Elemental Analysis of Environmental Samples - ACS Publications

Elemental Analysis of. Environmental Samples. Using Chromatography Coupled with. Plasma Mass Spectrometry. Toxic trace elements may be found in the ...
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E l e m e n t a l A n a l y s i s of Environmental Samples 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 e l e m e n t 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 i m p o r t a n t area of r e s e a r c h in t h e field of elemental analysis. Analytical atomic s p e c t r o m e t r i c techniques may yield detection of a variety of metallic and nonmetallic species, but they provide only total e l e m e n t a l c o n c e n t r a t i o n information. In combination with chromatographic techniques such as high-performance liquid chromatography (HPLC), gas c h r o m a t o g r a p h y (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 (2). A b s o l u t e d e t e c t i o n l i m i t s for HPLC-inductively c o u p l e d p l a s m a 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 m a g n i t u d e lower. T h u s , p l a s m a 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 (i). Plasma MS can be operated on-line in real time, and n e w software advances allow users to acquire data at more than one 528 A

F R A N C I N E A. BYRDY J O S E P H A. C A R U S O University of Cincinnati, OH

Cincinnati 45221-0172

mass versus time in a single acquisition. This means that with each acquisition several masses are monitored simultaneously, a n d 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 obt a i n i n g s p e c i a t i o n i n f o r m a t i o n . It can also be used to investigate only one species in a complex sample. Despite the potential applications, there are few s t u d i e s using HPLC-, GC-, or SFC-ICPMS for the speciation of trace elements in real environmental samples. Most studies i n v o l v e f u n d a m e n t a l w o r k on m e t h o d s development w i t h the potential for analysis of samples of environmental interest. Ordinarily, a s t a n d a r d reference material (SRM) is used to assess the viability of the method. (A n u m b e r of environmental SRMs are available from the Nat i o n a l I n s t i t u t e of S t a n d a r d s a n d Technology [NIST], including inorganic metal constituents in various e n v i r o n m e n t a l matrices a n d trace e l e m e n t s in fossil fuels a n d sediments.) In this article, w e will prim a r i l y d i s c u s s m e t h o d s t h a t fall into the SRM or real sample analysis categories, rather than solely investigate m e t h o d s d e v e l o p m e n t . Table 1 lists some of the environmental samples analyzed by chromatography-plasma MS that are discussed in this paper. Toxicity and speciation Several of t h e m o s t c o m m o n l y studied trace elements of toxicologi-

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cal interest—arsenic, tin, lead, and chromium—make their way into the e n v i r o n m e n t via i n d u s t r i a l p r o cesses. 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 (2). Speciation is necessary to obtain an adequate toxicological sample assessment w h e n all five of these species are present. Organotin c o m p o u n d s 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 c o m p o u n d s are more toxic than those containing phenyl or octyl g r o u p s . T h e most toxic organotin compounds include the tri- and tetraspecies (3). Lead is also well k n o w n 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 c u r r e n t l y c o n s i d e r a b l e interest in c h r o m i u m c o m p o u n d s because of their w i d e s p r e a d presence in the environment via industrial w a s t e s (e.g., t a n n e r y w a s t e s , corrosion inhibitor manufacturing, and municipal sewage sludge). The two most c o m m o n forms of chrom i u m are Cr(III), an essential trace nutrient found in m a n y foods, and Cr(VI), w h i c h is k n o w n to cause lung cancer and produce mutations. Cr(III) is present in most soils and is less mobile in the environment than Cr(VI) (2).

0013-936X/94/0927-528A$04.50/0 © 1994 American Chemical Society

FIGURE n e u r i t 1ι

Instrumental set-ups for ICPMS and MIPMS systems with chromatographic coupling schemes

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HPLC

_l 1_ _l L_

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-S

JJL. GC

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11

1 PI

ICP 1 A

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τι SFC

The chromatogram shows the potential of multielement capabilities. The separation was achieved by SFC-ICPMS under the following pressure program and temperature: initial pressure. 80 atm held for 2.5 min; pressure ramp, 150 atm/min; final pressure, 400 atm held for 5 min; oven temperature 75 "C. Illustration courtesy of Nohora Vela and Lisa Olson.

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 oscil­ lating magnetic field is established by passing radio frequency (rf) en­

ergy through a load coil with a gen­ erator t y p i c a l l y o p e r a t e d at 27.12 MHz. Once conductive, ener­ gies become sufficient to ionize gas­ eous atoms in the magnetic field. With further ionization resulting from collisions with gaseous atoms and ohmic heating, the plasma be­ comes 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

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529 A

TABLE 1

Some environmental samples analyzed using chromatography coupled with plasma-MS detection Sample

Element Investigated

Reference

Arsenic Arsenic

20, 2 1 , 22 22

Arsenic Arsenic Tin Tin Mercury Tellurium Rare earth elements Lead Chromium

24 28 29 31 34 35 38 39 40

GC-Plasma M S N I S T - S R M 1637 Lead in Reference Fuel Harbor sediment Pesticides (diazinon, malathion)

Lead Tin Phosphorus and sulfur

41 42 45

SFC-Plasma MS N I S T - S R M 2715 Lead in Reference Fuel

Lead

51

HPLC-ICP-MS DORM-1 dogfish muscle reference material Fish: 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 R M - 5 0 Wastewater N I S T - S R M 1633a Coal Fly Ash N I S T - S R M 2715 Lead in Reference Fuel Wastewater sludge incinerator emissions

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 ICPMS {6-8). None of these methods, however, uses chromatographic separation (although Method 200.10 does require a chelating column to preconcentrate the analytes of interest [Cd, Co, Cu, Pb, 530 A

Ni, U, and V] by first selectively separating Group I and II 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, 10). 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 (11). However, He MIPs have a lower tolerance for wet aqueous aerosols compared to ICP discharges, and there are no commercially available MIPMS systems in the U n i t e d 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-

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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 (12). 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 CF 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 mg/L for Cr). To analyze these samples by atomic absorption spectrometry, many standards would have to be run to establish a calibration curve (12). 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 interfaced directly to the outlet of an HPLC column because the continuous high liquid flow rates tend to quench the MIP (14). Thus, MIPs have not been extensively studied as HPLC detectors. Ways to circumvent these problems have been investigated (15) and show promise for reversed-phase HPLC [14). Various HPLC modes have been used to achieve separations with ICPMS detection. Although traditional HPLC has been used predominantly for the analysis of organic compounds, reversed-phase and ion-exchange columns can separate inorganic anions and cations, metal chelates, and organometallic species (13). Most of the elemental speciation separations utilizing ICPMS detection have, to date, employed ion-exchange chromatography (sometimes referred to as ion chromatography), which can simultaneously separate free ions, neutral species, and complex ions (3). Other techniques that have been used are reversed-phase liquid chromatography, which includes ion-pairing and micellar liquid chromatography, and size-exclusion chromatography (16), which separates metal species on the basis of molecular size. Several problems may hinder the development of an HPLC method for ICPMS detection. Mobile phases containing total dissolved salt concentrations > 0.2% will generally lead to salt deposition in the nebulizer, in the inner tube of the torch, and on the sampler and skimmer orifices. Clogging of the torch tip and orifices of the ICPMS system may decrease sensitivity and degrade detection limits. Decreasing the buffer concentration, using mixed-gas plasmas, increasing the rf power, and using a H N 0 3 wash between runs can circumvent these problems. High concentrations of organics generally used in HPLC mobile phases may also decrease sensitivity because of plasma instability and carbon deposition on the sampler and skimmer. Using mixed-gas plasmas and cooling the spray chamber can help to alleviate this problem. Finally, a major problem is the inefficiency of the HPLC plasma interface; generally, only 1—5% of the sample reaches the plasma. Some investigators have used direct-injection nebulizers with microbore columns to minimize postcolumn band broadening and improve chromatographic reso-

lution and detection limits (17-19). Sample c o n t a m i n a t i o n from metal HPLC components is possible, particularly in studies of trace metals. Corrosion of stainless steel components is a concern when Cl~ is present (e.g., in seawater). Therefore, Teflon or glass-lined stainless steel tubing is preferable; HPLC pumps constructed from inert materials are suggested for routine analyses; and fittings made from Teflon, polypropylene, or Kel-F should be used (13). HPLC-ICPMS applications. Although commercial ICPMS systems have been available only for a decade, there are a variety of HPLCICPMS applications in the literat u r e . In p a r t i c u l a r , e l e m e n t a l analysis of environmental samples (e.g., water, wastewater, fish and m a r i n e - b a s e d p r o d u c t s , soil leachates, coal fly ash, and incinerator emissions) has seen tremendous growth. Several publications investigate As in environmental standards such as the dogfish muscle reference material (DORM-1) with detection limits in the 50-300-pg range (20-22). (Arsenobetaine is the predominant As species in spiny dogfish flesh.) This SRM contains about 18 ppm As, but only 0.2 ppm is in the toxic inorganic As or monomethylarsenate forms. Because of the significant difference in the distribution of species, it is obvious that speciation is necessary to assess the toxicity of this material (23). Fifteen As compounds, including arsenosugars purified from a brown alga, were separated in another study that used ion-pairing chromatography (24). In this same study, arsenobetaine and cacodylate (dimethylarsinate) were detected in human urine following the consumption of fish. No differences were noticeable in ICPMS response for the different As species, and the CI" in the urine was resolved from the arsenic. The molecular ion 40 Ar 35 Cl + interferes with the determination of As at m/z 75 (25). Chromatographic conditions must, therefore, resolve this chloride species from the analytes of interest (26). Alternatively, adding N 2 gas to the nebulizer flow minimizes this polyatomic interference (27, 28). Using N 2 , As in soil extracts from a polluted land site was determined by ion-exchange HPLC with on-line ICPMS detection (28). ICPMS has been used in the analysis 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 (29-31), 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 (33). 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) (34), tellurium in wastewater (35), metals in soil leachates (36), 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 (39). Finally, Cr(VI) 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

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nebulization. For this reason, detection limits for gas chromatographic s a m p l e i n t r o d u c t i o n are typically better than for liquid sample introduction. Because the latter requires both desolvation and vaporization in the plasma, less energv is available for i o n i z a t i o n . On the o t h e r hand, gaseous sample introduction may provide additional plasma energy for the ionization process [23). Few GC-ICPMS studies have been reported. An MIP ion source is typically used for GC analysis because He plasmas ionize nonmetals more efficiently t h a n the Ar ICP. Comp o u n d s must be volatile and thermally stable for GC separation, although derivatives can meet these r e q u i r e m e n t s . To date, GC-ICPMS has been limited to the analysis of organometallics with methyl, ethyl, propyl, or butyl groups [23). GC-ICPMS a p p l i c a t i o n s . GCICPMS h a s not b e e n e x t e n s i v e l y studied, and environmental applic a t i o n s are few. A c a p i l l a r y 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 pg/s w e r e achieved. In a follow-up study, volatile organometallics containing Sn, Fe, and Ni were detected bv capillary column GC-ICPMS (42). In addition, the presence of volatile organotin c o m p o u n d s was confirmed in a s a m p l e of h a r b o r s e d i m e n t . Capillary c o l u m n GC-ICPMS h a s improved resolving power over packed-column GC-ICPMS [43, 44), which is of importance for the separation of complex mixtures found in many environmental samples (e.g., water, sediment, biota) (42). GC-MIPMS a p p l i c a t i o n s . 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 s i m p l e instrumental designs. Unfortunately, entrainment of air in the plasma produces interferences at low masses. Because GCMIPMS 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 nitro532 A

gen. These polyatomic interferences, however, may be eliminated by using reduced-pressure MIPs. High-purity He may also help to alleviate interferences. Both reduced-pressure helium and nitrogen MIPs have been evaluated as ion s o u r c e s for the analysis of phosphorus and sulfur (45). 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 c o m p o u n d s . Additionally, there are many variables that can be changed to alter s o l v a t i n g p r o p e r t i e s . SFC has been coupled with both MIPMS a n d ICPMS d e t e c t o r s . W i t h ICP, h o w e v e r , polar modifiers used to c h a n g e m o b i l e p h a s e polarity app a r e n t l y h a v e n o effect o n t h e plasma (46, 47). Finally, capillary SFC-ICPMS offers the advantage of using a low C 0 2 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 supercritical conditions constant through the c o l u m n . To p r e v e n t e l u e n t 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). S F C - p l a s m a MS a p p l i c a t i o n s . SFC-plasma MS is at an early stage of development. Only two applications of SFC-ICPMS, to date, have i n v e s t i g a t e d e n v i r o n m e n t a l samples. In the first study, the feasibility of the SFC-ICPMS combination as a multielement detection scheme w a s e v a l u a t e d (51). D i e t h y l m e r cury, t e t r a b u t y l l e a d , and tributyllead acetate w e r e s t u d i e d a n d , to verify results, NIST SRM 2715 was investigated. It was found that the single-ion monitoring m o d e of the mass spectrometer produced detection limits lower than those from multielement experiments. Preliminary work using SFC-helium M I P M S on a s t a n d a r d p e s t i c i d e

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m i x t u r e of 15 c h l o r i n a t e d c o m p o u n d s was achieved with absolute detection limits of 15 pg for CI and 0.75 pg for Br in t w o test c o m p o u n d s : 1 - c h l o r o n a p h t h a l e n e and l-bromo-2-methylnaphthalene (52). Conclusions Several reviews from our group have recently discussed ICPMS as a c h r o m a t o g r a p h i c d e t e c t o r for a w i d e range of samples (3, 23, 49). M a n y e n v i r o n m e n t a l s a m p l e s include c o m p o u n d s amenable to analysis by HPLC-ICPMS, GC-plasma MS, 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 (53). Commercially available realworld speciation standards w o u l d aid toxicity assessments because researchers w o u l d benefit from the ability to e v a l u a t e their m e t h o d s with s t a n d a r d s containing k n o w n a m o u n t s of different species. Developing these standards, however, requires that the integrity of the indiv i d u a l s p e c i e s be m a i n t a i n e d , w h i c h may be difficult in certain matrices. Until a way is found to reliably produce these samples, reContinued on p. 534A

Francine A. Byrdy is completing her Ph.D. in analytical chemistry at the University of Cincinnati. She received her B.S. degree in chemistiy from Youngstown State University, OH. Her research interests include the application of HPLC-ICPMS to the speciation of toxicologically important metals and the use of electrospray MS for elemental analysis. Joseph A. Caruso has been on the faculty of the University of Cincinnati since 1968. and since 1987 he has served as Dean of the McMicken College of Arts and Sciences. He received his Ph.D. from Michigan State University. Caruso's research interests include trace metal speciation investigations and traditional and alternative plasma source studies.

s e a r c h e r s w i l l h a v e to look at s p i k e d s a m p l e s , total e l e m e n t a l c o n c e n t r a tion standards, and interlaboratory r e s u l t s to a s s e s s t h e v i a b i l i t y of a method. Acknowledgments We are grateful to the National Institute of Environmental Health Sciences for partial support of this work through grants ES03321 and ES04908. We acknowledge the NIH-BRS Shared Instruments Grants program for providing the VG PlasmaQuad through grant number S10RR02714 and the U.S. Environmental Protection Agency for partial support of this work through grant number CR818301. Additionally, we wish to thank Drs. Vela and Olson for Figure 1. References (1)

(2)

(3) (4) (5)

(6)

(7)

(8)

(9)

(10)

(11) (12)

(13)

(14)

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Reporting Experimental Data Selected Reprints EDITED BY

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This practical guide will tell you NOW AVAILABLE everything you need to know about the proper handling of numeric data. Compiled from widely scattered sources, Reporting Experimental Data is a definitive reference volume which offers information on coordinating data from many disciplines. The volume gathers the recommendations for publication of data for measurements of a physical chemical nature together with current IUPAC recommenda­ tions for quantities, symbols, and units. A unified reference source for all authors of scientific publications! Contents: • Describing and Presenting Measurements • Items of General Use in Chemistry • T h e r m o d y n a m i c s Including Biothermodynamics • Chemical Kinetics and T r a n s p o r t Properties • Electrochemistry • Colloid a n d Surface Chemistry • Photochemistry • Analytical Chemistry • Crystallography and Electron Diffraction • Spectroscopies • Automated Products Howard J. White, Jr., Editor 360 pages (1993) Clothbound ISBN 0-8412-2529-X— $ 8 9 . 9 5 O r d e r from: American Chemical Society · Distribution Office, Dept. 74 · 1155 Sixteenth Street, NW · Washington, DC 20036 Or CALL TOLL FREE AT 1 - 8 0 0 - 2 2 7 - 5 5 5 8 (in Washington, DC, 202-872-4363) and u s e your credit card!

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