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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
bonate alkalinity and p H of the solution affect the reversibility of the reduction, possibly through variations in buffer capacity and rate of C 0 2 hydration. Sensitivity of ASV in these solutions may be related to this mechanism. I n light of these results, it is recommended that shifts in stripping peak potentials or changes in stripping current magnitude with gross changes in solution composition of p H and alkalinity not be used to indicate changes in metal speciation. Reversibility of the overall electrode reaction can affect these measurements in a manner that is complex and uninterpretable unless the reaction mechanism is known. ASV theory of complex mechanisms needs further development and mechanisms of metal reduction in carbonate solutions need further investigation before shifts in potential can be used with certainty. Anodic stripping used with an amperometric titration for studying metal complex formation is preferred. Any change of reversibility that affects ASV sensitivity is directly observable in this procedure and reflected in the data subsequent to t h e titration end point (11).
LITERATURE CITED (1) W. R. Matson, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1968. (2) W. L. Bradford, Limnol. Oceanogr., 18, 757 (1973). (3) R. Ernst, H. E. Allen, and K. H. Mancy, Water Res., 9, 969 (1975). (4) T. A. O'Shea and K. H. Mancy, Anal. Chem., 48, 1603 (1976). (5) H. E. Allen, Ph.D. Thesis, University of Michigan, Ann Arbor. Mich., 1974. (6) A. Zirino and M. L. Healy, fnviron. Sci. Technol., 6, 243 (1972). (7) A. Piro, M. Bernhard, M. Branica, and M. Verzi, in "Radioactive Contamination of the Marine Environment", IAEA, Vienna, 1973. (8) R. W. Baier, J . fnviron. Qual., 6, 205 (1977).
(9) Y. K. Chau, R. Gachter, and K. Lum-Shue-Chan, J . Fish. Res. Board Can.. 31, 1515 (1974). (10) Y. K. Chan and P. T. S. Wong, in "Workshop on Toxicity to Biota of Metal Forms in Natural Water". International Joint Commission, Windsor, Ont.. 1976, p 187. (11) M. S. Shuman and G. P. Woodward, Jr., Environ. Scl. Technol.. 11, 809 (1977). (12) J. C. Duinker and C. J. M. Kramer, Mar. Chem.. 5, 207 (1977). (13) M. S. Shuman and G. P. Woodward, Jr., Anal. Chem.. 45, 2032 (1973). (14) F. Morel, R. E. McDuff, and J. J. Morgan in "Trace Metals and MetalOrganics Interactions in Natural Waters", Ann Arbor Science Publishers, Ann Arbor, Mich., 1974, p 157. (15) A . Zirino and S. Yamamoto, Llmnol. Oceanogr., 16, 779 (1972). (16) J. D. Hem and W. H.Durum, J. Am. Water Works Assoc., 65, 562 (1973). (17) D. T. Long and E. E. Angino, Geochim. Cosmochim. Acta, 41, 1183 ( 1977). (18) J. Vuceta, P h D Thesis, California Institute of Technology, Pasadena, Calif., 1976. (19) L. Meites, J . Am. Chem. Soc., 72, 184 (1950). (20) R . S. Nicholson and I.Shain, Anal. Chem., 36, 706 (1964). (21) R. S. Nicholson, Anal. Chem.. 37, 1351 (1965). (22) R. H. Wopschail and I.Shain, Anal. Chem., 39, 1514 (1967). (23) P. Dehhey, "New InstrumentalMethods in Electrochemisby", Interscience, New York, 1954. (24) R. S. Nicholson, Anal. Chem., 37, 667 (1965). (25) A. A. Vicek, in "Progress in Inorganic Chemistry", F. A. Cotten, Ed., Interscience, New York, 1963, p 211. (26) L. Meites, "Polarographic Techniques", Wiley-Interscience, New York, 1963. (27) W. Stumm and J. J. Morgan, "Aquatic Chemistry", Interscience, New York, 1970. (28) D. M. Kern, J , Chem. fduc., 37, 14 (1960).
RECEIVEDfor review June 1,1976. Resubmitted May 11,1978. Accepted October 5 , 1978. Work supported by funds from the Oceanographic Section, National Science Foundation, NSF Grant OCE73-21045
Measurement of Organomercury Species in Biological Samples by Liquid Chromatography with Differential Pulse Electrochemical Detection W. A. MacCrehan" and R. A. Dursl Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
A new measurement approach for organomercury cations is described employing liquid chromatography with electrochemical detection. Special considerations in constructing apparatus to optimize reductive electrochemical measurement are outlined. The added selectively of the differential pulse mode of detection is demonstrated. A charge-neutralization reversephase separation of methyl-, ethyl-, and phenylmercury cations has been developed. The limit of detection for methylmercury is 2 ng/g or 40 pg (190 fmol/20 1 L sample). Methylmercury is determined by the technique in two research materials-tuna fish and shark meat.
Many metal and metalloid elements can be converted by biological processes into organometals (I, 2). Also the alkyl and aryl compounds of Hg, Sn, P b , and As are finding increasing use in industrial manufacturing and pest control (2). In order to investigate fully the role of toxic metals in environmental and biological systems, it is necessary to measure the exact chemical form of the element. T h e measurement of organometals in complex "real world'' samples presents two difficult analytical requirements: high selectivity and very low detection limits. Perhaps the most
frequently used approach for the analysis of the cationic organometals is derivatization with a halogen and subsequent gas chromatography (GC) with electron capture detection (3, 4 ) . Detection limits with this approach are quite good, 2 ng/g for CH,HgBr, for example (3). However, the relatively poor selectively of this detection approach requires rather extensive sample "cleanup" before analysis ( 4 ) . Recently, the selectivity problem has been overcome by the use of element-specific detection such as microwave cavity emission detection ( 5 , 6). However, the gas chromatographic separation requires thermally stable, strong complexes of the cationic organometals to be made by derivatization before analysis. An alternate approach for the measurement of organometal species is to use liquid chromatographic (LC) separation coupled with selective detection. Graphite furnace atomic absorption (GFAA) has been used as a detection system with an automatic, periodic sampling and atomization of the chromatographic eluent. With the autosampled mode, the eluent can be sampled, desolvated, and atomized only every 50 s ( 7 ) . This sampling period is much too long to preserve the integrity of a high efficiency separation. T h e detection limits for the GFAA approach for LC detection are relatively poor (7). This reflects the difficulties in sampling, the poor atomization efficiency of volatile organometals, and the
This article not subject to U.S. Copyright. Published 1978 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
relatively poor detection limits of GFAA for many environmentally important elements such as Hg, As, Sb, and Sn. This paper describes an LC detection approach based on the electrochemical reduction of the cationic organometals. Previous work has shown that many important organometal compounds of mercury, tin, lead, arsenic, and antimony are reduced a t potentials below -1.1 V (8,9). An amperometric mode of detection has been used to monitor the reverse-phase separation of such species as methyl-, ethyl-, and phenylmercury and also trimethyl-, and triethyllead (9). T h e selectivity of the electrochemical detection approach can be further enhanced by the use of a differential pulse waveform
2109
DIFFERENTIAL PULSE DETECTION
(10, 11).
There are several important considerations in the development of a system to employ reductive electrochemical detection in liquid chromatography. First, the separation modes are limited, by the requirement of a conductive electrolyte, to only ion-exchange and reverse-phase chromatography. Second, a working electrode material must be chosen t h a t combines high sensitivity with a wide negative potential range. Early work in the reductive detection of liquid chromatographic effluents (12, 13) employed the familiar dropping mercury electrode (DME). Besides being very cumbersome to use, this electrode is characterized by relatively poor detection limits (IO4 mol/L). However, solid electrodes often provide superior sensitivity. T h e negative potential range of solid materials is generally relatively poor. However, a layer of mercury greatly extends the useful negative potential range. Platinum has been used as a substrate for a coating of mercury in recent work (14, 15). In our work, a gold electrode has been used because the hydrogen overvoltage on gold amalgam is greater (0.80 V vs. 0.10 V) than on a platinum amalgam (16). T h e gold amalgamated mercury electrode (GAME) provides a negative potential range to about -1.2 V for amperometric detection (at pH 5.5) and gives reproducible results for an entire day’s operation. Another important consideration is the purification of the chromatographic solvent of reducible species. Oxygen can be removed to very low levels by purging the solvent reservoir continously with ultrapure nitrogen (0, < 0.5 ppm). Other reducible impurities are removed by electrolytic reduction a t a mercury pool. In order to maintain the purity of the solvent, it is necessary to enclose the entire chromatograph in a nitrogen-purged box to avoid reentry of oxygen through Teflon components in the pump, sampling valve, and detector cell (17). T h e final consideration in reductive electrochemical detection is the type of waveform applied to the cell. T h e simplest approach is to use amperometry where a constant potential is applied to the working electrode and the resulting current is monitored. In this approach, all species with reduction potentials below t h a t applied will give a response. Amperometric detection provides excellent detection limits mol/L) for use with a solid electrode (10) and (about 5 x has good selectivity for easily reduced analytes. However, when the analytes of interest have reduction potentials approaching -1.0 V, the number of possible interfering electroactive species becomes larger. Frequently, improvements in the chromatography or prior separations can be used to decrease the interference problem (18). However, more selective detection offers the advantage of minimum sample “cleanup” prior to analysis. T h e differential pulse mode of detection offers a substantial increase in selectivity over amperometry (10, 11). In Figure 1, for example, electroactive species B will have a large current response when the potential is pulsed from the applied potential E to E + AE. T h e differential readout of the current measured before and a t the end of the pulse will provide a signal proportional to the
APPLIED POTENTIAL
Figure 1. Hydrodynamic voltammetry of three reducible, coeluting specI e s concentration of B only, A and C will not be detected at potential E with the pulse height ( Semployed ) in the figure.
EXPERIMENTAL Apparatus. The apparatus is described in detail elsewhere (9). A commercially available electrochemical detector cell design (Bioanalytical Systems Inc.) was used with some modification. The electrode holder was made from a inch X 1 inch X 1 inch Plexiglas block. A hole was drilled to accept a 1.0-mm diameter high purity gold wire. positioned in the same place as the original carbon paste well. The wire is epoxied into place and the block is smoothed with wet no. 600 emery paper, followed by polishing with Raybrite A gem polish. A shiny layer of mercury is applied by floating the electrode block in a pool of mercury for 5 min and wiping off the excess adhering mercury. The reference (Ag/AgCl with 3 mol/L C1-) and auxiliary electrodes have been mounted by drilling a Teflon adapter (1/4 inch-28 to 1 / 2 inch NPT) and press fitting the electrodes. This modification positions these two electrodes closer than 1.0 cm to the working electrode and provides an exit dead volume of the detector cell of less than 100 pL (the effective dead volume to the working electrode is less than 1pL). A Princeton Applied Research model 174 Polarographic Analyzer is used to apply a constant potential or differential pulse waveform with sensitivity of 10 V output for a 20-nA signal, and pulse heights of 5 to 100mV with pulse times of 0.5 and 1.0 second. Reagents. The reagents used to prepare the ammonium acetate buffer were Merck Suprapur. The methanol was Burdick and Jackson distilled-in-glass, The methyl- and phenylmercuric acetates and ethylmercuric chloride were obtained from Alfa Inorganics Inc. and were used without further purification. The methylmercury solutions were standardized by constant-potential coulometry. It was necessary to prepare fresh solutions of the organomercury cations weekly. All other reagents were of ACS reagent grade. Sample Preparation. Organomercury cations are not stable to the acidic/oxidative digestion procedures frequently used for trace metals in biological materials. However, alkaline hydrolysis can be applied, providing a method for removing most of the solid matrix. The hydrolysis is followed by acidification with hydrochloric acid. The organomercury cations can then be extracted from the aqueous solution with toluene as the neutral chloride complexes. The procedure employed in this work for the fish sample preparation for the determination of methylmercury follows the recommendation of the Analytical Methods Committee of the Chemical Society ( 4 ) ,with some modification. The sample and reagent amounts were reduced by 4 / 5 . The aqueous backextraction solution used was 0.01 mol/L disodium thiosulfate (3) buffered to pH 5.5 with 0.05 mol/L ammonium acetate. This extraction solution was compatible with our chromatographic separation, and the determination was performed directly on this aqueous extract after filtering through a 0.2-i~rnsyiinge filter. The procedure eliminates the final toluene extraction. In all cases, a standard additions procedure was used for the determination with known amounts of diluted CH3Hgf solution added to the solid material before the hydrolysis step. The
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1
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
T
1 nA
1
~ INJECT
2
I
4
I
6
I
8
I
I
I
I
10 12 14 16
MtHg'
I
MINUTES Separation of three organomercury cations. Conditions: column, Altex Spherisorb ODS (5 Fm), 25 X 0.46 cm; eluent, 40% methanoVwater, 0.06 moVL NH,OAc (pH 5.5), 0.01 % v/v ME; flow rate, 1.0 mL/min; pressure, 800 psi, detector, differential pulse mode mol/L at -0.70 V, 100 mV pulses (-), 0.5 s/pulse; sample, 1 X MeHg', EtHg', and PhHg' Figure 2.
recovery was checked by comparison to a standard curve and found to be about 95%.
RESULTS AND DISCUSSION Separation of Organomercury Cations. T h e liquid chromatographic separation of methyl-, ethyl-, and phenylmercury can be efficiently accomplished in a reverse-phase system by the formation of their neutral 2-mercaptoethanol (ME) complexes (9). Figure 2 shows the separation and detection of these species in the differential pulse mode. This type of "charge-neutralization reverse-phase" separation is more advantageous than simple cation-exchange chromatography for work with real samples. Commercially available bonded-phase ion exchangers employ sulfonate and carboxylate anionic groups to act as fixed ligands for the analyte cations. However, for the organomercury cations (and many heavy metal ions), the sulfonate and carboxylate complexes are very weak, especially in comparison to ligands such as halides, cyanide, sulfide, and mercaptans. Such strong ligands in the sample will directly interfere with the chromatography of these cations unless a prior separation is performed to eliminate the complexing anions. A more elegant approach employs strong complexation of the analytes in the chromatographic separation, either by adding them to the stationary phase or to the solvent in a reverse-phase system. In our separation, M E forms very strong complexes with the organomercury cations (e.g., the formation constant for CH,HgME is 10l6) (19). Weaker ligands such as halides and cyanide do not interfere with the separation (17),leaving only other strong ligands such as mercaptans and sulfide as potential interferents. This remaining interference problem can be overcome in the sample preparation ( 4 ) . Another important type of interference encountered in this chromatographic separation is the simultaneous co-elution of other reducible sample constituents with the analytes. Figure 3A illustrates the interference of reducible metals (Cd2+,Pb2+, and Cu2+) on the measurement of methylmercury in the amperometric mode of detection. This interference can be eliminated by any of three methods. T h e chemistry of the separation can be improved, so that the divalent ions elute a t less critical elution volumes. Figure 3B illustrates this approach, where EDTA has been added to the samples prior t o the injection. T h e EDTA apparently forms stronger complexes with t h e divalent ions than does the ME, but methyl- and ethylmercury do not. A second method to avoid
k
h
6
8
INJECT
MINUTES
Eliminating the interference of heavy metals on the organomercury measurement. (A) illustrates simple amperometric detection at -0.83 V of a 5 X mol/L solution of MeHg', Cu2+,Cd2+and Pb2+. (B) shows the same conditions except lo-, mol/L EDTA has been added to the sample. (C)is the chromatogram of the same sample as A except the detection is in the differential pulse mode of detection at -0.74 V (100-mV pulses (-)). Figure 3.
v)
t L
a
&
20
U
a
c s m
a
g
10
2 w I
x
2 n o APPLIED POTENTIAL (IN V )
Current-potential response of the three organomercury cations in the differential pulse mode. Points taken with the same conditions as Figure 3 except the pulse height is 5 mV Figure 4.
this type of interference is to use a prior separation step (11). Finally, the selectively of the detector can be increased by employing a differential pulse waveform, which will limit the detector response to a small "potential window". Figure 3C illustrates the ability of this mode of detection to discriminate against other reducible species. This final approach has the advantage of minimizing sample manipulation before analysis. Differential Pulse Detection (DPD). There are several parameters that must be optimized to achieve maximum sensitivity (or selectivity) in DPD. The detector potential can be optimized by repetitive injection a t different applied potentials (as shown in Figure 4) or by slow scanning vol-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
O
I
2111
Table I. Methylmercury Content of Fish Samples m e r c u r y species in p g / g ___sample MeHg' EtHg' PhHg' total Hg nda nd 0.95 t 0.1 0.93 I0.1 RM 5 0 A l b a c o r e
_ _ _ _ I _
Tuna Japanese paste
a nd =
5
25
100
PULSE H E I G H T ( I N m V )
Figure 5. Signal-to-noise ratio X lo-' mol/L MeHg' sample
as a function of pulse height for a 5
tammetry in a stopped-flow mode. The electrochemical reduction potentials of these three organomercury species are so close, it is possible to choose a compromise detection potential of 4 . 7 0 V for simultaneous measurement (as shown in Figure 2) with only a small penalty in sensitivity for each. T h e best detection limits are obtained using the largest pulse heights as shown in Figure 5. However, little is lost in detection limits when 25-mV pulses are used rather than 100-mV. As a practical matter, the nature of the sample may determine the optimum pulse height depending on the selectivity required to measure the analyte without interference. T h e pulse repetition period should be as short as possible, 0.5 or 1.0 s (without subsequent RC filtering) to preserve the fidelity of the detector response to the chromatographic separation. We have observed little distortion of the CH3Hg+ peak (at about 10 effective plates/s) using the 0.5-s pulse period (and associated time constant of the analog sampleand-hold circuitry), as compared to the unfiltered amperometric response. However, 1.0-s period produces some peak broadening a t these efficiencies. The D P D provides very good detection limits. A solution of 1 X niol/L which is 2 ppb of methylmercury will provide a peak about two times the peak-to-peak noise level for a 20-gL sample injection (17). The detection limits in the amperometric mode are similar in our system, making the selectivity the primary criterion for choice of detection waveform. A linear dynamic range of measurement has been observed between lo-' to inol/L with DPD (17). One further advantage of electrochemical detection is the qualitative information inherent in the detector response. The current response variation with potential will be fairly unique, as it depends on the number of electrons transferred in the electrode reaction per mole of analyte, the reduction (or oxidation) potential of the species, and the reversibility of the electrochemical reaction. T h e analyte current-potential response can be evaluated even a t very low analyte concentrations, by multiple injections of the sample a t different applied potentials. The response can then be compared to standards providing further confirmation of the analyte identity over and above the retention volume. Sample Analysis. The biological materials that were taken for methylmercury determination are the two first attempts at a reference material for trace metals in fish. The National Bureau of Standards has prepared Research Material 50Albacore T u n a from carefully homogenized tuna fillet. The tissue was ground and mixed using stainless steel equipment, lyophilized, and finally packaged in polyethylene bags under nitrogen (20). RM 50 is available from the Office of Standard
shark not
8.41
3
0.1
(20)
nd
nd
'7.4 (22)
detected.
Reference Materials of NBS. Another material, a '*Wet" Shark reference material was prepared by the Department of Agricultural Chemistry of the University of Tokyo. The white shark muscle was homogenized and NaC1, starch, and a preservative AF-2 were added. The paste was then steamed (as in Japanese cuisine) and placed in sterilized Pyrex bottles (21). We have used a lyophilized sample of the shark referenci: material in this work. Both fish research materials have been analyzed for a number of trace metals including total mercury. The homogeneity of these samples is quite good for Hg, for RM 50 about 1070 (RSD) for a 0.25-g sample size and about 6% (RSD) for a 2-g sample of the shark meat reference. For the methylmercury determination in these two niaterials, duplicate 1.00-g samples were taken and also two other duplicates were spiked with known amounts of methylmercury. The sample chromatograms were characterized by a siiigle response for methylmercury with a high signal-to-noise ratio. Ethyl- and phenylmercury were not detected in these saiiiples under conditions similar to Figure 3C. T h e identity of the peak corresponding t o methylmercury was verified by comparison of the potential-current response function to a standard. The standard additions curves were drawn for quantitative analysis. A linear result was obtained for each. The results obtained (see Table I) for the methylmercury content of the fish samples were in fairly close agreemait to the total mercury (as measured by alternate technique such as atomic absorption and neutron activation analysis). This high proportion of methylmercury to total mercury in tissues is consistent with the results of other workers (23-251, T h e electrochemical detection approach in liquid chromatography shows great potential for the selective m a surement of trace orgaiiometals in environmental samples. Work is in progress on the separation and measurement of other organometal cations, including tri-n-butyltin, trimethyllead, and triethyllead.
LITERATURE CITED (1) W. P. Ridley, L. J. Dizikes, and J. M. Wood, Science, 197,325 (lY77). (2) J. S . Thayer, J . Organometal Chem., 76,265 (1974). (3) C. J. Cappon and J. C. Smith, Anal. Chem., 49, 365 (1976). (4) Analytical Methods Committee, Analysf (London), 769 (1977). ( 5 ) Y. Talmi. Anal. Chim. Acta, 74, 107 (1975). (6) D. C. Reamer, T. C. O'Haver, and W. H. Zolier in "Methods and Sta#;&rtis for Environmental Measurement", Natl. Bur. Stand. Spec Pub/, 464
609 (1977). (7) F. E. Brinckman, W. R. Blair, K. L. Jewett, and W. P. Iversoii, J Chromatogr. Sci., 15, 493 (1977). (8) S. G. Mairanovskii, Russ. Chem. Rev., 45, 298 (1976). (9) W. A. MacCrehan, R. A. Durst, and J. M. Bellama, Anal. Lett ., 10, 1175 (1977). (IO) D. G.Swartzfager, Anal. Chem., 48, 2189 (1976). (11) W. A . MacCrehan, R. A. Durst. and J. M. Beilama, in "Trace Orgaflic Analysis: A New Frontier in Analytical Chemistry". Natl. Bur-. Stand. Spec. Publ., in press. (12) W. Kemula, Rocz. Chem.. 26, 281 (1952). (13) E . Pungor, K. Toth, 2s. Feher, G. Nagy, and M. Varadi, Anal. Len.. 6 (12),ix (1975). (14) T. Wasa and S. Musha, Bull. Chem. Soc. Jpn., 48, 2176 (1975). (15) R. C. Buchta and L. J. Papa, J . Chmmafogr. Scl., 14, 213, (1976). (16) A. G.Stromberg and E. C. Zakharova, Zavod. Lab.. 30, 261 (1964). (17) W. A. MacCrehan, P h D Thesis, University of Maryland, College Park. Md., 1978. ( 1 8) P. T. Kissinger, "Measurement of Catecholaniines and Their M e i a i x l k s in Tissue and Physiological Fluids Using Reverse-Phase i i q d d Giro mstography with Electrochetnical Detec:ion", 9th Ma:&"kis Resce:::,
2112
(19)
(20) 1211 (22) (23) (24)
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978 Symposium, National Bureau of Standards, Gaithersburg, Md., April 10, 1978. R. C. Zepp, G. F. Baughman, N. L. Wolfe. and D. M. Cline, Environ. Left 6, 117 (1974). P. D. LaFleur and W. P. Reed, Report of Investigation- Research Material 50-Albacore Tuna. Y. Dokiva. M. Taouchi. s. Toda. and K. Fuwa. Anal. Chem.. 50. 533 (1978) T C Rains, Report of Analysis G Westoo, Acta Chem Scand , 20 213 (1966) G Westoo, Scence, 181, 567 (1973)
(25) C. A. Bache, W. H. Gutenmann, and D. J. Lisk, Science, 172, 951 (1971).
.I
RECEIVED for review July 28, 1978. Accepted September 19, 1978. T h e authors thank the ~ ~ ~ , i ~ Protection Agency for Partial support of this work through t h e Interagency Energy/Environment Program (EPA-IGA-D5-E684). T h e specification of commercial products does not imply endorsement by the National Bureau of Standards.
Atmospheric Pressure Helium Microwave Detection System for Gas Chromatography Bruce D. Quimby, Peter C. Uden," and Ramon M. Barnes Department of Chemistry, GRC Tower I, University of Massachusetts, Amherst, Massachusetts
A microwave emission detection system for gas chromatography has been devised which utilizes the TMoraresonant cavity to sustain a plasma in helium at atmospheric pressure. The effluent from the gas chromatograph is split between a flame ionization detector and a heated transfer line directing it to a small auxiliary oven containing a high temperature valve. The valve allows the effluent to be directed either to a vent or to the plasma. Atomic emission from the constituent elements of species entering the discharge is observed axially with an echelle grating spectrometer. The system allows for highly selective and sensitive detection of elements of interest by monitoring an appropriate wavelength corresponding to that element. Performance of the system is described in terms of linearity, selectivity ratios, and detection limits for the halogens, silicon, phosphorus, sulfur, lead, mercury, and manganese.
Element selective detectors have been an important means of simplifying difficult separation problems in gas chromatography for many years. When the compound or compounds of interest contain a particular element not present in the other components of t h e sample matrix, the utilization of a detector which responds solely to that element generally leads to a significant reduction in the chromatographic requirements of the analysis, since only those compounds containing the element of interest need be separated from each other, and not necessarily from other components in the sample. T h e desirable characteristics of an element specific GC detector are that it should be capable of monitoring any of a large number of elements with a high degree of selectivity and sensitivity, simple to operate and maintain, and compatible with the wide range of gas chromatographic techniques currently being employed. A microwave emission detector (MED), first described by McCormack, Tong, and Cooke ( I ) , appears to be well suited for these purposes. With this detector, compounds eluting from the gas chromatograph are directed into a microwave discharge which is sustained in either argon or helium. Observation of the optical emission spectrum resulting from the fragmentation and excitation of compounds entering the plasma affords sensitive, element selective detection. The MED has been demonstrated to be useful for the selective 0003-2700/78/0350-2112$01.00/0
0 1003
detection of several metallic elements including Hg ( 2 , 3 ) ,Cr ( 4 , 9), A1 (8, 9), Cu, Ga, Fe, Sc, V (81,and Be (9) in addition to many nonmetallic elements such as Se ( 5 ) ,As (6, 7 ) ,Sb (a, Si (IO),P , S, Br, C1, I(11, 12), C, H, D, N, and 0 (12). With the types of resonant cavities employed in the above studies, discharges can be sustained in argon at either atmospheric or reduced pressure (usually 5-50 Torr) and in helium a t reduced pressure only. Line emission is observed for all elements in the helium plasma, while in the argon plasma F, C1, Br, N, and 0 exhibit only diatomic molecular spectra. Since the measurement of band spectra has presented problems when applied for selective detection ( I , 13), the helium plasma, although somewhat less convenient to operate, is preferred because of its wider range of applicability. T o overcome the necessity of operating the helium plasma at reduced pressure, Beenakker (14-16) introduced the TMol0 cylindrical resonant cavity. Owing to its increased efficiency for transfer of microwave power to the discharge, an atmospheric pressure helium (or argon) plasma can be sustained a t the same low power levels as used with previous cavities. A further advantage offered by this design is the ability to view light emitted from the plasma axially. With earlier cavities in which the helium plasma is operated a t reduced pressure, the plasma is usually viewed transversely through the walls of the quartz discharge tube. Deposition of materials on the discharge tube walls and devitrification of the quartz when using the helium plasma result in gradual attenuation of sample response with time. The addition of small amounts of oxygen or nitrogen to the helium to act as a scavenger gas (12)reduces carbonaceous deposits, but deposition of metals and devitrification still present limitations. Since the plasma can be viewed directly when the TMolocavity is employed, situations of this type are avoided. Using an exponential dilution flask, Beenakker ( 2 5 ) assessed the analytical performance of the atmospheric pressure helium plasma for the selective detection of C, H, C1, Br, I, and S, and found the detection limits to be considerably lower than those attained with earlier cavity designs. The construction of a GC-MED system incorporates three fundamental units, the source, the monochromator-readout system, and the interface to the gas chromatograph. T h e atmospheric pressure helium plasma was chosen for reasons discussed above. T o accommodate the wide range of elements to be monitored, the monochromator operates over a waveC 1978 American Chemical Society
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