution aerosols

comment

ref

treatment with "OB, HzS04,Hz02 quartz tube furnace used 0.07 pg detection limit, interferences studied Nz0-C2Hzflame used electrically heated quartz tube used chelating resin separation heated silica tube used vaporization from nonaqueous solution digestion procedures described flow system described wet digestion with "OB, H3P01,H202 NO, interference eliminated by H2NSOBH influence of organics discussed flow injection technique used nitric and sulfuric digestion various methods described use of nonaqueous solution after extraction sample preparation scheme described CzHz-air flame used matrix effects and interferences studied instrumental problems discussed KI, AlC13, ascorbic acid modifiers 58 and 36 ppb detection, respectively separation with Chelex 100 resin collection and preconcentration in liquid nitrogen cooled trap 0.25, 0.13, 0.25 ng/m3 detection limit

D49 D50 D51 D52 D53 D54 D55 D56 D57, D58 D59 D60 D61 D62 D63 D64 D65-D68 D69 D70 D71 D72 D73 D74 D75 D76 D77 D78

discussed a des' strategy for microcomputer-controlled data acquisition angrocessing of an ion-exchange/AA system. Gas chromatography (GC) was a good mate for AAS, with a majority of the applications involved in the determination of Pb and alkyl-Pb species using FAAS (E6-EIO)and others employing a GC-ETA-AAS system ( E l l ) and hydride generation (E12). A miniature FAA was combined with a GC to determine alkyl-Pb, Se, and Sn (E13). Other applications using GC include the determination of tri-n-butyl-& (E14), methyl-As (E15), and organo-Hg (E16, E17). The interfacing of a high-performance liquid chromatograph (HPLC) to FAAS as well as the benefits to be achieved have been discussed (E18-E20). Applications of HPLC-FAAS have included the determination of Zn, Cu, and Cd in blood (E21), Mg and Zn in urine (E22),metallothionines (E23),As (E24, and E25), Cr in aqueous solutions (E26),Sn in water (E27), Zn in liver (E28). The construction of an HPLC-GFAAS system was described by Haswell et al. (E29),and Irgolic and Brinckman (E30) looked at correlating elution rates of compounds with their biological activity. Other applications of HPLC-GFAAS include As in fish and shellfish (E31),As in tissues (E32),Sn in mammal tissue (E33),Cr in biological samples (E34),and P b in mammal tissue (E35). Liquid chromatography with FAAS was used to determine triorganotin compounds (E36) and to preconcentrate Cr (E37). Reviews also appeared on E39). LC with a variety of spectrophotometric detectors (E38, Speciation of metals is often done by using special sample preparations followed by AA detection. Alkyltin and alkyllead compounds were determined by using chromatography and ETA (E40). Keirsse et al. (E41) used size-exclusion chromatography and ETA to determine various A1 species. Speciation of organoarsenic compounds and As in various oxidation states was done by using hydride generation techniques (E42-E44). Cr3+ and Cr6* were determined by ETA after extraction by trifluoroacetylacetone (E45, E46),after copreci itation (E47),and after separation by ion exchange (E48).8olvent extraction was used to speciate Cu (E44-E51). Oxidation states of Hg were determined by using extraction and cold vapor techniques (E52,E53), while the free Mn(I1) in lake water em loyed ion exchange resins and GFAAS (E54). Febo et al. (E557 determined the amount of tetralkyllead in the air by usin a denuder diffusion technique and ETA. Sb(II1) and Sb(6) were determined by using separation procedures (E56,E57) and hydride generation (E58). By use of column chromatography (E59) and electrolytic processes (Ern),trimeth lselenium, Se(IV),and Se(VI)were determined. Butyltin (E617and alkyltin (E62)were determined by using extraction and hydride generation techniques, respectively. 286R

ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

Butyltin was also determined by a separation based on differences in boiling points (E63). 2. Flow Injection Analysis. FIA with AA detection was FIA has been used, in many cases, reviewed by Ruzicka (EM). as the "front end" of FAA systems as a useful means of removing interferences such as NaCl, KC1, NH&l (E65),and Na (E66).A dual-phase gas diffusion FIA hydride generation system decreased interferences from transition metals (E67). Instrumental parameters were discussed by Appleton and and 169 sample-solvent carrier combinations Tyson (E68), were investigated by Attiyat (E69). A variable dispersion FIA manifold is described by Tyson et al. (E70)which allows for single standard calibrations and sample dilution. General method development problems for a FIA-FAAS system were discussed by Tyson et al. (E71),and Lopez-Garcia et al. (E72)investigated air compensation in FIA-FAAS. They found the main advantages were due to an increase in nebulization efficiency. Fundamentdand practical considerations in the design of preconcentration approaches were presented (E73),and the analytical potential of continuous precipitation in the indirect determination of chlorides and oxalate appeared (E74). A novel approach described by Burquera et al. (E75) involved the passage of the flow system through a microwave oven which allowed for "in-line" sample decomposition of blood. FIA-FAAS is also useful for indirect determinations and speciations: e.g., nitrates and nitrites by Cu (E76),sulfides chloride by Ag (E79),A1 by Cd (E77),cyanide by Cu (E78), by Fe (E80), and chloride and iodide by Ag (E81).Other applications include the determination of Na, K, Ca, Mg, Fe, Cu, and Zn in cerebrospinal fluid (E82), Cu, Zn, and Fe in saliva (E83),Mn and Co in blood (E84),A1 in hemodialysjs Pb in fluids (E85,E86),Cd in soil (E87),Cr in steels (E88), and Zn in serum (E91). urine (E89),Pb in gasoline (E90), Alonso et al. (E92)used a sequential FIA system to determine Ca potentiometrically and Mg by using AA.

F. APPROACHES TO SPECIFIC ANALYSES In the last two years the majority of the papers dealt with specific applications of various stages of the analysis using AA, AF, or FE. Many of the non-English applications papers were not included, yet there still remained literally hundreds of articles which were reviewed. These papers have been divided into the following categories: sample preparation, indirect analysis, eneral applications of AA including both FAAS and , solids analysis (including both direct solids and slurry analysis), isotopic analysis, and finally general applications of furnace emission, FE, and AF. Altogether over 500 papers on approaches to specific analysis are included.

ETA-AAPS

ATOMIC ABSORPTION SPECTROMETRY

Table 111. Liquid Extractions element

cum cum

method

solvent

sample

ref

F21 F22 acidic aqueous solutions F23 In F24 N-benzoyl-N-phenylltluene AAS geological Mo F25 ETA water and environmental diisobutyl ketone Mo trioctylmethylammonium chloride F26 ETA plant materials Sb 18-crown-6complex F27 FAAS picric acid Sr F28 N-benzoyl-N-phenylhydroxylamine ETA rocks Sn N-phenylbenzohydroxamicacid F29 ETA general sample Te F30 N-benzoyl-N-phenylltoluene FAAS rocks W F31 xylene/fIN08 FAAS alloys, water, hair Zn l-phenyl-3-methyl-4-stearoyl-5-pyrazolone/ F32 solar salts AAS Ca(II), Sr(I1) trioctylphosphine oxide (TOPO) in benzene TOPO in MIBK F33 FAAS geological Sn. Mo trichloroethane F34 salt water Cui Cd, Pb, Ni ETA DPTH F35 Fe(II), Cd(II),Zn(II), Co(II), Ni(II), Cu(I1) FAAS steel

AAS

alloys and human hair

AAS AAS

tap water

Several review articles (FI-FIO)cover a variety of applications for several techniques. EPA protocol was discussed by Aleckson et al. (F11)for 24 different metals in various matrices, and an interlaboratory study comparing ICP and AAS for the analysis of plants (F12)showed similar results. 1. Sample Preparation. General Sam le handling techniques were discussed by Ericaon et al. (F137and Tramontano et al. (F14)for controlling contaminations without clean room conditions. Koscielniak offered a simple and quick method for sample dilution (F15)and standard additions (F16).A comparison of direct sample introduction versus various sample preparation schemes was examined for Co in blood (F17)and Mg and Zn in erythrocytes (F18).Other general sample preparation schemes were examined (F19,PN).Other preparation techniques are also covered in earlier sections of this review. l a . Liquid Extractions, Ion Exchange. Table I11 shows various liquid extraction procedures for determining several metals. One of the most common liquid extraction schemes involves the use of methyl isobutyl ketone (MIBK) (F36-F44). MIBK was also used to extract Fe(III) for determination (F45) and to remove it as an interferent in analysis of food for Co (F46).Na was removed as an interferent in various tissues using hydrated antimony pentoxide (F47).CHC13was used to extract several metal complexes in the presence of seawater (F48),Te (F49),and low-grade ores (F50). Satake and Co (F52), and Cd, Pb and Zn coauthors extracted Pd (F51), (F53)on microcrystalline naphthalene. Pavoni et al. (F54) showed that without complete dissolution, occluded analyte could be lost from analysis in various marine sediments during an extraction. Interestin ly, Krivan and Schaldach (F55)used this to their advantage y extracting surface metals in pine needles and distinguishing these from total metal concentrations. Similar work was conducted by Porter on leaves (F.56).Mandal and Mandal (F57)looked a t the sequential extraction of various forms of Zn from soils. Several extraction processes were evaluated by Kheboian and Bauer (F58). Puchyr and Shapiro (F59)determined that the extraction of several metals in foods was superior to wet ashing techniques. Another useful extraction technique used ion-exchange resins. Hadi (F60)investigated absorption behavior of ZeoKarb-226 for a number of metals ions. This method was used In (F62),Ni (F63),Ba (F64), Au (F65, to separate Re (F61), F66),Ag (F67),Li (F68), Bi and Cd (F69), and Sn ( n o )from various matrices. 2a. Electrodeposition, Coprecipitation, and Other Preconcentration Techniques. Preconcentration is useful when low levels of analyte make detection difficult. Some preconcentration schemes also separate the analyte from interfering matrices. Liquid extraction procedures which preconcentrated analyte, as well as extracted it from difficult matrices, were discussed (F71-F78).Khavezov et al. (F79) looked a t different extraction methods for preconcentrating 10 elements for analysis by FAAS. Impurities in semiconductors were determined by Graziulene et al. (F80)using preconcentrationschemes. Along the same lines, ion-exchange and adsorption techniques were used for preconcentration (F814’97).Activated charcoal was used in several matrices

6

molten naphthalene

bismuthiol derivatives bis(2-ethylhexyl)phosphate in MIBK

Table IV. Indirect Analyses (Atomic Absorption Used in All Cases Except Where Noted) species sought anionic surfactants bromohexine Cinchona alkaloids Cl-

analyte

cu

co Hg Ag

Concanavalin-A desferrioxamine glucose Ilactose lanthanides malathion nicotine O2 in water

Fe Fe, A1

OH-

Ag

pilocarpine SCN; SeCN-

Hg

Mn Hp“ Mn Ca cu

Cu, Pb, Zn or Ag T1

cu

ref

F152 F153 F154 F155 F156 F157 F158 F159 F160 F161 F162 F163 F164 F165 F166 F167

Determination by cold vapor AAS. to preconcentrate a variety of metals (F98-FlO3). Electrolytic preconcentration was reviewed by Sioda et al. (F104).A1 was determined in high-purity Fe by electrolytic dissolution and deposition (F105). Bo-Xing et al. (F106) detected sub-ppb Hg in water by electrolytic deposition. Veber et al. (F107)reported 4 pp-trillion on the detection limit for Cd by electrochemicaldeposition on graphite, and Ag was determined in Cu by internal electrolysis on Pt gauze (F108). An electrochemical flow cell suitable for use with dual detection in an FIA system was characterized by Schulze et al. (Flog).They increased sensitivity an order of magnitude for the determination of P b in water. Coprecipitationwas used to concentrate Cd (FllO), Au and and a variety of elements in Ag (Fill),Cd and P b (F112), water (F113),solder (F114),and glass (F115). Ueda and Yamazaki (F116)used Hf(OH)4to coprecipitate Cu for determination by AAS. P b was continuously precipitated in an unsegmented-flow AAS system (F117).Stec et al. (F118)used osmosis to enrich transition-metal concentrations, and Le Houillier and De Blois (Fl19)examined concentrating trace precious metals in Au and Ag beads. SenGupta (F120)determined the l a n w d e s and Y in reference materials by using precipitation or fluoride and oxalates. IC. Wet Ashing, Dry Ashing, Microwave Digestions. Sample digestions are commonly done 00 a variety of solid samples to get the desired analyte in solution form. Autoclaving was used to decompose cluster compounds for the determination of Ru and Rh (F121). General wet ashing (F122-Fl28)and dry ashing (F129-Fl32)procedures were examined. Dry ashing techniques, when possible, were recommended because they are faster and less dangerous (F1334’136). The use of “bombs” or pressurized wet ashes was examined (F137-Fl39)as they usually speed up lengthy wet ashing procedures. Microwave digestions were also exANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

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ATOMIC ABSORPTION SPECTROMETRY

Table V. Atomic Absorption Analyses in Specific Matrices (a) Blood, Serum, Plasma Ag: ETA, in urine also (F168) Au: ETA, use Ni and Pd-Mo modifier with Zeeman correction (F169) Al: ETA, similar results for deuterium of Zeeman correction (F170);ETA, hemodialysis patients, C1 interferences, contamination problems (FI 71-Fl75); ETA, best precision using uncoated graphite tubes (F176);ETA, Mg nitrate, Triton X-100 used as modifiers with L'vov platform (F177);ETA, antihemophilic preparations discussed (F178);ETA, standard additions with STPF (F179) Ca: ETA, analysis of red cells; nitric extraction (F180) Cd: ETA, nitric acid deproteinization (F181);ETA, urine also, Pd(N03)2and NH4N03modifier (F182);digestion with HN03 then H202for ETA (F183);ETA, total vs intracellular (F184) Cr: ETA, Triton X-100 (F185, F186); ETA, Zeeman correction, enzymatic degradation (F187) Cu: ETA, in hemodialysis solutions (F188) Li: ETA, platform with Zeeman (F189);ETA, Triton X-100 (F190) Mg: FAA; compared to x-ray, comparable results (F191) Mn: ETA; Triton X-100 without deproteinization (F192);ETA; errors from HN03 precipitation (F193);ETA, operational parameters (F194);ETA, platform with Zeeman (F195);FAAS, preventive medical check-ups (F196) Ni: ETA, Zeeman background (F197);FAA (F198) Pb: ETA, NH3, NH4H2P04,(NH4)*HEDTAmodifiers with O2 ashing (F199);ETA and FAA, deuterium and Zeeman correction (F200);Standard Reference Material (F201); ETA, Triton X-100 solutions stable for >3 h (F202);ETA, chelation, and extraction (F203);clinical significance and techniques discussed (F204);ETA, comparison of HNO, deproteinization and NH4HzP04,Triton X-100 modifier (F205);ETA, Triton X-100 Ni modifier, L'vov platform, Zeeman correction (F206);ETA, Cu, Mg modifiers, platform atomization, air ashing (F207-F209) Se: ETA, Ni, Pt modifiers, wall atomization (F220). Zn: review Cu and Zn sample collection and preparation (F211); FAA, HN03 digestion (F212);FAA, Zeeman correction (F213) Al, Cr, Co, Fe, Ni: FAA and ETA: digestions and addition of Triton X (F214) Zn, Cu, Fe, Mg: discrete nebulization technique for FAA (F215) many elements: ETA, tabulated results (F216) (b) Urine As: ETA, Zeeman correction, Ni, Mg modifiers (F217) Be: ETA, STPF, MgN03, "OB, Triton X-100 modifiers (F218) Cd: ETA, comparison of wall vs platform, HN03 modifier (F219);ETA, (NH4)2HP04-HN03modifier (F220) Co: ETA, Zeeman correction, Mg(N03)2,HN03 modifiers (F221) Mn: ETA, perspiration also, direct analysis (F222) Ni: ETA with Zeeman correction (F223) Pb: ETA, Zeeman correction with platform atomization (F224); ETA and FAA, extraction methods used (F225) Sn: anodic stripping and ion exchange (F226) T1: ETA, Zeeman correction, no sample preparation (F227) Ni, Cr: ETA, direct dilution (F228) (c) Biological Tissues Ag: ETA, bovine liver, Zeeman correction (F229) Al: ETA, human tissue (F230);ETA, human tissue, plasma and urine, Zeeman correction (F231);ETA, review, limitations of ETA (F232) Au: ETA, cysteine enhanced tissue analysis, Zeeman correction (F233) Zn: FAA, leukocytes (F234) C d extraction procedure in mice liver (F235) Cr: ETA, bovine liver, special sheath gas treatments (F236) Co: ETA, fish flesh, extraction, Zeeman correction (F237) Ni: ETA, human tissue, Zeeman correction (F238) Mn: ETA, review (F239) Mo: rat organs, extraction method (F240);ETA, digestion and extraction in blood (F241) Pb: ETA, in teeth (F242) Pt: ETA, serum and urine, analyte loss with dry ash (F243) Sb: ETA and ASV (F244) 2381

ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

Se: ETA, liver, Ni and Ag modifiers (F245);ETA, Zeeman correction, blood and tissue (F246) Si: ETA, significance of platform and oxygen discussed (F247) T1: ETA, platform, atomization, deuterium correction (F248); ETA and FAA, review (F249) AI, Cd, Pb: ETA, STPF, direct and decomposition procedures (F250) Cd, Cu, Pb, Zn: spike-height FAA, interferences assessed (F251) Cd, Cu, Fe: FAA, mussels, extraction by HN03 (F252) Cr, Cu, Zn: ETA and FAA, marine samples (F253) Ni, Al: ETA, skin analyses, depth profiling, direct solids (F254) Pb, Cd: FAAS, Delves cup (F255) Se, As: ETA, animal tissue, Ni(N03)Z modifier (F256) many elements: ETA, human reference materials (F257); nerve tissues, solubilization schemes (F258) (d) Other Clinical Cu: ETA, saliva, platform atomization, "OB, ",NO3 modifier (F259) Fe: intraocular fluids (F260) Mn: use W probe to reduce interferences in clinical samples (F26l) ~--_-, Se: ETA, direct analysis with Pt-Ni modifier (F262);in seminal fluids (F263) Y: ETA, duodenal digests and feces, Ta coated graphite tube (F264) K, Na: ETA, biological fluids, 5 nL of fluid used (F265) (e) Environmental Samples Ak ETA, in soil leachates, uncoated tubes better (F266) Au: soils, digestion and extraction (F267) Be: ETA, soils, Zr-coated tube removed interference (F268) Bi: ETA, river sediment, H2 sheath gas, direct determinations (F269) Ca: FAAS, in fertilizer (F270) Cd: ETA, air particulates, (NHJ2HP04, ",NO3 modifiers (F271) Co: ETA and FAA compared, soils (F272) Ge: ETA, coal, Zr coated graphite tube (F273) Mo: FAA, in clay supported catalysts (F274);ETA, soils and plants with thermally shielded furnace (F275) P b ETA, air particulates, Zeeman correction (F276) Se, Te: ETA, coal, coprecipitation (F277) Su: ETA, marine samples, direct analysis and hydride generation (F278) T1: ETA, environmental and botanical, Pd and ascorbic acid modifiers (F279) As, Cd, Pb, Se: ETA, marine tissue, matrix modifiers, STPF (F280) Bi, Cd, Pd: FAA, soils, atom trapping, extraction (F281) Cu, Zn, Pb, Mn: in sewage sludge using electrophoresis (F282) Pt, Ir: ETA, seawater, sediments, organisms (F283) Zn, Cu, Fe, Mn: FAA, soils, extraction, effect of pH (F284) many elements: ETA, fly ash, STPF (F285);ETA, sediments, bomb digestion, STPF (F286); ETA, snow and air (F287) (f) Food Stuffs

As: ETA, food, comparison of sample preparation (F288, F289); ETA, animal feed, sample preparation (F290) Cu: ETA, milk powder, probe atomization (F291);ETA, infant formula, Triton X-100 (F292) Cr: ETA, wines, Ni also determined (F293);FAA, food, extraction (F294);food, water samples, extraction, wet digestion (F295) Li: FAA, wine, direct nebulization of wine (F296) Si: ETA, fats and oils, air ashing step (F297) Pb, Cd: ETA, in milk (F298) Cd, Pb, Cu: FAA, NaCl, table salt, extraction (F299) Cu, Sn, Zn, Cr, Ni, Pb: ETA and FAA, infant formula (F300) Mn, Fe, Cu, Zn: foods, sample digestion by aqua regea and H20, (F301) Pd, Cd, Mn: ETA, honey, extraction (F302) many elements: in sugar using various techniques (F303) (g) Geological Samples Au: ETA, ores, extraction and precipitation, Fe interference

ATOMIC ABSORPTION SPECTROMETRY

Table V (Continued) (F304);ETA, geochemical references, extraction (F305) Co: separation by ion exchange (F306) P: ETA, rocks, Zeeman and deuterium correction, Zn coated (F307);oils, ETA with Zr coated tube and PdClzmodifier

Mn, Al, Ca, Cr, Cu, Mg: FAA; high-purity Pt (F343) Cr, Si, W: FAAS, in Cr, Si, W containing materials (F344) many elements: FAAS, in Se (F345);ETA, in Cu, Mg(NO&, NH40H modifiers (F346)

(F308)

Pb: in gasoline (F309, F310); Mg-Al-ailicate, complexation examined (F311) Sb: ETA, geochemical samples, Pd is best modifier (F312) Sn: FAA, fusion and extraction (F313);ores, fusion (F314);ETA, geological samples, fusion, NH3 modifier (F315) W: FAA, ores, sulfate modifier (F316, F317) Au, Pd: ETA, digestion and extraction (F318) Co, Ni, Mn, Cr: ores, extraction technique (F319) Co, Ni: ETA, silicate rocks, removal of interferences by NH4F (F320)

Fe, Mn, Ni, Cu, Co: polymetallic seabed nodules (F321) Na, K: FAA, rocks, separation by ion exchange (F322) Pt, Pd: ETA, removal of Pt, Pd from geological - - samules with ion exchange (F323) Rb, Se, Ba: ETA, FAA, barsito, dissolution in HF, H2S04(F324) V,. Ni.. Fe.. Cu. Pb: in Detroleum ETA, furnace conditions examined (k325) rare earths: ETA, HC104,HF digestions (F326) many elements: ETA, FAE, rocks (F327);ETA, extraction matrix modification (F328);Zeeman and deuterium correction compared (F329) (h) Metallurgical Samples Ag: ETA, ZnSe and CuSe (F330)

Bi: ETA, in Ni alloys (F331) Ca: ETA, in steel, GFAES also used (F332) Cr: ETA in steel using both pyrocoated and Ta coated tubes (F333);FAA in steel (F334) Se: FAA in steel (F335) Si: FAA, in A1 (F336) Y: FAA, in Zr (F337) Nb, Ta: FAA, in niobate-tantalate samples (F338) Ag, Au, Bi, Cu, Pd: in Pb alloys (F339) As, Sb, Bi: in Cu (F340) Au, Ag: in Cu and species (F341) Ba, Ca, Fe, K, Ma, Na: FAA, in Nb, bomb digestion (F342)

amined as a faster and safer form of wet ashing (F140-Fl42). 2. Indirect Analysis. The use of atomic spectroscopy to determine nonatomic species demonstrates the mental flexibility of the analytical chemist. Kovatsis (F143) reviewed the use of indirect determinations in AAS in toxicology studies. Sulfates (F144, F145), nitrates and nitrites (F146, F147), phosphorus (F148, F149), and nitrogen based drugs (F150, F151) were all determined by indirect analysis. Various other indirect analyses are shown in Table IV. 3. Atomic Absorption. Many papers reviewed for this article were difficult to categorize with respect to their umajor thrust". Although a paper may be referenced in the general AA section, it may, for example, involve special sample preparation or discuss fundamental properties. Analyses are listed by element in Table V where, at a minimum, we have tried to note the element, matrix, and any special feature noted in the article. AA methods for inorganic analyses were reviewed by White (F391). A general review of the analysis of high-purity metals for trace contaminants was conducted by Yudelevich et al. (F392). The analyses of several environmental samples were investigated for a variety of metals (F393, F394). The determination of Cd was reviewed by Stoeppler (F395) for a variety of samples and techniques. Dittrich et al. (F396) discussed multiple techniques and samples for the determination of halogens. Se was extracted and preconcentrated from hair and water by Ejaz and Qureshi (F397). Fused melts were used to dissolve samples in LiBOz and H2S04(F398, F399), in NazO (F400),and in a new device called a PLASMASOL (F401 3a. Flame Atomic Absorption. Several factors pertaining to FAAS were investigated by Ihnat (F402) including prep-

p.

(i) Plant Matter Al: ETA, pine needles and leaves, Triton X-100 modifier (F347)

Co: ETA, Zeeman correction, direct determination (F348) Cu: FAA, tea, extraction process (F349) Mo: ETA, factors influencing atomization discussed (F350); ETA, extraction process and digestion in HN03-HzS04-HC104 (F351, F352)

P: ETA, use of Pd and Ca nitrate as modifier (F353) Se: ETA, Se collected in acetic acid, Cu modifier used (F354) Pb, Cr, Cu: ETA, algae, (NH4)zHP04modifier (F355) Pt, Pd: ETA, ashed plant samples with extraction procedure (F356)

many elements: in algae, NAA also used (F357) (j) Water

Ca: use of precipitation and HPLC (F358) Cd ETA, use of Zeeman and STPF (F359) Co: ETA, microextraction (F360) Mo: ETA, direct determination (F361) P: ETA, decomposition and extraction procedures (F362) Sn: ETA, KZCrz0,and NH4H2P04as modifiers (F363) As, Se, A1 use of atomization cuvette, aerosol deposition, Smith-Hieftje correction (F364) Cd, Cu, Ni: ETA, Ni modifier, sub-ppb detection in seawater (F365)

Li, Mg, Ca, Sr, Ba: standard method for analyzing potable water (F366)

Na, K, Zn, Ni, Co, Sr, Mg: FAAS, in water, study of flame fuel (F367)

Pd, Cd: ETA, La modifier used, interferences assessed (F368) Pb, Cr: ETA, modifiers investigated (F369) many elements: in rain and snow, collecting and handling methods described (F370);FAA, computer program for statistical analysis (F371);reduction of precipitation with SnClZ(F372)

aration of standards, calibration techniques, sampling and sample decomposition, solution preparation, instrumental parameters, and measurement protocols. The influence of organic emulsions on sensitivity was studied by Vidal and dela Guardia (F403). Interference effects in FAAS are often investigated, and the past two years have been no exception. CN radicals have been shown effective in the reduction of interferences for Mo (F404,F405), Cd (F406),Cu, Zn, and Cd (F407),and Mn (F408). The determination of Pt (F409) and Pd (F410) is aided by the addition of n-butylamine when cation interferences are a problem. Motojima et al. (F411) reported a 60-fold enhancement in sensitivity of Ru by the addition of Ce(rv). Phosphate interference on Ca was reduced by adding Triton X-100 (F412)or a glass-frit nebulizer (F413). Phosphate and silicate interference was reduced in the determination of Ca, Mg, and Sr by the addition of catechol or pyrogallol (F414). A mixture of ascorbic acid and tri-K citrate reduced interferences on Sr by sulfates (F415). Several matrix modifiers have been investigated for removal of Fe and A1 interferences in Cd determinations (F416). The general interference of cations OD Co (F417),sulfides on Ca, Fe, Pb, and Zn (F418),Cu and other metals on Pt (F419),and PH3 on several elements (F420)was investigated. Fraser et al. (F421) used atom trapping to determine Cd. They found that CaClZ reduced interferences of Mg, K, and Na. Castillo et al. (F422) also used atom trapping to determine Cr in P-diketonate complexes. Several other general FAAS analyses are included in Table V. 3b. Electrothermal Atomization. Most of the publications covered in this review are on general a plications of ETA. Kumer et al. (F423) reviewed ETA-AAg for trace metals in environmental samples. Wassall (F424) reviewed the growth ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

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ATOMIC ABSORPTION SPECTROMETRY

Table VI. Direct Solids and Slurries Analysis matrix

comment

element

ref

Direct Solids aquatic plants and animals Cd, Pb, Cu biological As Pb A1 Hg Cd, Hg, Cr, TI cement Se coal, fly ash

F465 F466 F467 F468 F469 F470 F471

Cu (high purity) GaAs semiconductors liver

F472 F473 F474 F475 F476 F477 F478 F479 F480 F481 F482 F483 F484 F485 F486 F481, F488 F489 F490 F491 F492 F493 F494

liver, blood margarine Ni alloys orchard leaves papers placenta plastic polyethylene reference materials renal tissue salivary calculi seafood seawater soil, bone, ash wheat

satisfactory results obtained cupped typed furnace, Ni mode, good results use of inner minature cup technique good agreement with dissolution procedures Zeeman correction use of Zeeman correction, graphite powder eliminated interference use of Zeeman correction, Ni and graphite powder as modifier, 0.1 ppm LOD continuum source correction many Smith-Hieftje correction, As removed during ash Cr NH,H2P0, mode, deuterium correction Cd, P b distribution investigated Pb, Cd, Zn comparison of wall and platform atomization Pb results were not reliable Cd, P b rapid and inexpensive method Ni precision and accuracy evaluated Pb homogeneity investigated heavy metals Cd, Pb, Cu, Cr, Hg direct vs dissolution methods compared direct vs dissolution methods compared Cd use cup-in-tube and Zeeman correction, acceptable results Cd, Cu, Mn, Rb Pb, U, Cu, Ni use Zeeman correction homogeneity investigations Pb, Cd problems evaluated Cd, Ni used Zeeman correction, not advisable for certain types of samples Pb, Cd Zeeman correction used Cd, Pb, Zn fast and accurate results Cd, Pb, Hg Cd, Cu, Mn, Pb, Zn inner cup used to analyze organic proconcentrator use of graphite capsule for flame analysis Pb direct microsampling and FAA Ca, Cd, Zn calibrated with aqueous standards Cd Slurries

biological

Zn, Cu, Cd

stirring techniques, precision, accuracy, sensitivity evaluated ammonium phosphate modifer; excellent results, precision equal to digestion procedures ascorbic acid modifier, in particular for Ca interference Pb wall vs platform atomization Cu partial wet oxidation with H2S04;carbonaceous slurry Pb, Cd various modifiers and stabilizing agents used; Smith-Hieftje was found As more effective than deuterium correction glycerin used to stabilize suspension As, Cd, P b oxygen ashing used; comparable to wet and dry ashing techniques Cd Cd, Cu, Pb, Zn, Mn thickening agent used to give stable suspensions, oxygen ashing used carbonaceous slurry many direct atomization of resin cu FE after slurry nebulization Na, K Au, Pt, Pd, Ir, Rh preconcentration on chelating sorbent then slurry analysis of sorbents slurry suspension stable for 1 h Pb, Cd FAA and aspiration of slurry Na, K, Ca, Mg

Cd

coal coal, fly ash food hashish ion-exchange resin meat organic sorbent plant tissue powdered milk

and develo ment of graphite furnace atomic absorption and

Slavin and) Carnrick (F4.25) reviewed applications of the stabilized temperature platform furnace (STPF). Multiple element analysis in atomic absorption was explored by using a simultaneous multielement atomic absorption continuous source (SIMAAC) spectrometer (F4.264429). Advances in ETA-AAS for use in geochemical labs were also discussed (F430), and a direct and near real time analysis of the atmosphere was investigated (F431). Problems in Cr determinations were examined by Yin and Bencze (F432). They recommended the use of pyrolytically coated tubes because of carbide formation in uncoated tubes. A1 determinations in blood were reviewed by Burnatowska-Hledin and Mayor (F433). A lot of interest in this area, in particular for hemodialysis patients, has been shown in the last two years (see Table V). The determination of several different metals in blood was reviewed by Subramanian (F434). An interlaboratory study showed ETA-AAS and hydride generation AAS as having relatively poor accuracy for Se in serums (F435). General interference problems in ETA-AAS have been investigated (F436-F440), and often these problems are handled through the addition of matrix modifiers. The newest and most popular matrix modifier is Pd due to its stabilizing effect on a large number of metals (F4414445). Another popular 240R

ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

F495 F496 F497 F498 F499 F500 F501 F502 F503 F504 F505 F506 F507 F508 F509

modifier for many metals is phosphoric acid and its ammonium salts (F446-F450). For P, Kunc (F451)tried La, Ca and Mg and Ta-coated tubes to increase reproducibility and sensitivity. Chloride interferences on Pb were eliminated by using an ammonium salt of EDTA (F452). Other matrix modifiers include BaF, for Mo (F453), ascorbic acid for B (F454),"OB for Mo (F455),thioureas for T1 and Mo (F456), and CaC12for removal of sulfate interferences on Mo (F457). Oxygen ashing was discussed by Mohl et al. (F458) for the determination of Pb and Cd in hard to analyze materials. 4. Direct Solids and Slurry Analysis. In the past two years a significant increase in the number of papers on solids and slurries analysis has been noticed. Two entire issues of Fresenius 2.Anal. Chem. (1985,332(7) and 1987,328(4-5)) were on direct solids and slurries analysis. Several reviews on slurries and solids have also been published (F45+F462). Carnrick et al. (F463)examined the effects of sheath gas flow rates on sensitivity for solids analysis. They also characterized 19 wavelengths of Cu, Cr, and Pb for use with Zeeman background correction in an effort to analyze materials with high levels of analyte. De Kersabiec and Benedetti (F464) examined matrix problems and tested matrix modifiers for geological analysis. Table VI gives a list of the types of solids investigated. It can be seen that the addition of graphite

ATOMIC ABSORPTION SPECTROMETRY

Table VII. Comparisons of Analytical Techniques element

methods

in brine, all acceptable, AAS greatest variances ICP, ETA-AAS Al in body fluids, ETA-AAS more sensitive A1 NAA, GFAAS in biological reference materials, NAA showed less variance As NAA, AAS results comparable As many techniques ETA-AAS with platform and Ni modifier worked best ICP, ETA-AAS Be in geological, ETA-AAS more sensitive ETA-AAS, ICP-MS Cd ETA-AAS good accuracy and precision, ICP-MS looked encouraging ETA-AAS, ICP Cd in sewage, ETA-AAS off 2.6%, ICP off 20% c1 FE vs ISE ISE unsuitable for clinical analysis of C1 co Zeeman vs deuterium correction in urine, Zeeman correction better Cr AAS, spectrophotometry absorption quicker, simpler, both comparable in precision and accuracy Cr NAA, AAS NAA results higher and slightly more variable Li ISE, AAS in serum, no comparable difference in blood, comparable results FAA, electron probe X-ray microanalysis Mg P FAA, ETA-AAS in wine, results comparable ICP, XRF, AAS Pb in soil, AAS most precise Pb ETA-AAS, ASU in blood, AA simple and fast Rb ETA-AAS, ICP in insects, ICP generally,better, at low levels ETA-AAS better sc ETA-AAS, ICP in coal, comparable results Se IDMS, ETA-AAS in blood, results comparable DCP, FAA Si in urine, results similar spectrophotometry,FAA Zn in milk, FAA convenient and rapid but less precise ICP, ETA-AAS ICP more precise and quicker Ca, Mg ICP, ETA-AAS Cd,,Mo ICP more precise, ETA-AAS more sensitive ISE. FAA Li, Na in blood, methods comparable deuterium vs Zeeman correction As, Se, Cd Zeeman better voltametry, AA Cd, Cu, Pb in seawater, voltametric for Pb Na, K, C1 FE, ion-selective electrode (HE) FE 5% low for K,others OK Sn, P, B ICP, FAA in alloys, IBP better Cu, Zn, Fe, Mn IC, FAA in wine, comparable results, IC showed no matrix effects Cu. Cd. Pb. Zn ASV. ETA-AAS in fertilizers, good correlation Cd; Cui Pb; Zn ICP,’ETA-AAS at low concentrations, ETA-AAS better Pb, Zn, Cu, Cd XRF, DCP, DPASV, AAS DCP slightly higher results, others comparable Cu, Zn, Se, Cd, Te, T1, Pb inverse voltametry, AAS in coal samples, comparable results Sb, As, Bi, Cd, Cu, Pb, Ag, Zn emission (total burn and SDectrograDhic).AAS good correlation between methods A1

spectrophotometry, AAS

comment

powder aided in several of the analyses listed in this table. Takada (F510) found that the addition of graphite powder reduced sputtering of the analyte in the determination of Cu in Al. Esser and Durnberger (F511)found that the addition of fine-grained Ni to inorganic samples eliminated Se losses with pretreatment temperatures up to 2000 “C; however, the method failed for organic matrices. Sam les were often collected on filter papers which were place! directly in the graphite furnace for analysis (F5124516). Chakrabarti et al. (3’517) collected airborne particulates on a graphite probe for direct insertion in a graphite furnace. Instrumental requirements for solid sampling were outlined by Kurfurst (F518). Several innovative furnace designs helped deal with some of the problems of doing a solids analysis. When solids are placed into a graphite furnace, problems due to blocking of the optical path may arise. Schmidt and Falk (F519) designed a “ring chamber tube” which allowed for the addition of large sample masses without blocking the optical path. Brown et al. (F520) and Scholl (F521) also described special tube configurations for solid Sam ling. Another problem which may be associated with soli& analysis is the slow rate of vaporization as the analyte diffuses out of the solid. Rettberg and Holcombe (F522) were able to collect slowly released analyte on an actively cooled “plug” which then allowed for the rapid release of the collected analyte. Solid sampling was also examined for routine use in industrial applications (F523, F524). Other novel methods for solids analysis include the sputtering of solids with high velocity gases (F525,F526) and laser desorption (F527) with atomic fluorescence and the use of GFAES (F528). Another means of solids analysis involves the use of slurries, which are finely ground samples which can be suspended

ref G7

G8 G9 G10

G11 G12

G13 G14 G15 G16 G17 G18 G19 G20,G21 G22

G23 G24 G25 G26 G27 G28 G29 G30 G31 G32 G33 G34 G35 G36

G37 G38 G39 G40 G41 G42

homogeneously in a solution. This approach offers the advantage of easy sample introduction and dilution. Jackson has coauthored several papers involving fundamental mechanisms of the release of Cd and P b from soils and soil-like slurries in a graphite furnace (F5294533). Again, the general improvement with the addition of carbon powder was investigated for slurries (F534). Mohamen et al. (F535) studied the effect of particle size in slurry methods for flame AA. Additional studies on slurries have been included in Table VI. 5. Isotopic Analysis. Several papers dealing with isotopic determination have appeared in the literature. Paithvatne and Lovett (F536) evaluated a computer-modeling system for FAA of lithium isotope abundances. Kushita (F537)measured Li ratios using “ultimate absorbance ratios”. This was found to be a better technique than the conventional absorbanceratio method. Deninger et al. (F538) were able to determine stable P b isoto es by using optical Zeeman scanning AAS. The amount of FgFein soils was determined by using a Ge-Li detector and AA (F539). 6 . Applications of Atomic Fluorescence, Flame Emission, and Graphite Furnace Emission. Laser-enhanced ionization (LEI) was used by Chaplygin et al. (F540) to determine alkali metals. Laser-induced or laser excited atomic fluorescence (LEAFS) has been used for several applications. LEAFS was used to determine Co in high-purity quartz glass (F541) and high-purity Sn (F542) using ETA. Monir (F543) used several techniques, including LEAFS, to analyze geological samples. LEAFS with an ICP for atomization was used by Huang et al. (F544)in determining precious metals. Tremblay et al. (F545) studied laser-excited ionic fluorescence of rare earths in an ICP. ICP-AF was used to ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988

241 R

ATOMIC ABSORPTION SPECTROMETRY

analyze waters (F546),nuclear fuels (F547),and geological samples (F548). Rigin (3'549) used GC-ICP-AF to determine heavy metals in a variety of materials. The effect of organic additives for the detection of refractory elements in ICP-AF was investigated by Greenfield and Thomsen (F550). Nakahara et al. (F551) used nondispersive AF to determine Pb with hydride generation into an Ar-H2 flame. Ekanem et al. (F552) reported improvements in the determination of Cd in blood by using flame AF, through better control of the EDL source temperature. Several papers reported determinations using FE and graphite furnace atomic emission (GFAE). Rojas (F553) studied various instrumental and chemical conditions affectin RU emission in an air-acetylene flame. Castillo et a]. (175547 used methyl borate generation and FE to determine B in water from a nuclear reactor. A solvent extraction scheme was described by Mohite et al. (F555)for the determination of Ca in picrate by FE. Alkali and alkaline-earth elements were determined in rock salt and brine by Matusiewicz (F556)using FE in a N20/C2H2flame. The same paper also looked at enhancement of effects of inorganic salts on the emission intensities. Aoshima et al. (F557)investigated Li uptake by oocytes using FE. Baxter et al. (F558)used GFAE to determine Cr in urine by using a probe. Using high heating rates (2500 K s-l) and high final temperatures (3290 K), GFAE was used to simultaneously determine 10 elements in blood (F559,F560). These studies reported detection limits of 1-80 ppb.

G. TECHNIQUE COMPARISONS Numerous papers have appeared over the last two years dealing specifically with comparisons of one technique to another. This differs from previous years when fewer comparisons were cited. Several techniques were compared by Corbin et al. (GI)for the analysis of Si and Al in zeolites. AA, NAA, and proton elastic scattering were found most reliable, whereas XRF, wet chemical, and ICP showed poor precision. Gouveia et al. (G2) compared NAA, AA, and other conventional methods for the analysis of eight different silicate rocks and two minerals. They found the techniques to be complementary. In the analysis of in-house control reference samples, Hall and Vaive (G3)determined which elements were best done by ICP, AA, and FE. Routh (G4) evaluated AA, ICP, and DCP and found each performed with acceptable accuracy and precision. The three methods were weighed against 12 general standards ranging from sample throughput to operating costs. In a review on multielement AA, Ottaway et al. (G5) suggested that the combination of a flame and isothermal ETA-AAS with a continuum-source AAS could provide a technique of comparable or better performance than ICP. In another review on ETA introduction into an ICP, Matusiewicz (G6) reported sensitivities superior to nebulization-based systems for ICP and comparable detection limits to ETA-AAS. Additional comparisons of a variety of techniques have been compiled in Table VII.

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

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C. FUNDAMENTAL STUDIES Procedural and Callbratlon Studler

(Cl) Rutledge. M. J.; Smith, B. W.; Wlnefordner, J. D. Anal. Chem. 1887, 5 9 , 1794-1797. (C2) Smlh. B. W.: Rutledae. M. J.; Wlnefordner. J. D. ADD/. Soectrosc. 1887, 41, 613-620. (C3) Hlrabayashl, A.; Okuda, S.; Nambu, Y.; Fujlmoto. T. ADD/. . . SDectrosc. . 1888. 40-, 841-847. (C4) Atnashev, Y. B.; Korepanov, V. E.; Muzgln, V. N. Zh. Prlkl. Spektrosk. 1886, 45, 737-744 (CA 706: 43003~). (C5) Grlnshteln. I.L.; Katskov, D. A.; Khodorkovskll, M. A. Zh. Prlkl. Spektrosk. 1888, 44, 728-733 (CA 105: 51267m). (C6) Gang, L. Fenxl Huaxue 1887, 15, 509-514 (CA 106: 189677t). (C7) Lovett, R. J. Appl. Spectrosc. 1885, 3 9 , 778-786. (C8) Manning, D. C.; Slavln, W. At. Spectrosc. 1888, 7 , 123-126. (C9) Manning, D. C.; Slavln, W. At. Spectrosc. 1886, 7 , 179-180. (C10) Welz, B. Fresenlus' 2 . Anal. Chem. 1888, 325, 95-101. (C11) Montlel, A.; W e b , B. Analusis 1888, 14, 167-172 (CA 105: 53561b). (C12) Andersen, J. R. Spectrochlm. Acta, 6 1887, 426. 929-932. (C13) Slavin, W. Spectrochlm. Acta. 6 1887 426, 933-935. (C14) Bysouth, S. R.; Tyson, J. F. J. Anal. At. Spectrom. 1888, 7 . 85-87. (C15) Baker, L. M. Proc.-AWWA Water Qual. Technol. Conf. 1886, 13, 547-570. (Cl6) Bayunov, P. A.; L'vov, B. V. Zh. Anal. Khlm. 1887, 42, 626-631 (CA 107: 126039r). (C17) Opfermann: J.; Martin, M. L.; Heynlsch, M.; Danzer, K. Fresenlus' 2 . Anal. Chem. 1887, 328. 13-17 (CA 107: 189876g). (C18) Klndevater, J.; O'Haver, T. C. J. Anal. At. Spectrom. 1986, 1 , 89-91. (Cl9) Kosclelnlak, P.; Parczewskl, A. Anal. Chlm. Acta 1885, 177, 197-202. (C20) Kaballa, P.; Lwln, T.; Strode, P. Analyst 1887, 112, 1541-1547. ('221) Kosclelnlak. P.; Parczewski, A. Chem. Anal. (Warsaw) 1885, 3 0 , 49-54 (CA 104: 141135a). (C22) Berndt, H.; Sopczak, D. Fresenlus' Z . Anal. Chem. 1887, 329, 18-27. (C23) Smith, S. B.; Sainz. M. A.; Schlelcher. R. 0. Spectrochlm. Acta, 6 1887, 426, 323-332. (C24) Barnett. W. B.; Bohler. W.; Carnrick, 0. R.; Slavin, W. Spectrochlm. Acta, 6 1885, 406, 1689-1703. (C25) Grobenskl. 2.; Lehmann, R.; Radziuk, B.; VoellkoDf. U. At. SDectrosc. 1886, 7 . 61-63. (C26) Bekyarov, 0.; Futekov, L.; Andreev. G. Fresenlus' Z . Anal. Chem. 1885, 322, 563-566. I

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(C54)

Falk, H.; Gllsmann, A. Fresenius' 2 . Anal. Chem. 1988, 323,

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A7R-75A .. . ..

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(01) Corbin, D. R.; Burgess, B. F., Jr.; Vega, A. J.; Farlee, R. D. Anal. Chem. 1987, 59, 2722-2728.

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Chemometrics Steven D. Brown,* Todd Q. Barker, Robert J. Larivee, Stephen L. Monfre, and Harlan R. Wilk Department of Chemistry and Biochemistry, Brown Laboratory, University of Delaware, Newark, Delaware 19716

INTRODUCTION Chemometrics, the application of statistical and mathematical methods to chemistry, continues to grow and mature as a field. This review covers some of the more significant developments in the field of chemometrics during the period from December 1985 to December 1987. This review is the seventh of the series, and the fifth with the title “Chemometrics” (Al). It is the custom for reviewers of chemometrics to rely heavily on the computer in obtaining suitable references, in preparing the manuscript, and occasionally in providing text. This review was prepared following that tradition. First, reference lists were generated from a computer search of the CAS Online databsae. To ensure obtaining as complete a list of references as possible, we selected keywords from lists given in earlier reviews (A2) and from words and phrases used in various summaries of the field. Approximately 10 OOO references were detected in all. Communications programs available with the Unix operating system were used to capture references in real time and then to store them on a superminicomputer. Then, various Unix utility programs were used to manipulate this reference set for printing and subsequent evaluation. Concurrently, a hand search was carried out on the major journals publishing papers in chemometrics. This second search proved worthwhile, as another set of references comprising about 20% of the final list were discovered. Some of the references obtained by the hand search were recently published, and not yet abstracted, while others were missed by the CAS search, apparently because of the author’s choice of keywords. The lesson here is that keywords are an important part of a paper, especially now that a hand search of all available literature on a subject is becoming infeasible. Many papers occurred on both the list from the hand search and the list from the CAS search, of course. Finally, other Unix programs were used to word-process the review and to generate a machine-readable disk for submission to the editorial office. During the period covered by this review, there has been substantial growth in the literature of chemometrics. Undoubtedly, much of this growth can be attributed to the ease 252 R

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with which analytical chemists can now gain access to low-cost, high-performance computers, as well as the growing familiarity of chemists with those “unit operations” needed to use computers effectively. Another contributingfador is the likelihood that the chemical data available to the chemist are already in digital form. With these favorable conditions, it is natural that the analysis of the data by statistical and mathematical methods would be investigated. Given the ready availability of many popular mathematical packages, especially “mainframelike”statistical packages on microcomputers,one might expect both the number and the diversity of applications of multivariate methods to mushroom. And mushroom it has. If one compares the results of a computer search of Chemical Abstracts for several subjects pertinent to chemometrics over the three-and-one-half-year period searched for the 1980 fundamental review (A2) with those same keyword subjects searched over the two-year period used for this review, the change is apparent. For example, the keyword “Fourier transform” produced 34 references from the period from January 1976 to October 1979, but from the period from December 1985 to December 1987, it produced 1315 references. The keyword “digital filtering” produced 4 references during the earlier period, but 505 references during this review period. These substantial increases clearly reflect the increased ease with which these computer-dependent methods can be accomplished, since the less-computer-oriented techniques do not show a similar trend: the keyword ”information theory” produced 227 references over the earlier period, but only 52 during this period. Chemometrics as a field extends well beyond the boundaries of analytical chemistry, as many of the previous authors of this review have noted. Consequently, many chemometric methods have seen use in a wide range of chemical applications, some of which are not in traditional analytical chemistry. Along with this widening base of application of chemometric methods, several new chemometric methods have appeared. While much of this work is in “mainstream”analytical journals (such as Analytical Chemistry, Analytica Chimica Acta, Applied Spectroscopy,and others), a suable fraction has been published in journals more toward the periphery of analytical 0 1988 American Chemical Society