Elemental Analysis of Environmental Samdes -
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Using Chromatography Coupled with Plasma Mass Spectrometry
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oxic trace elements may be found in the environment and can pose serious health risks to many organisms. Because the toxicity of an element depends on its chemical form and oxidation state, speciation (the determination and quantitation of the different chemical forms of an element of interest) has become an important area of research in the field of elemental analysis. Analytical atomic spectrometric techniques may yield detection of a variety of metallic and nonmetallic species, but they provide only total elemental concentration information. In combination with chromatographic techniques such as high-performance liquid chromatography (HPLC), gas chromatography (GC), and supercritical fluid chromatography (SFC), however, speciation information is provided. When plasma MS (mass spectrometry) is used as the element-selective detector, excellent detection limits are obtained ( I ) . Absolute detection limits for HPLC-inductively coupled plasma mass spectrometry (ICPMS) are in the subnanogram-to-picogram range, GC-plasma MS offers picogram-tosubpicogram limits, and SFC-plasma MS typically provides detection limits ranging from low-to-subpicogram levels. Compared with atomic emission spectroscopy, plasma MS detection limits are generally 2 to 3 orders of magnitude lower. Thus, plasma source MS is often considered the method of choice for the analysis of trace elements. As a chromatographic detector, the improved sensitivity of plasma MS often eliminates the need for postcolumn derivatization and other additional techniques ( 1 ) . Plasma MS can be operated on-line in real time, and new software advances allow users to acquire data at more than one
FRANCINE A. B Y R D Y J O S E P H A. C A R U S O University of Cincinnati Cincinnati, OH 45221 -01 72 mass versus time in a single acquisition. This means that with each acquisition several masses are monitored simultaneously, and as time goes on additional data at each mass are acquired. Chromatography coupled with ICPMS is useful for separating a variety of elements at trace levels (e.g., the rare earth elements) without obtaining speciation information. It can also be used to investigate only one species in a complex sample. Despite the potential applications, there are few studies using HPLC-, GC-, or SFC-ICPMS for the speciation of trace elements in real environmental samples. Most studies involve fundamental work on methods development with the potential for analysis of samples of environmental interest. Ordinarily, a standard reference material (SRM) is used to assess the viability of the method. (A number of environmental SRMs are available from the National Institute of Standards and Technology [NIST], including inorganic metal constituents in various environmental matrices and trace elements in fossil fuels and sediments.] In this article, we will primarily discuss methods that fall into the SRM or real sample analysis categories, rather than solely investigate methods development. Table 1 lists some of the environmental samples analyzed by chromatography-plasma MS that are discussed in this paper. Toxicity and speciation Several of the most commonly studied trace elements of toxicologi-
528 A Environ. Sci. Technol.. Vol. 28. No. 12, 1994
cal interest-arsenic, tin, lead, and chromium-make their way into the environment via industrial processes. Arsenic has been used in herbicides and pesticides and is mainly transported in the environment by water. The inorganic forms of arsenic (arsenite and arsenate) are more toxic than the organic species (monomethylarsonic and dimethylarsinic acid), with arsenobetaine being relatively nontoxic (21. Speciation is necessary to obtain an adequate toxicological sample assessment when all five of these species are present. Organotin compounds are highly toxic and for many years were used as antifouling paint agents, fungicides, and wood preservatives. Toxicity is affected by the alkyl group: Ethyl, methyl, propyl, and butyl organotin compounds are more toxic than those containing phenyl or octyl groups. The most toxic organotin compounds include the tri- and tetraspecies ( 3 ) . Lead is also well known to pose health risks, especially in young children. During the past 20 years, concern about lead toxicity has shifted from industrial to environmental exposure ( 4 ) . Harmful species include inorganic lead (the form found in older house paints), di-, tri-, and tetraalkyllead, with the last three being the most hazardous (2). There is currently considerable interest in chromium compounds because of their widespread presence in the environment via industrial wastes (e.g., tannery wastes, corrosion inhibitor manufacturing, and municipal sewage sludge). The two most common forms of chromium are Cr(III), an essential trace nutrient found in many foods, and Cr(VI), which is known to cause lung cancer and produce mutations. Cr(II1)is present in most soils and is less mobile in the environment than Cr(VI1 (21.
0013-936X/94/0927-528A$04.50/0 0 1994 American Chemical Society
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The chromatogram shows the potential of m~ltielementcapabilities.The Separation was achieved by SFC-ICPMS under the following pressure program and temperature: initial pressure. 80 aim held lor 2.5 min; pressure ramp. 150 atmimin: final pressure. 400 aim heid for 5 min; oven tempwature 75 ’C. llluStrafioncourtesy of Nahora Vela and Lisa Ol~on.
Plasma MS detection A plasma is a partially ionized gas. The hottest part, the plasma core, is > 5000 K. A plasma forms when a gas, usually Ar, is “seeded” by a spark with electrons. An oscillating magnetic field is established by passing radio frequency (rf) en-
ergy through a load coil with a generator t y p i c a l l y operated- at 27.12 MHz. Once conductive, energies become sufficient to ionize gaseous atoms in the magnetic field. With further ionization resulting from collisions with gaseous atoms and ohmic heating, the plasma becomes self-sustaining as long as the
rf field is applied (5). Sample-introduction into the plasma is generally accomplished with a nebulizer that creates an aerosol from solution. The sample next travels into the spray chamber where particles are separated and the large droplets removed. The fine sample droplets go to the plasma to
Environ. Sci. Technoi.. Voi. 28, No. 12, 1994 529A
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ome environmental samples analyzed using chromatography oupled with plasma-MS detection -1
PLC-ICP-YS DORM-1 dogfish muscle reference material Fkh cod, dab, haddock, mackerel, plaice, whiting Arsenosugars purified from a brown alga Soil extracts from a polluted land site
PACS-1 harbor sediment reference material Natural waters Albacore tuna sample NBS RM-50 Wastewater NET-SRM 163% Coal Fly Ash NIST-SRM 2715 Lead in Reference Fuei Wastewater sludge incinerator emissions
Element Investigsted
Reference
Arsenic Arsenic
20, 21,22
Arsenic Arsenic
24 28
Tin Tin
29
22
Mercury Tellurium Rare earth elements Lead
31 34 35 38 39
Chromium
40
Lead Phosphorus and sulfur
41 42 45
Lead
51
CPlasma US
NET-SRM 1637 Lead in Reference Fuel Harbor sediment Pesticides (diazinon,malathion)
Tin
FC-Plasma YS NET-SRM 2715 Lead in Reference Fuel
be vaporized, desolvated, and ionized. From there, the desolvated gaseous ions travel to the mass spectrometer to be separated according to their mass-to-charge ratios (m/z). Plasma ion source MS offers excellent sensitivity, selectivity, and the potential for isotope determinations. However, there are a number of potential disadvantages: high instrumental costs (about $200,000 per instrument; more for very high resolution systems), expensive gases (e.g., high-purity Ar), and the possible formation of interfering polyatomic species from matrix elements and atmospheric and plasma gases. Isobaric elemental interferences in ICPMS occur when isotopes of different elements form ions with the same nominal m/z as the element of interest; isobaric molecular ion interferences result when ions consisting of more than one atom or charge form from the plasma gas or as a result of atmospheric entrainment. Several EPA methods use ICPMS detection, including 200.8 for the determination of trace elements in waters and wastes, 200.10 for the determination of trace elements in marine waters, and 6020 (in a preliminary draft stage) for the multielemental determination of analytes by ICF'MS (6-8). None of these methods, however, uses chromatographic s e p a r a t i o n ( a l t h o u g h Method 200.10 does require a chelating column to preconcentrate the analytes of interest [Cd, Co, Cu, Pb,
Ni, U, and VI by first selectively separating Group I and I1 metals and most anions from the analytes). The microwave-induced plasma (MIP) is another plasma MS ion source, but it is a less popular alternative to the ICP because of perceived concerns with its operation. The MIP ordinarily employs a He plasma gas and is generally restricted to the analysis of the halogens, sulfur, and phosphorus because He plasmas ionize these elements more efficiently than Ar plasmas (9, IO).Suyani et al. used a tantalum-tip MIP torch to inject the column effluent directly into the He plasma. This approach suggests great potential for GC-MIPMS ( 1 1 ) . However, He MIPS have a lower tolerance for wet aqueous aerosols compared to ICP discharges, and there are no commercially available MIPMS systems i n the United States. Most existing systems are laboratory built. Figure 1 provides both ICPMS and MIPMS instrumental setups with the three most common separation schemes. The chromatogram illustrates the potential of multielement detection for chromatography. In this example, phenyltin and phenyllead compounds differ only in the metal species. Not only are the different metallic compounds separated by SFC. but the chromatogram supplies information on their various isotopes as well. By exploiting isotope ratio determinations, the time required to do multiple chro-
5 3 0 A Environ. Sci. Technol., VoI. 28, No. 12, 1994
matograms for calibration graphs is saved. Moreover, isotope ratios can compensate for matrix effects (I). For chromatographic sample introduction into the ICP or MIP mass spectrometer, the single-ion monitoring mode is typically used. In an investigation of effluents containing arsenic, signal intensity at m/z 75 [the most abundant isotope of As) would be monitored versus time. As the various species of As elute from the column, peaks corresponding to each As-containing species are identified. Vandecasteele et al. described applications of an ICPMS system specifically intended for environmental analyses (121. Although these applications use chromatographic techniques only for preconcentration, some important points regarding environmental sample analysis have been made. For example, the analysis of seawater is complicated because of the high salt concentration of the matrix and the low concentration of most trace elements. As a result, it may be necessary to preconcentrate the elements of interest. In addition, heavier elements are suppressed in the sodium chloride matrix. This suppression may be corrected through the use of an internal standard close in mass to the analyte of interest (12).In fresh water, polyatomic interferences can occur from CI- in samples, but the levels are generally below or close to the detection limits of the procedure. Finally, the large linear dynamic range of ICPMS is advantageous in studying wastewater samples. In general, wide concentration ranges are found in samples obtained from surface treatment of metals and plastics (e.g., 1.8-10,700 mglL for Cr). To analyze these samples by atomic absorption spectrometry, many standards would have to be run to establish a calibration curve (121.
Chromatographic sample introduction HPLC. The most common chromatographic technique currently used w i t h ICPMS detection is HPLC, likely because of the simplicity of the interface, use of flow rates compatible with most commercial ICPMS instrumentation (0.5-2.0 mL/ min), and the abundance of HPLC separations in the literature. HPLC stationary and mobile phases can be varied to enhance a separation, and analyses may be performed in solution at ambient temperature (13).
Low-power He MIPs cannot be in- lution and detection limits ( 2 7-2 9). Sample contamination from terfaced directly to the outlet of an HPLC column because the continu- metal HPLC components is possious high liquid flow rates tend to ble, particularly in studies of trace quench the MIP ( 2 4 ) . Thus, MIPs metals. Corrosion of stainless steel components is a concern when Clhave not been extensively studied as HPLC detectors. Ways to circum- is present (e.g., in seawater). Therevent these problems have been in- fore, Teflon or glass-lined stainless steel tubing is preferable; HPLC vestigated ( 1 5 ) and show promise pumps constructed from inert matefor reversed-phase HPLC (24). Various HPLC modes have been rials are suggested for routine analyused to achieve separations with ses; and fittings made from Teflon, ICPMS detection. Although tradi- polypropylene, or Kel-F should be tional HPLC has been used predom- used ( 2 3 ) . HPLC-ICPMS applications. Alinantly for the analysis of organic compounds, reversed-phase and though commercial ICPMS systems ion-exchange columns can separate have been available only for a deinorganic anions and cations, metal cade, there are a variety of HPLCchelates, and organometallic spe- ICPMS applications in the literacies ( 1 3 ) .Most of the elemental spe- ture. In particular, elemental ciation separations utilizing ICPMS analysis of environmental samples (e.g., water, wastewater, fish and detection have, to date, employed marine-based p r o d u c t s , soil ion-exchange chromatography (sometimes referred to as ion chro- leachates, coal fly ash, and incinermatography), which can simulta- ator emissions) has seen tremendous growth. neously separate free ions, neutral Several publications investigate species, and complex ions ( 3 ) . As in environmental standards such Other techniques that have been as the dogfish muscle reference maused are reversed-phase liquid chromatography, which includes terial (DORM-1)with detection limion-pairing and micellar liquid its in the 50-300-pg range (20-22). chromatography, and size-exclu- (Arsenobetaine is the predominant sion chromatography ( 2 6 ) , which As species in spiny dogfish flesh.) separates metal species on the basis This SRM contains about 18 ppm As, but only 0.2 ppm is in the toxic of molecular size. Several problems may hinder the inorganic As or monomethylarsendevelopment of an HPLC method ate forms. Because of the significant for ICPMS detection. Mobile phases difference in the distribution of specontaining total dissolved salt con- cies, it is obvious that speciation is centrations > 0.2% will generally necessary to assess the toxicity of lead to salt deposition in the nebu- this material ( 2 3 ) . lizer, in the inner tube of the torch, Fifteen As compounds, including and on the sampler and skimmer or- arsenosugars purified from a brown ifices. Clogging of the torch tip and alga, were separated in another orifices of the ICPMS system may study that used ion-pairing chromadecrease sensitivity and degrade de- tography ( 2 4 ) . In this same study, tection limits. Decreasing the buffer arsenobetaine and cacodylate (diconcentration, using mixed-gas methylarsinate) were detected in plasmas, inc:reasing the rf power, human urine following the conand using a HNO, wash between sumption of fish. No differences runs can circumvent these prob- were noticeable in ICPMS response lems. for the different As species, and the High concentrations of organics C1- in the urine was resolved from generally used in HPLC mobile the arsenic. phases may also decrease sensitivThe molecular ion 4oAr35C1+inity because of plasma instability terferes with the determination of and carbon deposition on the sam- As at m/z 75 (25).Chromatographic pler and skimmer. Using mixed-gas conditions must, therefore, resolve plasmas a n d cooling the spray this chloride species from the anachamber can help to alleviate this lytes of interest ( 2 6 ) .Alternatively, problem. Finally, a major problem adding N, gas to the nebulizer flow is the inefficiency of the HPLC minimizes this polyatomic interferplasma interface; generally, only ence (27, 28). Using N,, As in soil 1-5% of the sample reaches the extracts from a polluted land site plasma. Some investigators have was determined by ion-exchange used direct-injection nebulizers HPLC with on-line ICPMS detection with microbore columns to mini- (28). mize postcolumn band broadening ICPMS has been used in the analand improve chromatographic reso- ysis of marine reference materials
(coastal seawater CASS-2 and lobster hepatopancreas tissue LUTS-1) and a harbor sediment reference material (PACS-1)(29).Eleven trace elements were simultaneously determined in CASS-2 and seven trace elements were found in the LUTS-1 sample. Tributyltin (TBT) and dibutyltin (DBT) species were separated in the last reference material. HPLC was used only with the harbor sediment sample; the speciation of Sn was accomplished with a strong cation-exchange column. Two extraction methods were used for the analysis of this sample, and subnanogram absolute detection limits were obtained (29). Like As, Sn is a commonly determined metal by HPLC-ICPMS. Most procedures use ion-exchange chromatography (29-32), but ion-pairing (30) and micellar liquid chromatography (32) have been used as well. The separation of butyltin compounds using reversed-phase HPLC by Dauchy et al. appears to be the first HPLC-ICPMS work to add 0.1% tropolone to the organic mobile phase for the complexation of butyltin species ( 3 3 ) . Tropolone also serves to mask residual silanols on the reversed-phase column. Dauchy and co-workers are also the first, to our knowledge, to use triethyltin chloride (which is unknown in the environment) as an internal standard for organotin speciation with HPLC. Improved reproducibility using internal standards should be useful for the analysis of environmental samples, and applications of the method to sediment analysis are in progress (33). HPLC-ICP-MS has been used to determine methylmercury in an albacore tuna sample (NBS RM-50) ( 3 4 , tellurium in wastewater ( 3 5 ) , metals in soil leachates ( 3 6 ) ,pollutants in aqueous leachates (37),and rare earth elements in coal fly ash (38).NIST-SRM 2715 (lead in reference fuel) was used to investigate tetraethyllead and a water quality control sample from the U.S. EPA for inorganic Pb ( 3 9 ) . Finally, Cr(V1) has been determined off-line in wastewater sludge incinerator emissions using ion chromatography (40). A preconcentration scheme and postcolumn reaction were used, followed by the collection, acidification, and dilution of samples prior to ICPMS detection. Gas chromatography. Introducing samples as gases dramatically improves analyte transport efficiency to the plasma over solution
Environ. Sci. Technol., Vol. 28, No. 12, 1994 531 A
nebulization. For this r(?ason.(let tion limits for gas chromatograp sample introduction are typically better than for liquid sample introduction. Because the latter requires both desolvation and vaporization in the plasma, less energy is available for ionization. On the other hand, gaseous sample introduction may provide additional plasma energy for the ionization process (23). Few GC-ICPMS studies have been reported. An MIP ion sonrce is typically used for GC analysis because He plasmas ionize nonmetals more efficiently than the Ar ICP. Compounds must be volatile and thermally stable for GC separation. although derivatives can meet these requirements. To date, GC-ICPMS has been limited to the analysis of organometallics with methyl. ethyl. propyl. or butyl groups (23). GC-ICPMS applications. GCICPMS has not been extensively studied, and environmental applications are few. A capillary GCICPMS transfer line, constructed by Kim and co-workers. was applied to the analysis of five alkyllead species in a fuel reference material (41). Detection limits of 0.7 pgls were achieved. In a follow-up study, volatile organometallics containing Sn. Fe, and Ni were detected by capillary column GC-ICPMS (42). In addition, the presence of volatile organotin compounds was confirmed in a sample of harbor sediment. Capillary column GC-ICPMS has improved resolving power over packed-column GC-ICPMS ( 4 3 , 4 4 ) . which is of importance for the separation of complex mixtures found in many environmental samples (e.g., water, sediment, biota) (42). GC-MIPMS applications. T h e MIP uses lower gas flow rates and is a smaller. cooler plasma than the ICP ( 5 ) . Although peak broadening is less of a concern, MIP is relatively unstable in the presence of high organic concentrations. Generally, a valve between the column and the transfer line is used to vent solvent before reaching the MIP (45). The two main categories of MIPMS are low pressure and atmospheric pressure. Atmospheric pressure plasmas have simple instrumental designs. Unfortunately, entrainment of air in the plasma produces interferences at low masses. Because GCMlPMS is typically used to determine the halogens, phosphorus, and sulfur in the low mass region, this is a significant problem. For example, sulfur and phosphorus are prone to interferences from oxygen and nitro-
gen. These polyatomic: interferences, however, may he diniinatod hy using reduced-pressure MIPs. High-purity He may also help to alleviatc intcrferences. Both reduced-pressure helium and nitrogen MIPs have been evaluated as ion sources for the analysis of phosphorus and sulfur (4.5). These elements were determined in the organophosphorus pesticides diazinon and malathion with the use of a nitrogen plasma. Improved signal and reduced phosphorus interaction with the torch walls was observed in comparison to the He source. Supercritical fluid chromatography. SFC combines the best features of LC and GC: the solubility of liquids and the analysis speed of GC. Advantages include the potential to analyze thermally labile, nonvolatile. and high molecular weight compounds. Additionally. there are many variables that can be changed to alter solvating properties. SFC has been coupled with both MIPMS and ICPMS detectors. With ICP, however, polar modifiers used to change mobile phase polarity apparently have n o effect on the plasma (46, 47). Finally, capillary SFC-ICPMS offers the advantage of using a low CO, flow rate (48). Interfacing the ICP torch with the restrictor of the SFC has proven to be a challenge. The restrictor maintains the linear velocity of the mobile phase and holds the supercriti(:a1 conditions constant through the column. To prevent eluent from freezing at the restrictor tip because of rapid decompression of the mobile phase, the restrictor has to be heated (49). By directly heating the interface and auxiliary gas, the restrictor tip is indirectly heated. In addition, a heated transfer line is required, along with a thermocouple to monitor the temperature (50). SFC-plasma MS applications. SFC-plasma MS is at an early stage of development. Only two applications of SFC-ICPMS, to date, have investigated environmental samples. In the first study, the feasibility of the SFC-ICPMS combination as a multielement detection scheme was evaluated (51). Dicthylmercury, tetrabutyllead. and tributyllcad acetate were studied and. to verify results. NlST SRM 2715 was investigated. It was found that the single-ion monitoring mode of the mass spectrometer produced detection limits lower than those from multielement experiments. Preliminary work using SFC-helium MlPMS on a standard pesticide
532 A Environ. Sci. Technol., VoI. 28. No. 12. 1994
mixture of 15 chlorinated compounds was achieved with ahsolute detection limits of 15 pg for CI and 0.75 pg for Br in two test compounds: 1-chloronaphthalene and 1-bromo-2-methylnaphthalene ( 5 2 ) . Conclusions Several reviews from our group have recently discussed ICPMS as a chromatographic detector for a wide range of samples ( 3 . 23. 49). Many environmental samples include compounds amenable to analvsis by HPLC-ICPMS, GC-plasma M S , and SFC-plasma MS. The latter two have yet to be explored to their fullest potential. Alternatives such as GC-MIPMS and SFC-MIPMS may have important applicability to the analysis of pesticides (45, 52).Also, the use of low-pressure GC-ICPMS systems is likely to grow in the near future ( 5 3 ) . Commercially available realworld speciation standards would aid toxicity assessments because researchers would benefit from the ability to evaluate their methods with standards containing known amounts of different species. Developing these standards, however, requires that the integrity of the individual species be maintained, which may be difficult in certain matrices. Until a way is found to reliably produce these samples, reContinued on p. 534A
Fruncine A . Uyrdy is ,.ompI,!tin~lwr l’li.11. in nnnlytiiril chemistry of tlw Llniversity of Cincinnnti. She received her B.S. degree in chemistry from Youngstown Stote University, O H . Her reseorrh
interests include t h e opplicntion of HPLC-ICPMS to the speciation of toxicologically important metols and the use of rlectrosproy MS for elemental onolvsis. Joseph A . Caruso has been on the fncultv ofthe University of Cincinnoti since l9G8. nnd since 1987 he hns served ns Deon of the McMicken College of Arts ond Sciences. H e received his P1i.D. from Michigon Stote University. Cnruso’s reseorch interests include trocr nietnl specintion investigations ond troditionol and olternotive plosnio sotirce studies.
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Environ. Sci. Technol.. Vol. 28, No. 12, 1994 533 A
searchers will have to look at spiked samples, total elemental concentration standards, and interlaboratory results to assess the viability of a method. Acknowledgments W e are grateful to the National Institute of Environmental Health Sciences for partial s u p p o r t of this w o r k through grants ES03321 a n d ES04908. W e acknowledge the NIH-BRS Shared Instruments Grants program for providing the VG PlasmaQuad through grant number SlORR02714 and the U.S. Environmental Protection Agency for partial support of this work through grant number CR818301. Additionally, we wish to thank Drs. Vela a n d Olson for Figure 1.
References Hill, S. J.; Bloxham, M. J.: Worsfold, P. J. J. Anal. A t . Spectrom. 1993, 8 , 499. Klaassen, C. D.; Amdur, M. D.; Doull, J., Eds.; Casarett and Doull’s Toxicology: The Basic Science of Poisons, 3rd ed.; Macmillan: New York, 1986. Vela, N. P.; Caruso, J. A. J. Anal. A t . Spectrom. 1993, 8 , 787. Wixson, B. G.; Davies, B. E. Environ. Sci. Technol. 1994, 28, 26A. Montaser, A,; Golightly, D. W., Eds. Inductively Coupled Plasmas in A n a lytical Spectrometry, 2nd ed.; VCH Publishers: New York, 1992. USEPA Method 6020 CLP-M Version 8.0, preliminary draft; U.S. Environmental Protection Agency: Washington, DC, 1994. USEPA Method 200.8; Office of Research and Development. U.S. Environmental Protection Agency: Cincinnati, OH, August 1990. USEPA Method 200.10; Office of Research and Development. U.S. Environmental Protection Agency: Cincinnati, OH, April 1991. Heitkemper, D. T.; Caruso, J. A. In Trace Metal Analysis and Speciation; Journal of Chromatography Library Series, vol. 47; Krull, I. S . , Ed.; Elsevier: Amsterdam, 1991, Chapter 3. (10) Olson, L. K.; Heitkemper, D. T.; Caruso, J. A. In Element-Specific Chromatographic Detection b y A t o m i c Emission Spectroscopy; ACS Symposium Series, vol. 479; Uden, P. C., Ed.; American Chemical Society: Washington, 1992, Chapter 17. (11) Suyani, H. et al. J. Anal. A t . Spectrom. 1989, 4, 777. ( 1 2 ) Vandecasteele, C. et al. In Applications of Plasma Source Mass SpectrometIyII; Holland, G.; Eaton, A. W., Eds.; The Royal Society of Chemistry: Cambridge, UK, 1993; p. 48. (13) Batley, G. E.; Low, G. K-C. In Trace Element Speciation: Analytical Methods and Problems; Batley, G. E., Ed.; CRC Press: Boca Raton, FL, 1989; Chapter 6. (14) Uden, P. In Environmental Analysis Using Chromatography Interfaced with Atomic Spectroscopy. Harrison, R. M.; Rapsomanikis, S., Eds.; Ellis Horwood Limited: Chichester, West 534 A
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Environ. Sci. Technol., Vol. 28, No. 12, 1994
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