SOLIDS ANALYSIS BY GFAAS Nancy J. Miller-Ihli U.S. Department of Agriculture Nutrient Composition Laboratory Beltsville, MD 20705
Graphite furnace atomic absorption spectrometry (GFAAS) is one of the most cost-effective, sensitive techniques available. GFAAS provides sub-part-per-billion detection capability using microliter-sized samples and has been used extensively for metal determinations in a wide variety of sample matrices. Most analysts dissolve the sample so that it is in a liquid form for injection and analysis, but there are benefits to direct solids analysis by GFAAS. Advantages of solid sampling over conventional sample p r e p a r a t i o n procedures, such as acid digestion or fusion, include reduced sample preparation time, decreased analyte loss through volatilization before analysis, reduced analyte loss related to retention by an insoluble residue, reduced sample c o n t a m i n a t i o n , increased sensitivity (because samples are not diluted), elimination of hazards associated with the use of acids, and easier selective analysis of microamounts of sample. Unlike nebulization techniques, GFAAS does not suffer significantly from particle size effects because it offers longer residence times. As a result, samples may be placed directly in the graphite furnace atomizer and, in most instances, are completely atomized. Direct analysis of solids can provide analysts with special information that is not obtainable by conventional techniques requiring sample
dissolution. In addition, direct solids a n a l y s i s is i m p o r t a n t when only small amounts of sample are available or when there is interest in the distribution of analyte. Forensic applications, including analyses of gunshot residues, are well suited for solid sampling. Environmental monitoring often can be accomplished with direct solid sampling. The two principal methods for analyzing solids by GFAAS are to introduce the solid material directly into the furnace and to prepare a slurry or a suspension, then inject the sam-
REPORT pie by using conventional liquid sample delivery techniques. The merits of each technique will be compared. Because both methods provide for the analysis of solids, the following definitions apply: Direct solids analysis means that the solid is placed directly in the furnace; slurry analysis means that the sample is prepared as a slurry or a suspension.
Historical perspective Several reviews have focused on solid sampling using GFAAS (1-5). The most comprehensive of these is a recent paper by Bendicho and de LoosVollebregt (5), which includes more than 250 references. The idea of doing direct solids analysis was first reported by L'vov, who introduced the concept of graphite furnace determinations in the 1950s (6). Early pioneers included Kerber (7), who in 1971 reported the use of the first
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commercial tube-type furnace to determine gold in polyester fibers. Much of the early work was done with specially built atomizers or modified commercial graphite furnaces. Both resistively and inductively heated furnaces were used. A variety of commercial and noncommercial atomizers, some of which were specially designed for solid sampling, have been reported in the l i t e r a t u r e . Solid sample analyses have been performed with the following types of atomizers: Massmantype graphite tube, Hitachi's cup cuvette, platform, Grun-Optiks' boat system, Thermo Jarrell Ash's microboat, Perkin Elmer's c u p - i n - t u b e , Varian's transversely heated carbon rod, graphite probes, Schmidt and Falk's ring chamber, and Rettberg and Holcombe's second surface atomizer. The only commercial graphite furnace AA spectrometers designed specifically for direct solids analysis are the systems built in G e r m a n y by Griin-Optiks. More recently, Analyte Corporation has produced the Atomsource system, which is a commercial glow discharge AA system. A l t h o u g h e a c h of t h e a b o v e mentioned systems offers special advantages for solid sampling, a general improvement in g r a p h i t e furnace technology probably had a more significant impact on the direct analysis of solids. A noteworthy improvement during the past decade was the introduction of the stabilized-temperature platform furnace (STPF) concept by Slavin et al. (8, 9), which provided conditions leading to reproducible, high-accuracy determiThis article not subject to U.S. copyright. Published 1992 American Chemical Society.
nations that were not significantly affected by matrix effects. The STPF approach strives for more isothermal conditions for atomization using a conventional Massman-style furnace that is known to have a longitudinal temperature gradient. This concept facilitated the direct analysis of solid samples and also encouraged the use of aqueous standards for calibration. The key points of this approach include using good background correction (such as the Zeeman effect), platform atomization, and integrated absorbance measurements for quantitation. As Bendicho and de Loos-Vollebregt (5) point out, the annual number of solid-sampling applications reported has increased since 1970. The STPF concept was introduced in 1982, and in 1987 a record was set, when more than 20 papers on direct solid sampling were published. Interestingly enough, the average number of publications on direct solid sampling between 1970 and 1992 has been approximately 10 per year. Potential problems Although it seems obvious that solid sampling GFAAS is a powerful technique, it is rather limited for direct solids analysis. Many authors have reported disadvantages such as calibration, microweighing, and sample introduction difficulties; lack of solid standards; problems attributable to sample inhomogeneity; increased background (requiring good background correction); residue buildup; sample-dependent peak shapes; problems with refractory elements and with automation of sample intro-
duction; too much sensitivity; poor precision; and limited sample size. Admittedly, some of these problems justify serious consideration; successful determinations may be impeded if certain criteria are not met. However, many problems can be avoided by using s t a t e - o f - t h e - a r t technology and equipment. Calibration can be successfully performed in most instances with aqueous calibration standards, thereby avoiding the need for solid s t a n d a r d s for each sample matrix of interest. Sample inhomogeneity can be a problem if one is trying to get an accurate estimate of the mean concentration of analyte in the bulk sample; however, direct solid sampling can provide detailed information and facilitate homogeneity characterization of materials at milligram and submilligram levels. Use of Zeeman effect or S m i t h Hieftje background correction provides a d e q u a t e compensation for background in most instances. The buildup of residual material can often be avoided by using oxygen ashing during the char step in the furnace program. The fact t h a t solid sampling produces signals with sample-dependent peak shapes is typically unimportant as long as quantification is done u s i n g i n t e g r a t e d absorbance measurements. Poor precision is usually the result of inhomogeneous distribution of analyte in the solid material. Precision can be improved by reducing the particle size of the material being analyzed and by increasing the number of samples analyzed. Limited sample size can be a physical restriction, or it can be attributable to the analyti-
cal signal relative to the working r a n g e . In t h e l a t t e r case, l a r g e r amounts of material may be used when an a l t e r n a t e , less sensitive wavelength is available. The two disadvantages related to sample introduction are not easily dismissed. Analysts agree that microweighing is difficult and time consuming, and that transport of milligram quantities of material requires special care—even if the sample is weighed directly on a boat, a platform, or a cuvette that can be directly inserted into the furnace. Griin-Optiks developed a partially automated commercial solid sampler consisting of a powder sampler, a balance, and a transport system to be used with its Griin Zeeman AAS instruments (10). Powdered sample is transferred, using a piston and vibration, onto a graphite boat that is subsequently weighed and inserted inside the graphite tube. As one might expect, several factors affect the amount of material placed on the platform: particle size, moisture content, fat content, and specific gravity. Obviously, such a system works best with dried powders consisting of particles of uniform size and density. An alternative approach to the automation of direct solid sample introduction is the preparation of a slurry or a suspension that facilitates using conventional liquid sample-handling technology. This approach will be discussed later in this REPORT. Solid-sampling applications Table I lists 15 references in which direct solid sample insertion into the
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REPORT graphite furnace is used. These fairly recent references typically report data based on calibration against aqueous standards (11-25). Table I highlights the suitability of this ap proach for the successful determina tion of many elements in a wide va riety of sample matrices. Some other applications have been reported that indicate that strong matrix interfer ences have been observed, necessi tating the method of additions or cal ibration against matrix-matched reference materials. One serious problem with the use of reference materials for calibration is that analysts treat the mean refer ence value as absolute, ignoring the uncertainty. The certified concentra tion of Cd in Standard Reference Ma terial 1568 Rice Flour is 0.029 ± 0.004 μg/g (26). Thus the uncertainty is ±14% compared with the mean concentration. If calibration is per formed by using this material, then by definition, the computed analyti cal c o n c e n t r a t i o n s can h a v e no smaller uncertainty than ±14%. This level of uncertainty is not uncommon in commercial reference materials, and some uncertainties are as high as 25%. Also typically, minimum rec ommended amounts for commercial materials are on the order of 250 mg. When using these materials for di rect solids sampling, 0.5-5.0 mg is commonly used. The use of small amounts creates the possibility that the analyte will not be sufficiently homogeneously distributed to be rep resentative of the bulk material. If analysts want to use solid mate rials for calibration, special materi als must be developed that are well characterized with small elemental concentration uncertainties. In addi tion, these materials must be charac terized at milligram and submilligram levels. These concerns strongly support the benefit of calibration with aqueous standards whenever possible. Calibration using aqueous stan dards for direct solids analysis re quires absolute accuracy of the liquid sample delivery system, because the accuracy of the determination will depend on the volume delivered by the autosampler or the pipet. This method differs from digest analyses in which both samples and standards are pipetted and the absolute vol umes delivered do not affect calibra tion accuracy.
Slurry sampling As mentioned earlier, a convenient way to introduce solid material into the graphite furnace is to prepare a
Table 1. Solid-sampling applications Element
Sample
Reference
Comments STPF, cup-in-tube Cup-in-tube Milled Special platform Ringed chamber, integrated absorbance Constant - temperatu re furnace Wall, probe, platform, oxygen ashing Probe Cup-in-tube, oxygen ashing, alternate wavelength Graphite dilution, nickel powder modifier Ground, punched Filtered
Ni alloys Plastics Wheat Biological samples Biological samples
Tl, Bi, Te, Se, Pb Cd, Cu, Mn, Rb Cd Pb, Cd Cu
11 12 13 14 15
Biological samples
AI
16
Bovine liver
Pb
17
Airborne particles Coal, fly ash
Cd, Pb, Ni, Cu, Mn Cd, Ni
18 19
Inorganic materials
Se
20
Filter material Waterborne suspended matter Biological samples Fly ash, ash, filter paper Coal, fly ash
Cd Cd, Pb, Cu
21 22
As, Cd, Zn, Pb, Mn Cd, Pb, Tl, Cu, Mn, Ag As, Cd, Cr, Hg
23 24
slurry or a suspension. Conventional liquid sample-handling devices such as autosamplers and pipets may then be used to inject material into the furnace for analysis. Because of the ease of sample in troduction, Stephen and co-workers (27) concluded that slurry sampling combines the benefits of both solid and liquid sampling. To ensure that a representative aliquot is injected into the furnace, the slurry must be either stabilized or homogenized. Stabilization h a s been a t t e m p t e d with the use of thixotropic agents such as viscalex (27) and glycerol (28). In each instance, the authors reported difficulty with reproducible a u t o s a m p l e r pipetting because of material adherence to the exterior of the autosampler capillary. Problems with incomplete delivery of the sam ple with more viscous solutions were also reported. Such results suggest t h a t sample homogenization using agitation, such as a magnetic stir bar (29), vortex mixing (30), gas bubbling (31), or ultrasonic agitation, is pref erable. Reports of magnetic s t i r r i n g of slurries followed by manual pipetting are frequently found in the litera t u r e . Many r e s e a r c h e r s h a v e r e ported good r e s u l t s , but this a p proach is limited because manual pipetting provides poorer precision than that which can be achieved with
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Minicup Second surface atomizer Graphite dilution
an autosampler, and most modern laboratories require automated GFAAS analyses. Lynch and LittleJohn (32) developed a miniature stir bar mixing method t h a t was used with an autosampler to facilitate au tomatic mixing of food slurries in the a u t o s a m p l e r cup before a n a l y s i s . Hinds and Jackson (33) found mag netic stir bar mixing inadequate for some applications, because soil sam ples often contain particles that ad here to the stir bar due to their mag netic properties. Vortex mixing can provide ade quate agitation of slurries before analysis, but sample aliquots r e moved for injection must be handpipetted directly into the furnace to avoid rapid sedimentation of larger, dense particles that may occur when vortexed samples are subsequently placed in an autosampler cup (28). Because automated sample introduc tion using an autosampler is virtu ally impossible when using vortex mixing, this mixing technique has l i m i t e d a p p l i c a b i l i t y for modern slurry analyses. Workers in our lab tried gas bub bling for slurry mixing and found that this method typically provided low analyte concentrations when the analyte of interest was associated with larger, dense particles. Bendicho and de Loos-Vollebregt (31) also reported poor homogenization when
using argon mixing for glass slurries c o n t a i n i n g p a r t i c l e s in t h e 8 2 341 -μπι range, but they found that the argon bubbling method could be used to extract analyte from glass slurries into a 3% HF solution (34). Mixing with the use of a compact portable ultrasonic processor pro vides superior agitation of slurry preparations (28). High-power ultra sound depends on cavitation and its secondary effects. Ultrasound can be effective in dislodging mechanically interlocked particles, dispersing sol ids, and wetting particles; and the ef fectiveness often increases with de c r e a s i n g p a r t i c l e s i z e (35). I n addition, ultrasonic agitation can in crease the extraction of analyte into the liquid phase of the slurry. Com mercial 40-50-W units designed to process small samples (100 μ Ε - 2 5 mL) effectively disaggregate and dis perse particles. The titanium ultra sonic probe may be mounted on the autosampler tray and its operation synchronized with t h a t of the au tosampler, providing fully automated slurry analyses (36, 37). P e r k i n Elmer has commercialized this con cept and sells an ultrasonic mixing device (USS-100) (38) as an a u tosampler accessory to facilitate au tomated slurry analyses. Slurry-sampling applications Table II lists 13 references in which slurry graphite furnace determina tions were made. Like the applica tions cited in Table I, these references contain data based on calibration
against aqueous standards (28, 30, 32, 33, 39-47). Table II illustrates the utility of this approach and suggests that a wide range of samples can be accurately analyzed by using aqueous standards and modern furnace tech nology. All applications in Table II use one of the mixing methods discussed previously. The only mixing methods t h a t have been automated are the minibar (32) and ultrasonic agitation (28, 30, 44-46). In some instances, matrix modifi ers have been used. Oxygen ashing to avoid buildup of carbonaceous mate rial has also been used successfully in some s t u d i e s . B r a d s h a w a n d Slavin (45) reported a method that omitted both the pyrolysis step and use of a matrix modifier, providing r a p i d f u r n a c e a n a l y s e s . Such a method requires the use of an effi cient background correction system such as the Zeeman effect. Comparison of techniques Direct solids sampling and slurry sampling each have advantages and d i s a d v a n t a g e s . Both may require some sample pretreatment, such as grinding, if the sample is not in the proper form or if the analyte is not homogeneously distributed. Direct solids sampling is desirable when only a small amount of sample is available or when there is interest in the distribution of analyte in the solid. Most solid-sampling insertion devices are manual and require skill to obtain reasonable precision. Direct solids sampling may require the use
Table II. Slurry-sampling applications Sample
Element
Reference
Soil Airborne particulates Biological RMs
Pb V, Cd, Ni, Cu, Pb, Fe, Mn Cr, Co, Pb, Mn
Iron oxide
As, Pb
42
Glass
43
Spinach RM
Cu, Co, Cr, Mn, Fe, Ni Mn, Fe, Cu, Cr, AI
Coal, fly ash Sediment
As, Tl, Pb As, Fe, Mn, Pb
45 30
Biological samples Milk powder Food
Mn Zn, Cu, Mo, Pb Pb
46 47 32
Biological samples
Mn, Zn, Fe, Cu, Pb, Cr, Ni, Mo, AI, Ca
28
33 39 40,41
44
Comments Vortex mixing Ultrasonic mixing Manual agitation, oxygen ashing, zirconium beads Magnetic stirring, manual pipetting, nickel modifier 3% HF, argon bubbling Ultrasonic mixing, homogeneity study Ultrasonic mixing Ultrasonic and vortex mixing Ultrasonic mixing Magnetic stirring Minibar mixing, palladium modifier Ultrasonic mixing
of matrix-matched standards or the method of standard additions for cal ibration. Direct solids sampling can easily handle alloys, pure metals, and plastics. Slurry sampling is best suited for powdered samples such as sediments or soils. S l u r r y s a m p l e i n t r o d u c t i o n is straightforward when using conven tional liquid autosamplers and al lows matrix modifiers to be used eas ily because they are miscible with the liquid phase. Slurry mixing can be automated by using the mini-stir bar method or ultrasonic agitation. Calibration using aqueous standards may be applicable for a wide range of samples for both solid and slurry sampling. Slurry samples may be di luted easily to facilitate analysis in the linear range; solid samples can be diluted only with graphite powder, increasing the risk of sample con tamination. Both techniques, how ever, may take advantage of other methods for reducing sensitivity, such as selecting an alternate ab sorption line. Analysts using slurry sampling must have some knowledge of the an alyte distribution in the solid and liquid phases of the slurry to derive m a x i m u m benefit from the tech nique. If a large percentage of the an alyte is extracted into the liquid phase, the precision will approach that obtainable with a conventional liquid digest. If, on the other hand, none of the analyte is extracted into the liquid phase, slurry sampling can provide an easy and reproducible method for the introduction of micro g r a m a m o u n t s of solid into t h e graphite furnace. Factors of interest in optimizing either direct solids analysis or slurry sampling include homogeneity of the solid material, distribution of analyte in the solid, density, and particle size. Density and particle size should be used to calculate the number of particles in a mass. A large number of particles will reduce sampling er rors (48). Both techniques offer the unique opportunity for homogeneity characterizations of materials at mil l i g r a m a n d s u b m i l l i g r a m levels. When working with inhomogeneous samples, both direct solids analysis and s l u r r y sampling will benefit from smaller particle sizes. Several authors have reported a preference for either direct solids analysis or slurry analysis. In my opinion, slurry sample introduction should be used whenever possible be cause it is easy, it uses liquid sam ple-handling technology, and it pro vides both a c c u r a t e a n d precise
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REPORT results with calibration using aque ous s t a n d a r d s . Bendicho a n d de Loos-Vollebregt (5) state t h a t the slurry technique gives better analyt ical performance than direct solid sampling. Brady and co-workers (49) also endorse the use of slurries be cause of the ease of sample introduc tion. Schmiedel and co-workers (50) support slurry GFAAS and state that ultrasonic mixing provides optimum homogenization of the slurry before analysis. A generalized approach Compared with other techniques for direct solids analysis, GFAAS clearly offers the analyst a unique combina tion of excellent sensitivity and sim plicity at moderate cost. Modern methods must address the need for increased sample throughput, neces s i t a t i n g faster analyses. This r e quires minimizing chemical sample pretreatment, including decomposi tion procedures. Analysis of solids by either direct analysis or slurry sam ple introduction facilitates rapid analyses. Regardless of whether the analyst chooses direct solids or slurry analy sis, a systematic approach can be identified. Calibration with aqueous standards represents the most sensi ble approach for identifying general ized procedures for solids and slurry sampling. Calibration with aqueous s t a n d a r d s does, however, require that the absorption signal be inde pendent of the bulk matrix and be dependent only on the concentration of the analyte (5). Using peak area m e a s u r e m e n t s for c a l i b r a t i o n is therefore imperative, because peak area measurements minimize the ef fect of varying vaporization rates (at tributable to matrix differences). The a n a l y s t m u s t also be con cerned with sample homogeneity, be cause results will be affected by the distribution of analyte in the solid. If increased homogeneity is desirable, samples may be ground to a small (< 10 μιτι) particle size before analy sis (48). All s a m p l e s should be weighed on an electronic microbal ance, and care m u s t be t a k e n to avoid sample contamination. Analyt ical conditions must be optimized for each sample type. Wavelength selec tion will depend on analyte concen tration, and less sensitive nonresonance wavelengths may prove useful. GFAAS conditions must be sys tematically optimized, because ana lyte volatilities may be affected by the sample matrix. Charring and atomization optimization studies must be performed to identify optimum
(29) Karwowska, R.; Jackson, K. Vf. J. Anal. At. Spectrom. 1987, 2, 125. (30) Epstein, M. S.; Carnrick, G. R.; Slavin, W.; Miller-Ihli, N. J. Anal. Chem. 1989, 61, 1414. (31) Bendicho, C ; de Loos-Vollebregt, M.T.C. Spectrochim. Acta 1990, 45B, 679. (32) Lynch, S.; Littlejohn, D. / Anal. At. Spectrom. 1990, 4, 157. (33) Hinds, M. W.; Jackson, K. W. At. Spectrosc. 1991, 12, 109. (34) Bendicho, C ; de Loos-Vollebregt, M.T.C. Spectrochim. Acta 1990, 45B, 695. (35) Ultrasonics, Encyclopedia of Chemical Technology; 3rd éd.; John Wiley and Sons: New York, 1983; Vol. 23. (36) Miller-Ihli, N. J.J. Anal. At. Spectrom. 1989 4 295 References (37) Miller-Ihli, N. J. U.S. Patent 4,930,898, 1990. (1) Headridge, J. B. Spectrochim. Acta (38) Carnrick, G. R.; Daley, G.; Fotinopou1980, 35B, 785. los, A. At. Spectrosc. 1989, 10, 170. (2) Langmyhr, F. J.; Wibetoe, G. Prog. (39) Fernandez, Α.; Fernandez, R.; Car Anal. At. Spectrosc. 1985, 8, 193. rion, N.; Loreto, D.; Benzo, Z.; Fraile, R. (3) Van Loon, J. C. Anal. Chem. 1980, 52, At. Spectrosc. 1991, 12, 111. 955 A (40) Ebdon, L.; Fisher, A. S.; Parry, (4) Langmyhr, F. J. Analyst 1979, 104, H.G.M.; Brown, A. A. / Anal. At. Spec 993. trom. 1990, 5, 321. (5) Bendicho, C; de Loos-Vollebregt, M.T.C./ Anal. At. Spectrom. 1991, 6, 353. (41) Ebdon, L.; Fisher, A. S.; Parry, H.G.M.; Brown, A. A. / Anal. At. Spec (6) L'vov, B. V. Spectrochim Acta (Engl. trom. 1988, 3, 131. Transi.), 1984, 39B, 159; lnzh. Fiz. Zh. (42) Garcia, I. L.; Cordoba, M. H. / Anal. 1959, 2(2), 44. At. Spectrom. 1990, 5, 647. (7) Kerber, J. D. At. Absorpt. Newsl. 1971, (43) Bendicho, C ; de Loos-Vollebregt, 10, 104. M.T.C. Spectrochim. Acta 1990, 45B, 695. (8) Slavin, W.; Manning, D. C. Spectro (44) Miller-Ihli, N. J. Fresenius J. Anal. chim. Acta 1982, 37B, 955. Chem. 1990, 337, 271. (9) Slavin, W.; Carnrick, G. R.; Manning, (45) Bradshaw, D.; Slavin, W. Spectro D. C ; Pruszkowska, E. At. Spectrosc. chim. Acta 1989, 44B, 1245. 1983, 4, 69. (46) Jordan, P.; Ives, J. M.; Carnrick, (10) Kurfurst, U.; Kempeneer, M.; StoepG. R.; Slavin, W. At. Spectrosc. 1989, 10, pler, M.; Schuierer, O. Fresenius J. Anal. 165. Chem. 1990, 337, 248. (47) Wagley, D.; Schmiedel, G.; Mainka, (11) Irwin, R.; Mikkelsen, Α.; Michel, E.; Ache, H.J. At. Spectrosc. 1989, 10, R. G.; Dougherty, J. P.; Preli, F. R. Spec 106. trochim. Acta 1990, 45B, 903. (48) Miller-Ihli, N. J. At. Spectrosc. 1992, (12) Vôllkopf, U.; Lehmann, R.; Weber, 1, 1. D. / Anal. At. Spectrom. 1987, 2, 455. (13) Horner, E.; Kurfurst, U. Fresenius Z. (49) Brady, D. V.; Montalvo, J. G.; Glowacki, G.; Pisciotta, A. Anal. Chim. Anal. Chem. 1987, 328, 386. Acta 1974, 70, 448. (14) Brown, Α. Α.; Lee, M.; Kullemer, G.; (50) Schmiedel, G.; Mainka, E.; Ache, Rosopulo, A. Fresenius Z. Anal. Chem. H. J. Fresenius Z. Anal. Chem. 1989, 335, 1987 328 354 195. (15) Schmidt, K. P.; Falk, H. Spectrochim. Acta 1987, 42B, 431. (16) Freeh, W.; Baxter, D. C. Fresenius Z. Anal. Chem. 1987, 328, 400. (17) Chakrabarti, C. L.; Karwowska, R.; Hollebone, B. R.; Johnson, P. M. Spectro chim. Acta 1987, 42B, 1217. (18) Chakrabarti, C. L.; Xiuren, H.; Shaole, W.; Schroeder, W. H. Spectrochim. Acta 1987, 42B, 1227. (19) Schlemmer, G.; Welz, B. Fresenius Z. Anal. Chem. 1987, 328, 405. (20) Diirnberger, R.; Esser, P.; JanBen, A. Fresenius Z. Anal. Chem. 1987, 327, 343. (21) Schothorst, R. C ; Géron, H.M.A.; Spitsbergen, D.; Herber, R.F.M. Frese- Nancy J. Miller-Mi is a research chemist nius Z. Anal. Chem. 1987, 328, 393. (22) van Son, M.; Muntau, H. Fresenius Z. in the Nutrient Composition Laboratory at the USDA. She received her B. A. degree Anal. Chem. 1987, 328, 390. (23) Atsuya, I.; Itoh, K.; Akatsuka, K. in chemistry in 1977 from Shippensburg Fresenius Z. Anal. Chem. 1987, 328, 338. University (PA) and her Ph.D. in analyti (24) Rettberg, T. M.; Holcombe, J. A. cal chemistry in 1982 from the University Anal. Chem. 1986, 58, 1462. of Maryland under the direction of T. C. (25) Esser, P. Fresenius Z. Anal. Chem. 1985, 322, 677. O'Haver. Her research interests include (26) Standard Reference Material 1568 GFAAS, multielement AAS, slurry sam Rice Flour, National Bureau of Stanpling, sample preparation, plasma emis dards Certificate of Analysis; National sion spectrometry, and the development of Institute of Standards and Technology; Gaithersburg, MD, 1978. quality control materials. She has pub (27) Stephen, S. C; Littlejohn, D.; Ottlished approximately 35 papers and is the away, J. M. Analyst 1985, 110, 573. (28) Miller-Ihli, N. J.J. Anal. At. Spectrom. author of a patent describing the use of ul trasonic agitation for slurry mixing. 1988, 3, 73.
conditions. The use of matrix modifi ers or oxygen ashing may prove use ful to avoid carbonaceous buildup in the furnace. Modern furnace technol ogy should be used, including good background correction, platform atomization, and calibration with aqueous standards using peak area m e a s u r e m e n t s . Optimized furnace conditions make it possible to obtain accurate and precise analytical de terminations by using either direct solids analysis or slurry analysis.
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