Nohora P. Vela, Lisa K. Olson, and Joseph A. Caruso Department ofChemistry University of Cincinnati Cincinnati, OH 45221-0172 Elemental speciation is defined as the identification and quantitation of the chemical form of an element, including metals and nonmetals. This chemical form can be the oxidation state of an inorganic form; it can also depend on the type and number of substituents of organometallic compounds. From a risk assessment perspective, it is no longer sufficient to quantitate the total elemental content of samples to define toxicity. Thus elemental speciation offers a continuing challenge for the analytical chemist. Two complementary techniques are necessary for trace element speciation. One provides a n efficient and reliable separation procedure, and the other provides adequate detection and quantitation. The coupling of these techniques requires sample introduction compatibility and minor instrumental modifications with maximum efficiency and response of each technique. In addition, the possibility of real-time data acquisition for the separation process is desirable. Several separation techniques are available, but the ones most often used for elemental speciation are chromatographic techniques such as GC,LC, and SFC. Element-selective detection can be achieved with many techniques, including graphite furnace atomic absorption spectroscopy 0003-27W/93/0365-585A/504.00/0 0 1993 American Chemical Society
(GFAAS), flamellaser-excited atomic fluorescence spectroscopy, plasma atomic emission spectroscopy (AES), and plasma MS. GFAAS and flame/laser-excited atomic fluorescence spectroscopy do not allow the on-line detection desirable for chromatography. Plasma AES and plasma MS are compatible with chromatographic sample introduction and have been widely used for element-specific detection. Plasma spectroscopy offers low limits of detection with wide linear dynamic ranges. Compared with AES, detection with plasma MS offers increased sensitivity of 2-3 orders of magnitude (subnanogram to subpicogram levels) with the additional ca-
REPORT pability of isotopic analysis. Both inductively coupled plasmas (ICPs) and microwave-induced plasmas (MIPS) are useful as ion sources for MS and are easily coupled with various types of chromatography. In this REPORT we describe elemental speciation methods in which LC, GC, and SFC are coupled with ICPMS and MIPMS. Certain elements and species of interest are briefly mentioned, and plasma types and instrumentation are discussed. Special emphasis is placed on interfacing concerns, necessary instrumental modifications, and overall performance. Advantages and limitations of the different methods, particularly for LCIICPMS and GCI MIPMS-the most important
techniques-are discussed along with examples of applications. A current summary of detection limits and a n illustrative method comparison are presented. Finally, future directions for elemental speciation using other novel techniques coupled with plasma MS are discussed.
Elements and compounds of Interest Several metals, such as As, Hg, Pb,
Sn, and Cr, are well known as having varying toxicities based on their specific chemical forms, although there is no general rule or trend. For example, arsenic-containing compounds have widely different toxicities. The inorganic arsenite, AsUII), is the most toxic form, followed by arsenate, As(V), monomethylarsenate, and dimethylarsenate (1). Other organoarsenic compounde such a s arsenobetaine and arsenocholine have been found to be nontoxic ( I ) . As an example of the importance of elemental speciation, consider the spiny dogfish Canadian standard reference material (DORM-1). I t contains 18 ppm of arsenic. However, only 0.2 ppm is in the inorganic arsenic or monomethylarsenate form. If such distribution occurs naturally in samples, the risk assessment becomes markedly different when speciation methods are used. Mercury and lead in inorganic, methylated, or ethylated forms are commonly found in environmental samples. Methyl mercury and dimethyl mercury compounds are the most significant forms with respect to toxicity. In the case of lead, tetraethyllead, tetramethyllead, and the
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REPORT three mixed methylethyllead species are found most commonly. These Table 1. Typical operating conditions of the ICP and the MiP compounds, which act as gasoline I Panmetar ICP MIP antiknock agents, are still widely i Operating power used outside the United States. The 1.2-1.5-kW 50-400 W Radio frequency Microwave Centers for Disease Control of the i Plasmagas Ar. ArlHe, Arlo, He, Ar Department of Health and Human i Coolant flow rate 14-16 Llmin 0.1-8 Llmin Services recently revised the blood Intermediate flow rate NA lead action level in children to 10 pg/ Nebulizer flow rate 0.10-0.2 Llmin dL (100ppb) from a previous level of Sample intrcduction Gas. liquid 25 Fg/dL (250 ppb). type The toxicity of tin compounds is Compatibility GC, SFC, LC such t h a t tetra- and triorganotin Information Elemental Elemental, structural compounds pose the highest health Metals, nonmetals Nonmetals, metals Elements risks (1). As the number of organic Availability Commercially Lab modifications substituents decreases, toxicity also available needed decreases, and inorganic t i n may even be considered beneficial as ar mon choices i Y. essential trace nutrient. Anothei metal of interest is chromium, because the oxidation state defmes the extent of the hazard. CrS+is essenshaped plasma. Another function of eous samples; therefore, there is the tial in human nutrition, whereas this flow is to cool the inside walls of possibility of coupling ICPMS with Cra+ is toxic or a precursor to toxic, the torch and prevent it from meltGC and SFC as well as with other mutagenic, or carcinogenic forms (2). ing. The nebulizer gas, which flows sample introduction methods such as Metal speciation can also be useful through the inner tube, transports hydride generation or electrothermal for obtaining information for medical the sample (in the form of an aerosol vaporization. Note t h a t gaseous applications (e.g., to i d e n t i the role or gas) to the plasma. The third flow, sample introduction always leads to and metabolism of drugs in the huwhich comes between the inner and better detection levels with plasma man body or to determine concentrathe intermediate tubes, is termed the sources; the plasma energy is used tions of metalloproteins in biological auxiliary flow. Its function is to posimore efficiently to form ions, and ensamples). tion the plasma and keep the plasma ergy is not wasted on desolving and In addition to metal-containing removed from the torch. vaporizing the sample. compounds, nonmetallic compounds Power is coupled to the torch via a The most important characteriswith C1, Br, P, and S in the molecule water-cooled load coil by use of a ratics of the ICP include high tempera(present in pesticides, dioxins, and dio frequency generator that typitures, long analyte residence times, PCBs) can be toxic to nontarget orcally operates at 27.12 or 40.68 MHz high electron number densities, and ganisms, including humans. Elemenwith output levels of 0.5-2.5 kW. a relatively inert environment (4,5). tal speciation is important for these The plasma is produced when elecThe combination of these properties compounds because their toxicities trons from a n external source are leads to total desolvation (if soluvary. For example, the toxicity of “eeeded” into the region of the inductions are introduced), nearly comchloro-substituted dibenzodioxins tion coil and ionize the neutral plasma plete solute vaporization, and a high has been of concern, particularly gas. Once the argon conducts, the atomizationlionization efficiency. when i t is associated with t h e plasma forms spontaneously and The potential of simultaneous, multi2,3,7,8-isomer. It is desirable to obmaintains temperatures of 6000element analysis with the ICP is retain detection levels other than those 8000 K Typical operating conditions alized by using a suitable detection achieved with the usual mass specof the ICP and the MIP are shown in system. trometric methods. Table I. The efficiency of the ICP in proSeveral methods can be used for ducing singly-charged positive ions Pl88m8 MS introducing liquid or gaseous samfor most elements makes it an effecA plasma is a gas or a mixture of ples to a plasma. The most common tive ionization source for MS. The gases in which a fraction of the atmethod is solution nebulization from presence of doubly-charged ions is oms or molecules is ionized. The ICP a n aqueous sample. The main purexpected only from t h e elements is most commonly used for spectropose of a nebulizer is t o produce an with low second ionization potential metric analyses, and ita basic operaaerosol that can be introduced to the (3).The first use of the ICP as an ion tion has been described extensively plasma through the inner tube of the source for MS was reported in 1978, in the literature (3-5). The ICP is torch. Pneumatic nebulizers, includand t h e first instrument became formed in a quartz torch that coning concentric and cross-flow types, commercially available in 1983. sists of a n assembly of three concenare most common, although the adA schematic diagram of an ICPMS tric tubes. Plasma gas (usually arvantages of ultrasonic nebulizers instrument is shown in Figure 1. A a n ) is passed through these tubes at have also been demonstrated (5).A two- or three-stage differentially different flow rates. spray chamber is also necessary for pumped interface is used to extract Each tube has a specific function. separating larger droplets produced ions from the atmospheric-pressure The flow carried between the interby the nebulizer and reducing the plasma into the low-pressure mass mediate and t h e outer tubes is solvent load to the plasma. Both LC spectrometer. Ions pass through a known as the support flow or cooling and flow injection involve interfacing cooled sampling cone (typically gas. This plasma gas is introduced with the spray chamber-nebulizer nickel or another metal) with an oritangentially and forms a vortex flow system. fice - 1 mm in diameter. The gas exto the center, providing a toroidalThe ICP is also suitable for gaspands behind this first orifice, and a
i
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588 A * ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1,1993
portion passes through a second orifice in the skimmer cone. A series of ion lenses, maintained a t appropriate voltages, are used to focus the ions into the quadrupole mass analyzer. The ions a r e t r a n s m i t t e d through the quadrupole on the basis of their mass-to-charge (m/z) values and are then detected by using an electron multiplier. The use of a quadrupole mass analyzer gives better than unit mass resolution over a mass range up to m/z = 300. The ICPMS system is considered a sequential multielement analyzer that has scan times < 20 ms for one sweep. The signal intensity is a function of the number of analyte ions (both excited and neutral) in the plasma and the massdependent transport through the mass spectrometer.
drogen can combine with themselves or with any of the other elements to produce isobaric interferences ( 5 ) . The use of helium, which is essentially monoisotopic a t m/n = 4, can reduce the number of these interferences when comvared with arzon plasmas (6). At this time. no commercial MIP is marketed in &e United States as an alternative source for plasma MS; thus, the experimental setup and ope r a t i n g parameters used for He MIPMS vary among systems. The MIP is most often interfaced to existing mass spectrometers by replacing the torch box and matching network of an ICPMS instrument with a microwave cavity (Figure 2). The MIP is formed in a single discharge tube, which is placed in the resonant cavity or waveguide. A 2460-MHz generator is used to couMIP ple the microwave energy to the cavThe major reasons for using the heity and is operated a t powers generlium MIP are based on the limitaally < 400 W. The atmospherictions associated with the argon ICP. pressure MIP requires microwave First, the argon ICP is not an effipowers in the 200-400-W range, cient excitationlionization source for whereas the low-pressure MIP renonmetals such as certain halogens, quires powers < 100 W. MIP torches As, and Se (6).Gaseous helium has commonly used are tangential flow an ionization potential of 24.5 eV types. A tangential flow torch is concompared with that of argon (16.75 structed by placing a threaded insert eV), which suggests that a helium (e.g., aluminum or another metal) plasma should be a more efficient exthrough a fitting that holds the discitationlionization source for noncharge tube. Typically, one tangenmetals (6).Second, isobaric interfertial gas flow is used to form t h e ences are produced by polyatomic plasma in the microwave cavity respecies arising from the plasma gas gion. The atmospheric-pressure MIP and the atmosphere (6).The i ~ ~ t o p e s requires a maximum gas flow of of argon, oxygen, nitrogen, and hy8 Llmin, whereas a low-pressure
\
Coolant
Skimmer
uuaorupole Ionlenses
cone
, \
-
sarnpling’lo~ Auxiliary conel
flow I
MIP system requires gas flows of < 1 Llmin. Intermediate gas flows, if needed, can be incorporated through the center of the insert to help carry analyte to the plasma. Additional pumping and reduced sampling cone o f i c e size are necessary when using atmospheric-pressure MIPS (7). The actual operating conditions, including power and gas flow rate, depend on t h e type of 80u~ee(atmospheric-pressure or lowpressure) and the type of sample introduction (gaseous or aqueous). Examples of operating conditions for the He MIP are listed in Table I. Although the sample introduction scheme shown in Figure 1 applies to t h e MIP, introduction of gaseous samples is preferred because the MIP a t low powers is less tolerant than the ICP to aqueous sample introduction. The torch design used with MIP systems allows easy interfacing with GC and SFC. Aqueous samples can be introduced to t h e MIP, but power and flow must be adjusted to maintain the plasma stability. When using helium a s t h e plasma gas, special nebulizers such as the He concentric or MAK (Meddings-Anderson-Kaiser) must be used to form a n aerosol with t h e smaller, lighter gas (8).One special feature of the MIP is the ability to obtain information for nonmetallic elements and for metals (9). LC with plasma MS detection LC is commonly used for separating ionic, polar, and nonpolar compounds as well as complex ions and neutral species. It is the most popular technique for elemental speciation with ICPMS detedion. In reversed-phase LC, aqueous mixtures of methanol, acetonitrile, or tetrahydrofuran are frequently used as mobile phases. In addition, optimization of a reversedphase method often requires variations in buffer solution concentration, pH control, and the addition of
Flgure 1. Schematic diagram of an ICPMS system. Med lines show immduuion of gaseous samples: solid lines show intmducNond Hquid easmps.
Flgun 2. MIPMS interface
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13,JULY 1,1993 * 587 A
Table II. Detection limits for elements speciated using LCIICPMS
AS
Cr
Rare-earth elements
Reverred phase Ion pair Ion exchange Reversed phase Ion pair Ion exchange Ion pair MicellW Ion exchange Reversed phase ion pair Ion exchange Ion exchange
P
25-87
0.2-3900 Not detected 2-1000 26-126 400-1000 0.6-20 ppb 7 2
wb
1-5pptr 400-4000 7000 Not detected Not detected Not detected
salts to the mobile phase. Reversedphase ion-pair chromatography uses the eame columns and mobile phases as reversed-phase LC. The ionpairing mechanism is accomplished by adding a counterion reagent to the mobile phase. If the ion-pairing reagent is a detergent, the technique is known as micellar LC. Ion-exchange chromatography (IEC) can be applied to the separation of both ionic and non-ionic compounds. The formation of ionic complexes or the use of ligand-exchange reactions is necessary to resolve nonpolar compounds. Buffered aqueous salt solutions containing moderate amounts of methanol or acetonitrile are used as mobile phases, whereas the extent of ionization, sample retention, and selectivity is controlled by variations in pH. Separation in size exclusion chromatography (SEC) occurs aceording to the effective molecular size of the analyte in solution. Mobile phases are selected on the basis of sample solubility in the solvent and compatibility with the stationary phase. Interfacing. The major requirement for interfacing LC with ICPMS or MIPMS is a transfer line that connects the outlet of the LC column with the liquid flow inlet of the nebulizer. The transfer line is usually composed of an inert material, such as Teflon or PEEK tubing, and connections are made with conventional 588 A
300 50-300 20-300
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LC fittings. Tubing length and internal diameter must be carefully selected ta minimize extracolumn volume; using t h e shortest possible narrow-bore tubing will help to minimize peak broadening resulting from the transfer line. Typical LC flow rates, on the order of 0.5-2 mL/min, a r e within t h e range usually required for liquid sample introduction to the plasma with traditional pneumatic nebulization. However, one limitation of this conventional nebulization is the low analyte transport efficiency (1-58) to the plasma. Any increase in sample transport efficiency is desirable and would result in a corresponding improvement in detection limits. The use of other nebulizers, such as ultrasonic types, can increase sample transport but can also present additional extracolumn dead volume in the gas phase. One option that shows promise for LCIICPMS is the use of direct iqjection nebulization (5,10). By using this technique, peak broadening can be minimized and transport efficiency can approach 100% with mobile-phase flow rates up to 0.5 mL/ min for mimbore columns (10). LC mobile phases generally consist of some combination of organic solvents, s a l t s in buffer solutions, andlor ion-pair reagents. Organic solvents can lessen the performance of plasma MS because of the insta-
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1,1993
bility of the plasma to organic vapors and deposition of carbon on the sampling cone and torch. The use of a water-cooled spray chamber and a n increase in radio frequency power (up to 1.7 kW) can help to reduce the solvent load to the plasma and increase stability (11). The addition of oxygen (1-3%) to the nebulizer gas flow can help to minimize carbon deposition and clogging of the sampling cone; however, the cone lifetime is significantly decreased (11). Mobile phases containing salts can cause short-term signal drift, affect sensitivity ( S I N ) ; and give a more complex mass spednun, particularly for m/z < 80 (2).Signal enhancement or depression may be troublesome because of the matrix effect on the gas-phase chemistry of the analytes. In addition, the deposition of salts in the nebulizer and on the sampling cone can cause bloekage (12).Nebulization of 2% nitric acid between chromatographic runs can reduce some of the problems caused by the salts in the mobile phase and increase the time available for experimentation. In general, any LC method coupled to plasma MS requires the selection of chromatographic and plasma conditions that provide an efficient separation while maintaining adequate detector performance. Buffer concentrations > 0.2% dissolved solids and the use of organic solvents can s i g nificantly affect the LC/ICPMS experiment, and chromatographic conditions must be carefully selected. Applications. Several elements, including As, Hg, Sn, Pb, Cr, Cd, Cu, Zn, Au, P, and S, have been the focus of speciation studies coupling different LC modes with ICPMS (10-27). A summary of elements that have been studied, the types of chromatography used. and a range of detection limits are listed in Table 11. As stated earlier, arsenic-containing compounds have been investigated with great interest. Reversedphase LC and ion-pair LC have been used for t h e separation of AsOz-, As0,8-, monomethylarsenate, dimethylarsenate, and arsenobetaine species, with detection limits from 50 to 300 pg (13, 14). Similar detection limits are obtained by using IEC for the same species (12,14,15).Several samples have been investigated, including dog fish muscle, urine, wine, and club soda (12-1.5). One limitation of using ICPMS detection for A8 speciation is the presence of a n isobaric interference at m/z = 75. This interference results from formation of the polyatomic ion
4oA$6C1+when chlorine is present in a sample. One advantage of IEC is the possibility of separating this interference from the compounds of interest; for example, the ArCl' can be chromatographically separated from the arsenic species, as shown in Figure 3 (15).Because As is not efficiently ionized in an argon plasma, these authors described the use of He-Ar mixed-gas ICP with mass spectrometric detection (15).The use of this plasma type improved the detection limits for As by l order of magnitude and minimized sample cone blockage. Speciation of lead and lead-containing compounds h a s been t h e topic of several studies i n which ICPMS detection was used (10,16, 17).In addition to inorganic Pba*, other species of interest are trimethyllead chloride, triethyllead chloride, triphenyllead chloride (10,16),and tetraethyllead (17).Speciation of these compounds has been accomplished by using reversed-phase and ion-pair LC and IEC (10,16,17). Inorganic lead, ionic alkyllead, and tetraalkyllead compounds have been separated in one run by using ionpair LC with gradient elution (Figure 4, 17).Detection limits for all three compounds using reversedphase and ion-pair LC with ICPMS ranges from 0.2 to 3900 pg. Urine and water eamplea as well as refermce fuel have been analyzed for lead and lead-wntaining compounds (IO,17).
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Tin speciation has foeused mostly on triorganotin compounds because of their wide commercial use. IEC and ion-pair chromatography have been used for the separation of trimethyltin chloride, triphenyltin acetate, and tributyltin chloride by ICPMS with detection limits of 4001000 pg (11). Both inorganic tin and organotin compounds adsorb to some extent on silica-based columns, causing increased backgmund levels after several injections and limitine t h e detection" capabilities witK ICPMS (11. 18). A po?ymer-bamd cohnnn with ionpair LC reduced this adsorption effect and improved detection limits by 2 orders of magnitude (- 2 pg for all three compounds). Certified reference material consisting of fish tissues and noncertified tuna fish were extracted and analyzed for organotin compounds by using the polymeric column (18).Micellar LC with sodium dodecyl sulfate mobile phase has been used for the separation of trimethyltin chloride, triethyltin bromide, and tripropyltin chloride, a s well a s mono- and diorganotin species. Detection limits for this method were 26-126 pg Sn (19). The three most significant mercury species, besides H2+,are methyl mercury, ethyl mercury, and phenyl mercury. These compounds, which are often found a s chloride wmpounds, have been separated by u s i n g reversed-phase LC with
ICPMS; detection limits a r e 720 ppb (20). The use of postcolumn cold-vapor generation improved the sensitivity of ICPMS for Hg with detection limits ranging from 0.6 to 1.2 ppb (20).Both thimerosal solutions and certified reference tuna fiah were analyzed with this method (20).Ion-pair LC has also been used for speciation of these Hg compounds with detection limits of 7 pg; urine samples were also analyzed with this method (10). Several other publications on the speciation of Cr, P, S, and rare-earth elements with ICPMS detection have appeared in the literature. The inorganic forms of Cr (CrS+and Cra+) have been separated by using IEC and ICPMS with detection limits on the order of 2 ppb (21).Trace rareearth elements present as impurities in other rare-earth materials were determined by IEC coupled with ICPMS (22).Detection limits on the order of 1-5 pptr in solution were obtained. The detection of P and S using plasma MS is hindered because of the presence of isobaric interferences a t m/z = 31 ("NNleOH+) and d z = 32 ("O;),respectively (5).Elevated backgrounds a t these masses usually result in poor detection; however, hy using desolvation and alternative isotopes (for sulfur), improved detection can sometimes be achieved. Jiang and Houk described the use of ion-pair LC for the separation of an-
D )
Figure 3. Separation of four As species and ArCI' by ion-exchange chromatography with Ar ICPMS detection. O M :dimelhyIaraenale. MMA: monomeIhylamnale. All As species were a WKBmraliOns 01 100 @As in unne &luted 1:4. Moble p n w . 50 mmwL carbonate-hydrogen carbanme Rmei at pH 7.5; sampla 104, size, 100 pL; and mommnng a1 mz 75. ( M W w.Ih permmion hom Reterenm 15.)
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Flgum 4. HPLC/ICPMS chromatogram of organic lead, ionic alkyllead, and tetraalkyllead c o m ~ u n d s ' A Pb? B: IneIhylW ChlWe; C: InphenyilePdchloride: and D: telraelhy(lead. 5 ppm each. CnromatopraphicCondiPons: Nucleosil C,. column; mot4le phase,8 mmOVL sodium pentane sulfonate a1 p H 3; pradim eluent, 4040% m e I h m I wer 10 min. held 81 90% methanol tor 20 min. Flar ram. 1 mUmin. (Mapled wnm permmion Imm R e f e m 17.)
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1,1993
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ionic compounds containing P and S (23).An ultrasonic nebulizer with continuous desolvation was used to introduce column effluents to the ICPMS instrument. Detection limits of 400-4000 pg and on the order of 7000 pg were obtained for P and S, respectively. The low-abundance isotope (4.2%) of S a t m/z = 34 was used because of the remaining interference from '"0; a t m/z = 32. SEC/ICPMS h a s been used for speciation of metalloproteins. Cd, Zn, Cu, and Au present in high molecular weight compounds were separated and detected i n pig kidney samples and in other biological materials (.%-27). Gold-based drug metabolites in blwd samples have also been determined by SEC with weak anion-exchange columns (27). One limitation specific to the use of MIPMS as a detector for LC is the low tolerance of the MIP to liquid sample introduction (8). Only one publication to date has dealt with coupling LC with MIPMS: Heitkemper et al. (8)successllly coupled rev e r s e d - p h a s e LC with t h e He MIPMS for the detection of halogenated organic compounds. A He concentric nebulizer was used to produce adequate aerosol. Detection limits for Br and I were calculated to be 50 pg and 1 pg, respectively, and the detection limit for C1 was elevated at 10 ng because of polyatomic interferencea at m/z = 35 and m/z 37. The design of more efficient sample introduction devices (nebulizers and spray chambers) to improve sample transport and the discovery of ways to minimize peak broadening are still needed. Evaluation of new mobile and stationary phases could expand the range of applications for LClICPMS while maintaining the same level of response achieved with direct solution nebulization. MIPMS may be an additional tool, complementing ICPMS, for trace element speciation. In addition, the MIP shows promise as a n atmosphericpressure ion source for LC/MS, where low power conditions produce compound fragments (instead of the totally atomized and ionized compounds) that are more useful for structural determinations.
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GC with plasma YS detaction GC is used for the analysis of volatile and thermally stable compounds or for derivatives of the compounds with these same characteristics. The number of organometallics amenable to separation by GC is limited because of their typical chemical properties, which include high molecular
I
Atmospheric pressure Low pressure Atmospheric pressure Low pressure Atmospheric pressure Low pressure Low pressure Low pressure Atmospheric press
I
weights, low volatility, and some degree of polarity. However, organometallics that have substituents consisting of methyl, ethyl, or propyl groups (occasionally even butyl groups) are usually volatile and thus are good candidates for separation by
GC. Examples of organometallics that fall into this category are compounds containing Sn, Pb, and Hg. These compounds are also known as environmental contaminants and are subjected to different speciation studies (28,B). A review of the literature indicates that the applications of GClICPMS are limited. and most researchers using plasma MS have foeused on the determination of halogenated compounds. Interfacing. Gaseous sample intrcduction to the plasma is advantageous because it provides nearly 100% analyte transport efficiency (9,and additional plasma energynot used for desolvation and vaporization-is available for ionization. Coupling GC to ICPMS or MIPMS is easily accomplished by connecting the column to the inner tube of the torch using a transfer line between the GC oven and the plasma torch. The transfer line usually consists of a metal tubing into which the analytical column or a piece of deactivated fused silica is passed. Typically, the metal transfer line is maintained at a temperature that guarantees all compounds will remain in the gas phase (at least a t the highest oven temperature used for the separation). Fluctuations in the transfer line temperature can affect GC peak shape and resolution. The column and metal transfer line are positioned approximately flush with the end of the inner tube closest to the plasma. However, uneven heat-
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ing of the transfer line, particularly within the torch, can cause some analyte condensation if adequate heating is not provided (9). When using capillary GC columns with carrier gas flows of < 5 mL/ min, it is necessary to introduce a makeup gas approximating flow rates usually found for the nebulizer gas (up to 1Llmin). The makeup gas produces a central channel in the plasma and helps to carry analyte from the GC column into the plasma. However, this gas flow must be optimized to minimize peak broadening and maximize analyte transport. Several important factors must be considered when interfacing GC with MIPMS.Helium is the ideal carrier gas for MIPMS because the use of nitrogen or hydrogen can lower t h e plasma energy available to excite and ionize the eluent (most significantly for halogenated species). Because the MIP is unstable i n the presence of high organic solvent concentrations, venting is needed to prevent the plasma from being extinguished or signifcantly quenched as the sample solvent passes through the system. Usually a valve placed between the column and transfer line t u b q serves as the vent and is switched to the on-line position onee the solvent has vented to the atmosphere. Peak broadening is of less concern for GClMIPMS than for GC/ ICPMS because of t h e smaller plasma size and lower gas flow rates associated with the ME' (Table I). In addition, the MIP torch configuration allows the column to be placed closer to the plasma than with the ICP, which minimizes the diffusion processes. Applications. GC/MIPMS can be used for both nonmetal and metalcontaining species; however, i t is
most applicable to nonmetallic compounds. Studies describing the use of GClMIPMS are difficult to summarize under one set of standard conditions, because variations in torch design,operating conditions. MIP type, and instrumentation make each study unique (6, 7, 30-32). One major difference is the use of atmospheric-pressure or low-pressure MIPs. Both are applicable for most elements; however, the low-pressure plasma is somewhat easier to operate and is advantageous for elements such as C1, P, and S, in which isobaric interferences result from atmospheric entrainment. The advantages of GC/MIPMS for determining halogens, P, and S compounds make it useful for the detection of pesticides, PCBs, and dioxins at ultratrace levels. Table 111 summarizes the results obtained for these elements from several studies involving both atmospheric-pressure and low-pressure MIPs. In general, for the halogens (including I, Br, and C1) detection limits were found to be in the low-picogram range (6, 7, 30), whereas detection limits for P and S are in the low-nanogram range (31, 32).The application of this technique for the determination of pesticide compounds has been described (32). A standard mixture of pesticide compounds was separated by detecting C1 in the picogram range. The advantage of element-specificdetection available with the mass spectrometer is the ability to aid in peak identification by monitoring different mh values for the same sample injection In this specific example, the sulfurand chlorine-containing pesticides could be distinguished from those containing only chlorine, as shown in
Figure 5. One application of GCIMIPMS to the speciation of organometallic compounds has been reported (33).A mixture of tetraorganotin compounds was separated and yielded detection limits of 1-4 pg Sn using a standard tangential torch. A tantalum injector tube inserted into the torch improved the detection limits for a mixture of tetra- and triorganotin compounds. (The results ranged from 0.09 to 0.4 pg.) As previously noted, applications for speciation of organometallic compounds by GC are limited, even though the MIP is capable of ionizing these species. Few publications concerning GC; ICPMS have appeared in the litera. ture (28,B). In general, coupling GC with ICPMS requires no change from normal operating conditions for the ICP. Several tetra- and trialkyltin
compounds have been separated by GC/ICPMS (28)using a slightly modified ICP torch to-obtain-high transport efficiency, stable plasma operation, good chromatographic resolution, and detection limits in the picogram range. In addition, the organotin species present in a sample of harbor sediment were identified. The use of GC/ICPMS for the separation of tetraorganolead compounds was described recently (29). The detedion limit for tetraethyllead was calculated a s 0.7 pg/s. Organolead compounds identified as tetramethyllead, trimethylethyllead, dimethyldiethyllead, methyltriethyllead, and tetraethyllead were separated in a naphtha sample. The technique was also applied to the analysis of a tetraethyllead motor mix in reference fuel (NIST SRM 1637). Other compounds that have been separated include f e r m n e and nickel diethyldithiocarbamate. The detection limit for ferrocene was determined as 3.0 pg/s; the chromatogram obtained illustrated the advantage of using a "dry"plasma (with no aqueous sample introduction). No isobaric interference from 40Ar'BO+ was observed, thus allowing the determination of Fe at m/z = 56 (28). SFC with plasma MS detection SFC is rapidly gaining popularity because it provides an alternative to traditional GC and LC methods. SFC
combines the high diffusion coefficients of GC with the solubility properties of LC. It is possible to separate thermally labile, nonvolatile, and high molecular weight compounds (not easily separated by GC) with shorter analysis time and less solvent use than are required for LC (34).Temperature, pressure (or density), and mobile-phase composition can be controlled i n SFC. CO, is most commonly used as the mobile phase, and methanol or other suitable mobile-phase modifiers can be added to increase solvating power. Interfacing.An important characteristic of SFC is that while the separation is performed with the mobile phase in the supercritical state, the detection can be obtained after the decompression zone, where the supercritical fluid mobile phase changes to a gas. To maintain the supercritical conditions through the column and to keep a certain mobilephase linear velocity, a restrictor is connected to the end of the column. The restrictor consists of a length of fused-silica tubing (30-120 cm)with a porous f r i t end. As t h e mobile phase makes the transition from supercritical conditions (at least 70 atm and 40 "C for CO,) to atmospheric pressure, net cooling results and therefore suffcient heat must be provided to the restrictor (temperature above 150 "C) (34). Because the column eMuents are
5
Endosulfan I
I
Endosulfan II
Endosulfan sulfate
Figure 5. Single-ion chromatograms of a standard pesticide mixture. Top: single-ion chromatogram of chlorine a1 mlz= 35; M o r n : single-ion chmmamgram 01 suifur (rerponse magnified isx) at mlz = 34:80 ng of each compound injected.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY I , 1993 * 591 A
REPORT in the gas phase, coupling SFC with both ICP and MIP torches requires similar conditions as in GC,where a heated transfer line connects the oven with the torch. The restrictor is passed through the transfer line and positioned flush with the end of the inner tube for interfacing. In addition to the temperature control of the transfer line, a separate heating unit is required for the transfer linehrch connection. The combination of heating the torch and providing a heated makeup gas flow maintains proper restrictor temperature. As with GC, the general operating conditions of both the ICP and the MIP with capillary SFC require no additional modifications, and the makeup gas flow rate approximates the normal nebulizer gas flow rate in the ICP or the typical intermediate gas flow rate in the MIP. Optimization of the makeup gas flow rate and temperature is necessary. Using CO, as the mobile phase can cause backF u n d interferences, including '%+, 'C"O;, and mAr'aC'. The use of organic mobile-phase modifiers does not compromise the response of the detector for plasma MS (35). Applications. The use of SFC opens the door for the speciation of organometallic cornpounds not easily separated by GC and offers the advantage of gaseous sample introduction to the plasma. The coupling of SFC with plasma MS is a relatively recent development (35-39).Most studies have focused on the soeciation of organotin compounds b i SFC/ ICPMS (35-37).Tetrabutyltin, tributyltin chloride, triphehyltin chloride, and tetraphenyltin have been separated in a single c h m a t o gram (see Figure 6 ) with detection limits in the subpicogram range (363. The SFC/ICPMS system was also evaluated by comparing results from an SFC system with flame ionization detection (FID) (37).Better resolution was obtained with FID, and differences were attributed to fluctuations i n t h e temperature of t h e transfer line used in the SFC/ICP interface. However, ICPMS detection limits are superior to the FID results by 1d e r of magnitude. Lead, mercury, and arsenic-containing compo&& have been determined-by using SFC/ICPMS (38).Lead a n d m & w y compounds can be detected in the picogram range. The use of MIPMS as a detector for SFC has been discussed (39).To evaluate the SFC/MIPMS system and to demonstrate the possible application to pesticide determination, 1-chloronaphthalene and l-bromo-
2-methylnaphthalene were used. The presence of CO, lowers t h e plasma energy available for ionization and can affect the sensitivity. Chlorine and bromine were detected at picogram levels, comparable to those observed with GCIMIPMS. SFC, as an analytical technique, is still in the early stages of development and acceptance. Plasma MS shows promise as a very sensitive, element-specific detection method for SFC with applications to both metallic and nonmetallic compounds. The use of alternative mobile phases (including micellar mobile phases) and modifiers as well as packed SFC columns are topics for further studies with plasma MS detection. Extending the range of applications to indude larger molecular weight compounds-for example, metalloproteins-or catalysts is also an interesting area for future research. Method comparison Detection limits for selected elements that have been speciated using the three different chromatographic modes with plasma detection are compared in Table IV. Several species of Sn,Pb, C1, and Br have been separated by using GC,LC, and SFC coupled with ICPMS for metals and MIPMS for halogenated compounds. In general, detection limits in the low- to subpicogram range are obtained for GC and SFC with the ICPMS and MIPMS methods, presumably because of the nature of the
Figure 6. SFC/ICPMS chromatogram of organotin compounds. inbaionr ot 530 pg of each orgsnotin compound were eone using a 4-m SB-Biphenyl wiumn. inhiai pressure: 80 atm held lor 2.5 min; pressure ramp: 150 abnImin; find p~ssure:400 atm heM foi 5 min. Oven ternpratvm. 75 "C. Peaks identified as follows: TBT lelrabutyilln; TrBT -C : tnbutyiiln drbdde: TrPT-CI: tdohenvilin drbnae:
X: unidentified tin-cdmaiiq c o m p d ; and TPT: tetraphenyltin. (Adapted with permission lmm Referenca 36.)
592 A * ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1,1943
gaseous sample and direct intmduction of the column effluent into the plasma. In addition, fewer interferences from the mobile phase result in better detection limits. A wider range of detection limits (picograms to nanograms) is observed with HPLC coupled to plasma MS. In general, HPLC is somewhat limited by the inefficiency of liquid sample introduction to the plasma and by restrictions related to the use of certain solvents a n d mobile phases. However, HPLC accommodates a wide range of compounds and is important in speciation studies. Selection of the appropriate chromatographic mode and plasma type is dependent on the Characteristics of the analytes of interest, sample type, separation requirements, mobilephase properties, detection limits required, instrument response to the element of interest, and minimization of possible interferences. Future dlrectlons To date, most emphasis in trace elemental speciation has been placed on development of the actual separation techniques and interfacing to plasma MS. This is only a portion of the total pmeedure for determining species in a given sample. The optimization of these methods was initiated with the use of chemical standards, followed by validation using "real 88111ples," including certified standard reference materials. One limitation to validation of these speciation methods is the availability of reference materials for certified species concentrations. If a wider variety of these materials were available, more speciation techniques could be developed as approved methods for toxicological and environmental studies. One area that has received minor attention is sample preparation techniques specifically designed to retain all species in their original forms. If a sample is naturally found in aqueous forms or is easy to dissolve, little or no pretreatment is necessary. Many examples of this type, such as urine or water, have been studied. Most such samples may require some dilution, pH adjustment, or fdtration. Solid and biological samples usually require more complicated p d u r e s involving extraction or preconcentration. Chemical digestion procedures are usually not applicable when attempting to preserve species, and sample pretreatment procedures of any type require extreme care in handling to avoid contamination and interconversion of species.
35TH ROCKY MOUNTAIN CONFERENCE ON ANALYTICAL CHEMISTRY ATOMIC SPECTROSCOPY Mass Spectrometric Determination of Analyte Loss Mechanisms During ElectrothermalAtomization in Graphite Furnace Atomic Absorption Spectrometry. Garrett N. Brown, Jerry D. Harris and David 1. Styris (Invited)
CHROMATOGRAPHY Includes presentations on polymer characterization, psoralen photochemistry, photoacoustic spectroscopy, volatile organic pollutants, petroleum hydrocarbons, gas phase chemiluminescence, and sulfur selective detectors.
July 25-29,
1993 Hyatt Kegency Denver
PHARMACEUTICAL ANALYSIS
Highlights
Dissolution: Past, Present & Future. Marvin Delgado (hvited Speaker)
Extension of the Capabilities of ICP-MS for the Determination of Trace Metals in Waters. Diane Beauchemir (Invited Speaker)
EPR International EPR Society Award. Award Address: Multiquantum EPR. James S. Hyde
FTIRINIRIRAMAN Heterogeneous Photochemical Reactions involving Carbonaceous Particulate; Shedding Some Light on a Dark Subiect. Dwight M. Smith (Keynote Speaker)
Will cover subjects relating to values of mature compost, quality standards and testing, case studies, explosive contaminated soil composting and analysis, marketing opportunities, source separation, and procedures for converting waste to a resource.
Field Applications of Combined C hromatographic/Spectroscopic Techniques for Environmental Analysis. Henk Meuzelaar (Invited Speaker)
NMR
Conference
ICP-MS
ENVIRONMENTAL CHEMISTRY
Recent Developments in Electrospray Mass Spectrometry. (Keynote Speaker)
Denver, Colorado
Addressing Background Problems in Isotope Ratio ICP-MS. S. R. Koirtyohann, lijian Yu (Keynote Speaker)
Includes presentations on applications of polymer modified, nickel alloy, ion selective, and ultamicro electrodes, and electrochemical studies of composite films, conducting polymers, heterodinuclear complexes, and activity coefficients in acetonitrile.
MASS SPECTROMETRY
High Resolution NMR of Complicated Solids: Synthetic and Biological Composites and Complexes. Jacob Schaefer (R. W. Vaughan Plenary Lecture)
COMPOSTING
ELECTROCHEMISTRY
and Elise D. Bowman. (Keynote lecture)
LABORATORY SAFETY Panel Discussion-Common safety problems including establishment of laboratory operating rules and safe operating conditions, special safety equipment, and dealing with crises.
LUMINESCENCE
Thermal Analysis and Dosage Formulations Walter McCrone (lnvited Speaker) Applications of Calorimetry in an Industrial/Pharmaceutical Environment. Tom Hofelich (Invited Speaker)
QUALITY ASSURANCE Includes presentations on NlST SRMS, misuse and abuse of chemical analysis, and studies on analytical precision.
RADIOCHEMISTRY Includes speakers from the Colorado Department of Health, CSU, U. S. Geological Survey, U. S. Department of Energy and U. S. Environmental Protection Agency.
ROBOTICS Presentations on the applications of robotics to chemical analysis.
ACS SHORT COURSES Basic Principles of Mass Spectrometry and Interpretation of Organic Mass Spectra-Practical Analytical Atomic Spectroscopy: AAS, ICP/AES, ICP/MS-Spectroscopic Characterization of PolymersLaboratory Waste Management. contact: Tara Barrineau
Electrogenerated Chemiluminescence Detection using Tris (2,2' bipyridyl) ruthenium (11). Timothy A. Nieman (Keynote Lecture)
(303) 347-5486
Detection of Metabolites of Polycyclic Aromatic Hydrocarbons in the Urine of Psoriasis Patients Treated with CoalTar. Ainsley Weston, Regina M. Santella,
For further information, contact: Robert Wershaw, U. S. G. S., P. 0. Box 25046, MS 408, Denver, C O 80225
(303) 467-8280
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1,1993
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REPOR7 References
Table IV. Intermethod co
(1) Harrison, R. M.; Rapsomanikis, S. Envimnmcnfal Anal is Using Chmmatognzphy
Interfaced wifhA m i c Spectroscopy;John
.
.
Trim&yltin chloride,triphenynin lCPl
'b
I
:I
acetate. tributvltin chloride Tetrabutyltin, tributyctin chlorde. ICPMS triphenyltln chlonde. tetrmhenvnin Tetm'ethyliead, trimethylethyl- lCPl lead, ð Idimethyllead. methyltndyllead. tetraethyllead Trimethyllead chloride. triethyllead chloride, triphenyllead chloride, tetraethyllead, Pb2' Tetrabutyllead,tributyllead acetate Chlorotoluene, chlorobenzene, ch1orq)henoI Chlorobenzene 1 -Chiomnaphthalene Bromwctane, bromononana )ibiomomethane, bromobenzene I -Bromo 2-methvl-
HPLC 2-1OW
Wiley & Sons: New York, 1989. (2) h l l , I. S. Trace Metal Analysis and Speciafion;Elsevier: Amsterdam, 1991. (3) Houk, R. S. Anal. Chem. 1988, 58, 97 A-I05 A. (4) Olesik, J. W. Anal. Chem. 1991, 63, 12 A-21 A. (5) Montaser, A.; Golightly, D. W. Induc-
tively Coupled Plasmas in Analytical Spectrometty; VCH Publishers: New York,
1992. (6) Creed, J. T.; Mohamad, A. H.; Davidson, T. M.; Ataman, G.; Carum, J. A. ]. Anal. At. Spectram. l O S 8 , 3 , 923-26. (7) Mohamad, A. H.; Creed, J. T.;Davidson, T. M.; Caruso, J. A. Appl. Spectrosc. 1989,43,1127-31. ( 8 ) Heitkemper, D.; Creed, J.; Caruso, J. A,]. Chmmafogr. Sci. 1990, 28, 17581. (9) Uden, P. C. ElementSpecifc Chromato.
graphic Deiection by Atomic Emission Specfroscopy; ACS Sym osium Series 479;
American Chemicay Society: Washington, DC, 1992. (10) Shum, S.C.K.; Pang, H.; Houk, R. S.
Anal. Chen. 1992,64,244-50. (11) Suyani, H.; Creed, J.; Davidson, T.; Caruso, J. A. 1. Chromafogr. Sci. 1989, 27,139-43. (12) Heitkemper, D.; Creed, J.; Caruso, J.; Fricke, F. L. 1. Anal. At. Spectram. 1989,4,279-84. (13) Beauchemin, D.; Bednas, M. E,; Berman, S. S.; McLaren, J. W.; Siu,
K.W.M.; Sturgeon, R. E. Anal. Chem.
in= rn mm-i? ____,I "_ (14) Beauchemin, D.; Siu, K.W.M.; McLaren, J. W.; Berman, S. S. 1.Anal. AT. Spechom. 1989,4, 285-89. (15) Sheppard, B. S.;Caruso, J. A,; Heitkemper, D. T.; Wolnik, K. A. Analyst m a , 117,911-75. (16) AI-Rashdsn, A,; Heitkemper, D.; Caruso, J. A.I. Chmmatw. Sei. I991,29, _ ) _ _
One promising extraction technique for this field is supercritical fluid extraction (SFE). The possibility of performing extractions a t lower temperatures and in shorter times than those obtained with traditional Soxhlet extractions may prove advantageous for many thermally labile or easily oxidized compounds. By using modifiers, the range of applicability can be extended for the extraction of slightly polar and nonpolar compounds. Current research focuses on the use of SFE to extract organometallic compounds from solid samples such as fish tissue for subsequent speciation with a chromatographic plasma mass spectrometric technique. The continuons development of alternative separation procedures, including capillary zone electrophoresis (CZE), field-flow fractionation (FFF), and countercurrent chromatography (CCC) could provide additional opportunities for extending the development of new speciation techniques. These methods allow for separation based on other physical or chemical properties not considered in traditional separation methods such a s HPLC or GC. Variations in characteristics such as charge and particle size are important in CZE and FFF. CCC can be applied to nat596 A
ural products, compounds that can interact with solid phases, or those requiring extremely mild separation conditions. Preliminary research coupling FFF (40) a n d CZE with ICPMS is already in progress. Interfacing considerations, however, are somewhat more complicated when coupling t h e s e techniques w i t h plasma MS. Possibilities for extending the application of elemental speciation exist in collaborative work among toxicologists, biochemists, and environmental scientists who can, by using appropriate separation methods and low-level detection, determine the species of significant impact to living organisms. Plasma MS with the appropriate chromatographic technique shows great promise for achieving this goal. With improvements in interfacing and instrnmentation, as well as detailed procedures, these methods can become routine analysis techniques. We are grateful to the National Institute of Environmental Health Sciences for partial support of this work through grant numbers E503221 and ES04908. We acknowledge the NIH-BRSShared Instruments Grants pmgram for providing the VG PlasmaQuad through grant number SlORRO2114 and the US. Ennronmental Protection Agency for partial support of this work through grant number RF-2963.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1,1993
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(17) AI-Rashdan, A.; Vela, N. P.; Caruso, J. A.; Heitkemper, D. T.1.Anal. A f . Spectmm. 1992, 7,551-55. (18) Kumar, U.; Evans, H.; Dorsey, J.; Caruso, J. A,, submitted for publication
inJ Chromato
(19) Suyani, If Heitkemper, D.; Creed, J.;Caruso, J. A. Appl. Spectrox. 1989,43, 962-67. (20) Bushee, D. S. Analyst 1988, 113, 1167-70. (21) Roehl, R.; Alforque, M. M. At. Spect r a ~ISSO, 11, 210-15. (22) Kawabata, K.; Kishi, Y.; Kawaguchi,
0.; Watanabe, Y.; lnoue, Y. Anal. Chem.
1991,153,2137-40. (23) Jiang, S. J.; Houk, R. S. Spectrochim. Acta ISW,43B,405-11. (24) Dean, J. R.; Munro, S.; Ebdon, I..;
Crews, H. M.; Massey, R. C. I.Anal. At. Spectram. 1987,2, 607-10. (25) Mason, A. 2.; Storms,S.D.; Jenkins, K. D. Anal. Biochem. 1990, 186, 18790,
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(26) Crews, H. M.; Dean, J. 8.; Ebdon, L.; Massey, R. C. Analyst 1989, 114, 69599. (27) Matz, S.G.; Elder, R. C.; Tepperman, K.I. A d At. SDechom. 1089.4.767-71.
(30) Creed, J. T.; Davidsan, T. M.; Shen, W. L.; Caruso, J. A. J. Anal. At. Specirom. 1990,5, 109-13. (311 Story, W.C.;Olson, L. K; Shen, W. L.; Creed, J. T.; Carum, J. A. J. Anal. At. Specimm. 1990, 5,467-70. (32) Stow. C.: Caruso. J. A. I. Anal. At. Spectrom:,'in ,,ess. (331 Suyani, H.; Creed, J.; Caruso, J. A. J. Anal. At. Specirom. 1989, 4,777-82. (341 Lee,.M. L.;Markides, K. E. Analytical Supercniical Fluid Ckromaiography and Extraction; Chromatography Conferences, rnr. . prnvn .- .-, TIT - ., iesn - - - -. (35) Shen. W.L.;Vela, N. P.; Sheppard, B. S.: Carusa. J. A. Anal. Chem. 1991.63. . . 1491-96. (36)Vela, N.P.;Caruso, J. A. J. Anal. At. Specirom. 1992, 7, 971-77. (37) Vela. N. P.: Caruso. J. A. 1. Chromaiogr. in press: (381 Carey, J. M.; Vela, N. P.; Caruso, J. A. J. Anal. Ai. Spectrom. 1992, 7, 1173-81. (391 Olson, L.K.;Caruso, J. A.I. Anal. Ai. Specirom. 1992, 7, 993-98. (40)Taylor, H.E.;Garbarino, J. R.;Murphy, D. M.; Beckett, R. Anal. Ckem. 1992, 64.2036-41.
a&
losebh A. Camso recpiapd h i s Ph.D. from Miihigan State University in 196? He joined the faculty offhe University ofCincinnati in 1968. Since 1987he has sewed as Dean of the McMicken College ofArts and Sciences. His research interests include traditional and alternative plasma source studies as well as different trace metal speciation investigations.
Nohora t' V d a (IeP., received her Ph.11. from the hiversify of Cincinnati, where she is also a postdoctoral fellow. H e r research interests include the use of CC, SFC, and SFE coupled to plasma MSfor the determination oforganometallics. Lisa K. Olson received her Ph.D. from the University of Cincinnati, where she is also a postdoctoralfellow. Her research focuses on sample introduction methods, including chromatographic techniques, for plasma spectrometry a n d on inuestigations with alternative plasma sources.
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