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(3) Abboud. J. L.; Taft, R. W. Phys. Chem. 1977, 8 3 , 412. (4) Brady, J. E.; Carr, P. W. J. Am. Chem. SOC. 1985, 8 3 , 412. (5) Brady, J. E.; Carr, P. W. J. Phys. Chem. lS82, 8 6 , 3053. (6) Brady, J. E.; Carr, P. W. J. Am. Chem. SOC. 1985, 8 9 , 1813. (7) Brady, J. E.; Carr, P. W. Anal. Chem. 1982, 5 4 , 1751. (8) Bekarek, K.; Jurina. J. Collect. Czech. Chem. Commun. 1982,4 7 , 1060. (9) Abboud, J. L. M.; Karnlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1977, 9 9 , 6325.
(IO) Chawla, 8.; Pollack, S. K.; Lebrllla, C. B.; Karnlet, M. J.; Taft, R . W. J. Am. Chem. SOC. 1981, 103, 6924. (11) KamJet, M. J., personal communication, June 1967. (12) Karnlet, M. J.; Kayser, E. G.; Jones, M. E.; Abboud, J. L.; Eastes, J. w.; Tan, R. W. J . phvs. Chem. 1978,8 2 , 2477. (13) Melander, W. R.; Horvath, C. I n High Performance Liquid Chromatography-Advances and Perspectives ; Academic: New York, 1980;Vol. 2, p 113. (14) Kamlet, M. J.; Abboud, J. L.; Tan, R. W. Prog. Phys. Org. Chem. 1981, 13, 485. (15) Chastrette. M.; Ragzmann. M.; Chanon, M.; Purcell, K. F. J. Am. Chem. SOC. 1988, 107, 1. (16) Relchardt, C. I n Molecular Interactions ; Ratajczak, H.. Orville-Thornas, W. J., Eds.; Wlley: Chichester, 1982;Vol. 3. (17) Krygowski, T. M.; Wrona, P. K.; Zielkowska, U.; Reichardt. C. Tetrahedron 1985, 4 1 , 4519. (18) Krygowski, T. M.; Reichardt, C.; Wrona, P. K.; Wyszornirska, C.; Zielkowska, U. Chem. Res. ( S ) 1983, 116. (19) Johnson, B. P.; Khaledi, M. G.;Dorsey, J. G. J. Chromatogr. 1987,
384,221. (20) Johnson, 8. P.;Khaledi. M. G.; Dorsey. J. G. Anal. Chem. 1988, 5 8 , 2354.
(21) Sadek, P. C.; Carr, P. W.; Doherty, R. M.; Karnlet. M. J.: Taft, R. M.; Abraham, M. H. Anal. Chem. 1985, 5 7 , 2971. (22) Carr, P. W.; Doherty, R. M.; Kamlet. M. J.; Taft, R. M.; Melander, W.; Horvath, C. Anal. Chem. 1988,5 8 , 2674. (23) Tirnmerrnans, J. Physicochernlcal Constants of Binary Systems in Concentrated Solutlons; Intersclence: New York, 1960;Vol. 4. (24) Cheong, W. J.; Carr, P. W. J. L i q . Chromatogr. 1987, 10, 561. (25) Moreau, C.; Dougheret, G. Chem. Thermodyn. 1978, 8 , 403. (26) Langhals, H. Angew. Chem., Int. Ed. Engl. 1982. 21, 724. (27) Langhals, H. Nouv. J. Chim. 1981, 5 , 97. (28) Kolling, 0.W. Anal. Chem. 1985, 5 7 , 1721. (29) Kolling, 0.W. Anal. Chem. 1984, 5 6 , 2968. (30) Balakrishnan, S.; Easteal, A. J. Aust. J. Chem. 1981,3 4 , 943. (31)Nevecna, T.; Bekarek, V. Collect. Czech. Chem. Cornmun. 1988, 5 1 , 1942. (32) Davis, M. J.; Douhert, G. Thermochim. Acta 1986, 104, 203. (33) Nevecna, T.; Vymetalova, J.; Bekarek, V. Collect. Czech. Chem. Commun. 1988, 5 1 , 2071. (34) Marsh, K. N. Spec. Period. Rep. (Chem. SOC.,London) 1973, 2 , Chapter 1.
(35) Taft, R. W., unpublished work, July 1987. (36) Katz, E. D.; Ogan, K.; Scott, R. P. W. J. Chromatogr. 1986,352, 67. (37) Reichardt, C. Solvent Effects in Organic Chemistry, 1st ed.; Veriag Chernie: Weiheirn. 1979.
RECEIVED for review August 3, 1978. Accepted December 21, 1987. This work was supported in part by a grant from the Graduate School of the University of Minnesota and by the National Science Foundation.
CORRESPONDENCE Alternating Current Plasma Detector for Selective Mercury Detection in Gas Chromatography Sir: Gas chromatography (GC) coupled with elementspecific detectors has been employed in many applications to simplify the interpretation of complex chromatograms. Desirable characteristics of an element-specific detector are that it should be highly specific for a wide range of elements, sensitive for these elements over a wide linear range, simple to construct and maintain, and stable under many application conditions. Spectroscopic-based detectors, such as plasma emission detectors (PED), satisfy these requirements and are commonly employed as element-specific detectors. There are three principal plasma emission sources employed for GC/ PED, namely, microwave-induced plasma (MIP), inductively coupled plasma (ICP), and direct current plasma (DCP). The plasma emission detectors are well suited for element-specific detection due to their high selectivity, sensitivity, and compatibility with GC helium effluent flow rates (I). In addition, PED also have the advantage of being multielement detectors to provide valuable quantitative and qualitative data not readily available from the more conventional GC detectors. Selective detectors, such as the MIP detector, are commonly employed and exhibit high sensitivity and selectivity for both organometallics and nonmetals (2-7). However, these detectors do suffer from some disadvantages including solvent extinguishment and complexity in design. This work describes the design, construction, and evaluation of a novel mercury-specific helium alternating current plasma detector (ACPD) for GC. The ACPD was designed to overcome some disadvantages suffered by other element-selective detectors. The power source employed for the detector produces a stable, self-seeding plasma. The detector design includes a tee configuration discharge tube and a heated fused silica-coated megabore capillary tube as the effluent transfer 0003-2700/88/0360-0828$01.50/0
tube which reduces band broadening and offers improved sensitvity. The ACPD is evaluated for sensitivity, selectivity, and linear dynamic range with methylmercury(I1) chloride and ethylmercury(I1) chloride.
EXPERIMENTAL SECTION Discharge Tube Design. A schematic diagram of the components of the ACPD is shown in Figure 1, while the plasma emission tube, which is an essential component of the ACPD, is shown in Figure 2. The body of the tube is in a tee configuration and constructed of 'I4 in. 0.d. X lIg in. i.d. borosilicate glass tubing. The vertical electrode arms (3 in.) house the electrodes and transport the helium make-up gas to the plasma. The window arm (1.5 in. in length) is situated perpendicularly to the plasma discharge and electrode arm plane. The plasma discharge appears at the intersection of the arms; a quartz window or biconvex lens is fastened on the end of the window arm by silicon adhesive, forming an airtight tube. An important feature of the tee design is the window arm in which the helium flow carries degradation products produced by the plasma discharge up and away from the window arm which results in the reduction of signal attenuation. Straight tube designs often were coated by degradation products, resulting in a loss in sensitivity due to blocked emission and has been a problem in previous work (8). The airtight electrode holders, depicted in Figure 2, are a second feature of the discharge tube tee design. The holders are constructed of two 1/4-in.Teflon tee unions (Cole-Parmer Instrument Co., Chicago, IL, catalog no. T-6374-42)attached to the end of each vertical electrode arm via Teflon 'I4 in. ferrule nuts (Cole-Parmer Instrument Co., Chicago, IL, catalog no. 5-6374-82). in. Teflon The perpendicular arm of each tee is fitted with a 'I4 ferrule nut containing silicon rubber septa (Hamilton Co., Reno, NV, catalog no, 75805, 1 cm diameter) in which the electrodes are inserted. The electrodes are then positioned in the vertical 1988 American Chemical Society
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Table I. General Operating Conditions for the GC-ACPD Unit Chromatographic helium grade 99.99% molecular seive 5A and Supelpure HC 2-2445 and 0 2-2449 (Supelco, Bellefonte, PA) Hewlett-Packard 5890A gas chromatograph chromatograph 30 X 0.526 mm fused silica capillary column column, coated with 1.5 FM DB-1 (J and W Scientific, Folsom, CA, catalog no. 1251032) column flow (helium) 10 mL/min injection volume 1 rL split ratio 25 to 1 injector temp 210 "C column temp 150 "C interface temp 170-180 "C gas gas traps
U
Figure 1. A schematic diagram of the GC-ACPD system: (A) column oven; (B) low dead volume tee; (C) FID; (D) heated transfer tube and make-up tee; (E) ACP discharge tube; (F) optical mounts; (0)lens; (0) ac power source; (H) monochromator; (I) PMT; (J) PMT power source; (K) picoammeter; (L) low-pass filter; (M) recording integrators; (N)
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electrode arms of the tube and gapped at 4 mm. For this study, aluminum, in. X 6 in. long, electrodes were employed. This design provided an airtight seal and permitted easy electrode adjustment and removal. Design of the Detector Interface. The plasma discharge tube has an inherently large dead volume which produces excessive band broadening and loss in sensitivity. In order to minimize detector dead volume, a 2-m length of an actual fused silica megabore DB-1 capillary column served as the interface tube in all work done. The transfer tube was connected to the analytical megabore DB-130-m column in the oven via a low dead volume tee split. Half of the column effluent was directed to the ACPD while the balance was delivered to a flame ionization detector via an identical megabore tube. The connection of the transfer tube to the ACPD was done by inserting the end into the discharge tube, using the appropriate fittings, and positioning it so that the tube parallels the bottom electrode and terminates 3-7 mm from the plasma discharge (Figure 2).
Spectroscopy 4 mm Furnace Ignition Transformer (France, Fairview, TN, catalog no. 5 LAY 04) 40-290 W GCA/McPherson scanning monochromator series EU-700 300 pm 5 mm 8.0 A
253.67 nm 40-50 mL/min McPherson Model 7640, McPherson, Acton, MA -825 V dc RCA 1P28 75 mm biconvex lens, Oreil, Stratford, CT, catalog no. 1734
Instrumentation Model 414s (Keithley Instruments, Cleveland, OH) 0.20 s time constant Hewlett-Packard 3393A
The entire transfer tube is jacketed in a flexible metal tube in. i.d.) and wrapped with heating tape (Fisher Scientific, Pittsburgh, PA, catalog no. BIH101-020), glass wool, and glass electrical tape in order to minimize condensation. Three chromel-alumel thermocouples were employed for interface temperature monitoring throughout the experiment. For all organomercury separations, the transfer tube temperature was kept a t least 50 "C above the oven temperature. A helium make-up flow was added into the flexible metal tube of the heat jacket by means of a 1/4 in. tee. The helium flow traveled along the outside of the inner megabore transfer tube into the discharge tube itself. Teflon fitting nuts were used to fasten the jacket tube to the lower electrode holder tee. The flow continued along the transfer tube in the discharge tube until the GC effluent and make-up flows merged in the plasma. The make-up flow provided a stable plasma, even at low column flow rates. The plasma discharge tube was attached to a fully adjustable optical mount (Edmund Scientific, Barrington, NJ, catalog no. E60,816) such that the inlet arm was positioned at the bottom of the mount. The optical mount fastened onto a linear optical bench. This arrangement allowed the plasma tube assembly to be moved in every plane, providing fast and easy focusing of the plasma emission image onto the entrance slit of the monochromator. Chemicals. Methylmercury(I1) chloride, ethylmercury(I1) chloride, and diphenylmercury were purchased from Morton Thiokol, Inc. (Alfa Products, Danvers, MA). All analytical and calibration solutions were obtained by diluting the stock solution with pesticide-grade benzene (Fisher Scientific, Pittsburgh, PA) to the appropriate concentration.
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Detector Operation. The apparatus and optimal conditions for the GC/ACPD are listed in Table I. After the attachment of the interface to the ACPD, make-up helium gas was initiated in the plasma tube to allow the entire tube to purge free of oxygen and nitrogen. The GC oven, inlet, and FID were then heated to the desired temperature. Usually, the FID and inlet were maintained at 220 "C for this study. The plasma interface tube was then heated to at least 50 OC above the temperature of the column oven. Next, the current suppression of the picoammeter and full scale range were set to the appropriate levels. The plasma plume was initiated by starting the furnace ignition transformer set at 290 W ac. The ac power range for this detector was 40-290 W, at which all work was done. The entire system was allowed to warm-up for 15 min to provide a stable plasma for trace analysis. The system was optimized for sensitivity and standard solutions of methyl- and ethylmercury(II) chloride were then run to produce calibration curves and determine detector sensitivity and selectivity. Finally, an organomercury solution of gasoline was prepared by the dissolution of 92 and 39 ng/& of methyl- and ethylmercury(I1)chloride, respectively. A l-wL aliquot of the spiked gasoline was injected and detected by both the ACPD and FID.
RESULTS A N D DISCUSSION Plasma Stability. The present power supply provides a very stable, self-seeding plasma in the power range of 40-290 W ac. The plasma does not appear as a well-defined arc but as a blue, diffuse plasma plume. The intensity of the atomic emission line Hg I is greatly dependent on the ac power output, as shown in Figure 3. Above 40 W ac, a relatively stable and self-seeding plasma could be initiated; however, detector sensitivity was poor. As seen in Figure 3, the emission intensity of Hg I at 253.67 nm is maximized a t 290 W ac. All subsequent mercury detections were done at the upper limit of the power supply of 290 W in order to obtain the most stable plasma and the highest detector sensitivity. Detector Sensitivity and Linearity. Response data was obtained by injecting known amounts of methyl- and ethylmercury(I1) chloride into the GC/ACPD and averaging the peak areas of each mass of each mercury compound injected, as depicted in Figure 4. From the response data and calibration curve, detector sensitivity and linear dynamic range were determined. The sensitivity of the ACPD for ethylmercury(I1) chloride and methylmercury(I1) chloride was 20.0 and 3.5 pg/s, respectively, and was calculated by the expression described by Scott and Braman and Dynako (8,9). Higher sensitivities were found for the methylmercury(I1) chloride due to the fewer number of carbons which suggests that the extra carbon in ethylmercury(I1) chloride creates a higher fraction of diatomic species, such as diatomic carbon and CCl. The formation of these compounds produces a cooler plasma, resulting in an increase of molecular emission and, therefore, lower sensitivities for Hg I can be expected.
0.5 1.0 1.5 2.0 log Nanograms Injected
2.5
3.0
Flgure 4. Calibration curves for ethyl- and methylmercwy(I1) chloride demonstrating the detector linearity and dynamic range. The operating conditions were 290 W ac, a flow rate of 110 mL/min, and a wavelength of 253.67 nm.
Table 11. Selectivities of the ACPD at 253.67 nm for Methyl- and Ethylmercury(I1) Chloride over Various Elements" compound (element)
selectivity ratio
di-n-propylamine (N) 1,4-dioxane(0) carbon tetrachloride (Cl)
84 000 300 000 30 000 530 000
n-octane ( C )
"The band-pass, He flow rate, and ac power output were 8 A, 110 mL/min, and 290 W ac, respectively. As seen in Figure 4, the linear dynamic range, determined by the detector response, of the ACPD for the mercury compounds is found to be over 3 orders of magnitude. The electrodes did become coated over a period of time and may have contributed to the short dynamic range by enhancing self-absorption. Unfortunately, the employment of higher power levels to reduce self-absorption was not possible since the experiment was conducted at the maximum power output of the ac supply. In addition, the redesign of the sample introduction technique may provide not only a wider dynamic range but also higher sensitivity. Detector Selectivity, Mercury selectivity of the ACPD was evaluated by a response mixture. The mix contained the following compounds containing oxygen as 1,4-dioxane, nitrogen as dipropylamine, chlorine as carbon tetrachloride, carbon as n-octane, and mercury as methylmercury(I1) chloride. These elements were chosen because they have atomic or molecular emission lines near the mercury emission line. The detector selectivity was defined as the ratio of the response of the ACPD toward Hg I at 253.67 nm per unit mass of mercury injected to the response of the ACPD toward 0, N, C1, or C at 253.67 nm per unit mass of that element injected. Table I1 lists the selectivities of the ACPD toward Hg I vs 0, N, C1, and C at 253.67 nm and displays the high selectivity of the ACPD, although these data were generated under high band-pass conditions. It is interesting to note that the ACPD selectivity toward Hg I vs carbon as n-octane is approximately 5 times greater than the selectivity of a typical MIP detector for Hg I over carbon as dodecane, previously reported (2). However, the selectivity for Hg I over C1 was comparatively low in our study. This may be due to spectral interference caused by CC1 species that form in the plasma under these power conditions, since the band-pass was over 7 A. A higher power setting and smaller slit width may possibly reduce this interference and enhance the selectivity. Determination of Ethyl- a n d Methylmercury(I1) Chloride i n Spiked Gasoline. The resulting chromatogram
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
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F m e 5. Gas chromatogram of a mixture of gasoline, spked with 92.8 ng/pL methylmercury(1I) chlorlde and 39.6 ng/pL ethylmercury(I I) chloride. The separation was done at an oven temperature of 110 OC and a helium flow rate of 6 mL/min. Peak A represents 4.18 ng of methylmercury(I1)chloride and peak B represents 1.77 ng of ethylmercury(I1) chloride.
of the gasoline sample spiked with methyl- and ethylmercury(I1) chloride is seen in Figure 5. Between 0.5 and 3.0 min many hydrocarbons elute which causes the plasma to cool slightly, leading to a noisy base line and spiking due to the high fraction of molecular emission. I t is important to note that although the background matrix contains a large amount of organics, the plasma does not extinguish, due to the self-seeding aspect, and therefore solvent venting is not necessary. Peak A in Figure 5 represents 4.18 ng of methylmercury(I1) chloride, which is well resolved from ethylmercury(I1) chloride, peak B, which represents 1.77 ng. The chromatogram of the gasoline sample demonstrates the ability of the ACPD to selectively detect organomercwy compounds over other element-containing compounds in a complex matrix. In addition, the study demonstrated the fact that the ACPD does not extinguish in the presence of a large concentration of organic compoenents; an ability that MIP lacks. Gasoline is only one of many potential applications to the analysis of organomercury compounds in complex matrices by the ACPD.
C0NCLUSIO N The ACPD system described here provides a viable alternative to previous plasma emission detectors. Limitations experienced by MIP detectors are overcome by the ACPD. The 60-Hz ACPD power source is an inexpensive, compact size unit and provides a stable discharge that is self-seeding,
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therefore needing no Tesla coil to initiate the plasma. As a result, the plasma does not extinguish under high solvent backgrounds. The transfer tube design, therefore, did not require a heated venting valve for the solvent peak, which are commonly found in GC/MIP detection ( 2 , 6 ,7). The absence of such a valve further simplifies the overall design and reduces the possibility of any band broadening caused by the valve. The useful tee configuration of the discharge tube allows efficient sample peak throughput and provides a clean observation window, free from the coating of degradation products produced by the plasma. Soot and molecular species are carried away from the window and maintain a clean discharge tube. A new sample introduction design may lengthen the dynamic range of the ACPD for Hg(1) detection and increase the detector sensitivity. The heated transfer capillary tube provides continuous separation and minimal band broadening due to the stationary phase within. Detector sensitivities and selectivities for mercury demonstrate that the ACPD has many potential applications for organomercury and other organometallics and nonmetals. Further detector optimization should also improve the overall performance of the ACPD. Registry No. Hg, 7439-97-6; MeHgC1, 115-09-3;EtHgC1, 107-27-7.
LITERATURE CITED (1) Unden, Peter C. Chromtogr. Fourm 1986, 1 , 18. (2) Quirnby, Bruce D.; Uden, Peter C.; Barnes, Ramon M. Anal. Chem.' 1878, 50, 2112-2117. (3) Ballantine, D. S.; Zoller, W. H. Anal. Chem. 1984, 56, 1288-1293. (4) Estes, S.; Uden, Peter C.; Barnes, Ramon M. Anal. Chem. 1983, 53, 1336- 1340. ( 5 ) Chiba, K.; Haraguchi. H. Anal. Chem. 1983, 55, 1504-1508. (6) Tanabe, K.; Haraguchi, H.; Fuwa, K. Spectrochim. Acta, Part B 1981, 368, 633-639. (7) Slatkavltz, K.; Hoey, L.; Uden, P.; Barnes, R. Anal. Chem. 1985, 57, 1846-1853. (8) Braman, R. S.; Dynako, A. Anal. Chem. 1968, 40, 95-107. (9) Scott, R. P. W. J . Chromatogr. Sci. 1971, 9 , 641.
Robert B. Costanzo Eugene F. Barry* Department of Chemistry University of Lowell Lowell, Massachusetts 01854
RECEIVED for review September 9,1987. Accepted December 9,1987. The authors wish to acknowledge the University of Lowell Graduate School for Summer Research Support of this work. This work was presented in part a t the 38th Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Alantic City, NJ, March 1987.
