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(15) Cromartle, W. J.; Craddock, J. G. Science 1088, 154, 285-287. (16) Hunt, D. F.; Crow, F. W. Anel. Chem. 1078, 50, 1781-1784. (17) Munoz, E.; Ghuysen, J.-M.; Heymann, H. Blochemistry 1967, 6 ,
3659-3670.
(18) Gllbart, J.; Fox, A. Infect. Immun. 1087, 55, 1526-1528.
RECEIVED for review May 23, 1988. Accepted November 14,
1988. This work was supported by an equipment grant from the Department of Defense (to S.L.M. and A.F.) and grants from the National Institutes of Health (to A.F. and S.L.M.), the Army Research Office (to S.L.M. and A.F.), the American Heart Association (to A.F.), and the National Science Foundation (travel award to A.F.).
CORRESPONDENCE Microelectrodes Coated with Ionically Conducting Polymer Membranes for Voltammetric Detection in Flowing Supercritical Carbon Dioxide Sir: Voltammetry in supercritical and near-critical fluids has been receiving considerable attention ( I , 2). These fluids are becoming increasingly important solvents, especially as chromatographic mobile phases (3). Carbon dioxide is a convenient supercritical solvent because of its easily accessible critical temperature and pressure. We have recently reported the voltammetry of ferrocene in supercritical COZ modified with small quantities of water (2). Because of the large ohmic resistance of the fluid, ultramicroelectrodes were used as they are relatively immune to solution resistance effects (4, 5). Nevertheless, ohmic potential drop was observed unless an electrolyte, tetrahexylammonium hexafluorophosphate, was also used. The use of such electrolytes, however, is unlikely to be practical in supercritical fluid flow systems due to problems such as their precipitation at the fluid outlet. Several recent reports concerning voltammetry in resistive media suggested to us a way of achieving the goal of a voltammetric sensor in flowing supercritical fluid. Metal electrodes supported on ionically conducting polymer membranes have been used to conduct voltammetry of species extracted from gases (6) or electrolyte-free nonpolar organic solvents (7,8). In these studies, the membrane served as a separator between a vapor or solvent containing the electroactive substrate and an aqueous electrolyte solution containing auxiliary and reference electrodes. Solutes retained in similar membranes have also been voltammetrically characterized in the absence of a contacting liquid solvent with the working, reference, and auxiliary electrodes contained in the film (9, 10). Furthermore, such a device has been used for the analysis of electrochemically active additives extracted from an oilbased lubricant (11). Voltammetric detection of gas-phase solutes has been accomplished by providing for ionic conduction across an insulator between an ultramicroelectrode and adjacent counter electrode (12). The experiments described below were aimed at developing an electrochemical sensor for use in a supercritical COz flow system using a combination of these approaches. EXPERIMENTAL SECTION For studies of polymer-modified electrodes in supercritical COz, an electrochemical probe consisting of two coplanar platinum disks sealed in glass was used. The probe was constructed by heat sealing a 5 pm radius Pt wire in a 600 pm 0.d. soft glass capillary. The capillary and a 26-gauge Pt wire were heat sealed into a 4 mm 0.d. soft glass tube. The probe tip was ground flat to expose the cros9 section of the two wires and polished with 600.grit paper and 1-,0.3-, and 0.05-pm alumina. The separation between the disks was approximately 500 pm. The 5 pm radius Pt disk served as the working microelectrode and the 26-gauge Pt disk was used 0003-2700/89/038 1-0270801.50/0
as a quasi-reference electrode (Pt-QRE). The polished face of the probe was coated with 0.5p L of 0.5 w t 70 Nafion (H' form) in isopropyl alcohol by using a Hamilton syringe. The alcohol was evaporated at 80 OC for 1 h following which a vacuum was applied at 80 "C for 15 min (23). This produced a film on the probe approximately 1-2 pm thick which completely covered both electrodes. Preliminary static fluid experiments were conducted with Nafion-coated probes in the apparatus previously described (2). For flow stream experiments, a system was constructed as illustrated schematically in Figure l. The apparatus was placed inside a gas chromatographic oven to maintain constant temperature. The electrode was positioned down stream from a high-performance liquid chromatography (HPLC) injection valve (Rheodyne 7010) that was used as a three-way switching valve. The valve was used to direct one of two fluid streams to the electrode. One flow line contained unmodified C 0 2 supplied directly from an HPLC syringe pump. The other flow line contained COz that had been passed through a sample vessel containing ferrocene and water. The lower portion of Figure 1is a detail of the detector. The Nafion-coated probe was positioned in the body of a 1/16 in. to 1/4 in. stainless steel union. The inlet to the detector cell was a in. stainless steel tube with 0.01 in. i.d. The probe was positioned approximately 1mm from the end of the inlet tube. The distal end of the outlet tube was connected to a Swagelok tee that had been drilled so the probe could pass through. The probe was sealed into the tee with epoxy. The side arm of the tee was connected to a capillmy flow restrictor which allowed a fluid pressure of 88 atm to be maintained at a flow rate of approximately 200 pL/min. Flow experiments were carried out at 50 O C ; the maximum temperature rating of the valve. RESULTS AND DISCUSSION The voltammogram shown in Figure 2 was recorded with the Nafion-coated probe in static supercritical C02containing water and ferrocene at 80 "C and 88 atm. A potential scan performed under identical conditions, but without ferrocene present, revealed only background current. Without water present, the membrane was nonconductive and no current flowed at any applied potential irrespective of the presence or absence of ferrocene. (Note that water is not required to dissolve ferrocene in supercritical COz.) This demonstrates that cation solvation by the polar modifier is required for ionic conduction in the Nafion film. This is not a general property of ionically conducting membranes. Conduction in poly(ethylene oxide) membranes, for instance, depends on segmental motion of the polymer chains and occurs a t elevated temperatures in solvent-free bathing gases (9). The voltammogram of Figure 2 allows a qualitative description of the behavior of ferrocene at a Nafion-coated microelectrode in supercritical Cot. The sigmoidal anodic 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 3, FEBRUARY 1, 1989
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Figure 1. Top: Schematic of supercritical fluid flow system. Bottom: Detail of the electrochemicaldetector: (A) Nafion membrane, (B) R microelectrode, (C) R-QRE, (D) glass tube, (E) 0.01 in. i.d., 'Ire in. 0.d. stainless steel inlet tube, (F) stainless steel 'Ile in. to ' I 4in. union, (0) outlet tube.
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E (V vs Pt-QRE) Flgure 2. Cyclic voltammogram of ferrocene (200 pM) in supercritical COBcontaining 0.13 M water at 80 OC and 88 atm with a scan rate of 0.1 V I S using a Nafion-coated two-electrode probe.
current is typical of radial steady-state diffusion at disk-shaped microelectrodes (14). Under such conditions, the dimension of the diffusion layer is approximately equal to 6r0, where ro is the radius of the disk (15),which amounts to 30 pm in this experiment. The diffusion layer, therefore, is not confined to the 1-2 pm thick Nafion membrane and extends well into the supercritical fluid. However, the film does act as a partial barrier to ferrocene because the current amplitude is attenuated relative to that obtained at a bare electrode (2). The peak-shaped current observed for the reduction of the ferricinium during the cathodic potential sweep is a consequence of the slow diffusion typically observed for hydrophobic cations in Nafion membranes (16). The asymmetric shape of the current indicates that ferricinium is soluble in the membrane. Ferricinium is known to be insoluble in moist COz (2), so it is retained in the membrane. The consequence of this observation will be discussed below. The sigmoidal shape of the anodic voltammogram shown in Figure 2 implies that ferrocene is able to diffuse from the fluid to the microelectrode on the voltammetric time scale (i.e. several seconds). This implies the probe should be able to respond to concentration changes of electroactive substrate in the fluid on a similar time scale. This is demonstrated by the results shown in Figure 3 which were obtained in the flow system described in the Experimental Section and Figure 1. In Figure 3A, results obtained by using constant potential detection are shown. The electrode potential was poised at
Flgure 3. (A) Constant potential detection of ferrocene in the supercritical fluid flow system. Three injections (left to right) from a small vessel containing 500 pM ferrocene dissolved in supercritical COP saturated with water were performed with an applied potential of 4-0.5 V vs Pt-QRE followed by two injections at -0.3 V vs R-ORE (i = 0.02 nA, t = 5 min). (6)Potential scanning detection of ferrocene In the supercritical fluid flow system. The potential was continuously scanned from -0.3 to +0.5 V vs R-QRE at 0.5 V/s. The current observed at the anodic switching potential of each cycle is shown (i = 0.1 nA, t = 5 rnin). The horizontal bars in A and B indicate the 1-min periods during which the electrode was exposed to the C021ferrocenelH20.
+0.5 V vs Pt-QRE. At 4-min intervals the electrode was exposed to a 1-min bolus of the COz/ferrocene/water mixture. During the intervening 3 min, unmodified C02 passed over the electrode. The electrode responded rapidly to the introduction of the bolus, reaching a maximum anodic current in approximately 10 s. The signal decreased continuously for the duration of the 1-min bolus. During two subsequent exposures to the electroactive mixture, the current initially reached a value approximately equivalent to the current at the end of the previous bolus, and then declined further. After the third bolus in Figure 3A, the electrode potential was returned to -0.3 V vs Pt-QRE. As the potential was adjusted, no substantial current was observed. During a subsequent bolus of the COz/ferrocene/water mixture a large cathodic current was recorded. The cathodic signal was apparently derived from ferricinium retained in the Nafion membrane from the previous oxidation of ferrocene. The fact that no reducible material was present in the bolus itself was demonstrated by a final exposure to the electroactive mixture during which no cathodic current was observed (Figure 3A). The results of this experiment are consistent with the static fluid results described above. The ferricinium cation is retained by the Nafion and is not reduced until water is introduced which enables the film to become conductive. This also illustrates that the Nafion membrane is rapidly hydrated and dehydrated under these conditions. Rapid entry of solvent into other polymer films has been observed by others (17).
Thus, the continuous decrease of the anodic current at constant applied potential is due to the retention of ferricinium by the membrane. The continuous accumulation of ferricinium in the vicinity of the microelectrode causes the displacement of hydrogen ions to maintain electroneutrality. T h e low mobility of ferricinium, as indicated by the shape of the reduction wave in Figure 2, limits ionic conduction in the vicinity of the electrode. The mobility of cations in Nafion has been shown, in some cases, to decrease with increasing concentration due to the limited availability of fixed anionic sites in the membrane (1419). To overcome this, a potential scanning procedure was employed. A triangular waveform from -0.3 to +0.5 V vs Pt-QRE was applied continuously to
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the electrode at a sweep rate of 0.5 V/s. The current recorded at the anodic switching potential of each potential cycle during consecutive exposures of the electrode to the electroactive mixture is shown in Figure 3B. It is clear that the potential scanning scheme greatly improves the stability of the response. The remaining instability may be further decreased by the inclusion of a delay time between anodic scans to more thoroughly reduce the ferricinium. CONCLUSION The results presented in Figures 2 and 3 demonstrate that microelectrodes coated with ionically conductive polymer membranes provide practical devices for electrochemical investigations in flowing supercritical COz without added supporting electrolyte. The requirement for a polar modifier does not seem to present a major drawback as modifiers are widely used in chromatographic systems for which these detectors are of interest. Furthermore, as mentioned above, other available ionically conducting membranes do not require such modifiers. Particular attention has to be paid to the potential waveform used for detection to circumvent deleterious effects of electrochemical products in the film. Electrochemical detection in supercritical fluids is anticipated to be quite sensitive because of the gaslike diffusion coefficients of solutes. Diffusion through membranes, however, is often very slow. If the membrane is sufficiently thin compared to the dimension of the diffusion layer, as in Figure 2, the voltammetry becomes characteristic of the fluid diffusion coefficient (20). Thus, the use of polymer films is not expected to significantly counteract this attractive aspect of supercritical fluids. In liquid flow systems, microelectrodes coated with very thin Nafion films display response times that are very similar to uncoated electrodes (21). These latter two points suggest that membrane-coated microelectrodes have the requisite properties for an electrochemical detector of supercritical fluid chromatography that may match or surpass the performance of electrochemical detectors currently available for liquid chromatography (22). Registry No. Pt, 7440-06-4;COz, 124-38-9;ferrocene, 102-54-5; Nafion, 39464-59-0.
LITERATURE CITED Crooks, R. M.; Bard, A. J. J. Electroanal. Chem. 1988, 243, 117-131. Philips, M. E.; Deakin, M. R.; Novotny, M. V.; Wightman, R. M. J. fhys. Chem. 1987, 97,3934-3936. Novotny. M. V.; Springston, S. R.; Peaden, P. A,; Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1981, 53,407A-414A. Wightman, R. M. Science 1988, 240, 415-420. Pons, S.; Fleischmann, M. Anal. Chem. 1987, 59, 1391A-1399A. Beran, P.; Bruckenstein, S. Anal. Chem. 1980, 52, 1183-1186. Kaaret, T. W.; Evans, D. H. Anal. Chem. 1988, 60,857-862. DeWulf, D. W.; Bard. A. J. J. Electrochem. SOC. 1988, 735, 1977-1985. Reed, R. A.; Geng, L.; Murray, R. W. J. Electroanal. Chem. 1988, 208, 185-193. Geng, L.; Reed, R. A.; Longmire, M.; Murray, R. W. J. Phys. Chem. 1987, 97,2908-2914. Wang, S.S.J. Electrochem. SOC. 1988, 135, 15312, Abstract #461. Brina, R.; Pons. S.; Fleischmann, M. J. Electroanal. Chem. 1988, 244,81-90, Moore, R. E., 111; Martin, C. R. Macromolecules 1988, 27, 1334-1339. Aoki, K.; Akimoto, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1984, 177, 219-230. Wightman, R . M.; Wipf, D. 0. I n Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1988; Vol. 15. Martin, C. R.; Dollard, K. A. J. Electroanal. Chem. 1983, 759, 127- 135. Parcher, J. F.; Barbour, C. J.; Murray, R. W. Anal. Chem., submitted. Buttry, D. A.; Anson, F. C. J. Am. Chem. SOC. 1983, 705,685-689. Moran, K. D.; Majda, M. J. Electroanal. Chem. 1986, 207, 73-86. Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 172, 97-115. Kristensen, E. W.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1987, 59, 1752-1757. Weber, S. G.; Long, J. T. Anal. Chem. 1988, 60,903A-913A. Author to whom correspondence should be addressed.
Adrian C. Michael
R. Mark Wightman* Department of Chemistry Indiana University Bloomington, Indiana 47405
RECLWED for review September 23,1988. Accepted November 1,1988. This research was supported by the Chemical Analysis Section of the National Science Foundation.
Enhanced Detection of Sulfite by Inductively Coupled Plasma Atomic Emission Spectroscopy with High-Performance Liquid Chromatography Sir: Inductively coupled plasma atomic emission spectroscopy (ICP-AES) has become a standard analytical technique for sulfur determination in soil ( I ) , plant tissue @), and coal (3). In most cases, the sample is digested or extracted and aspirated directly from solution and the concentration determined. Speciation information is, however, unavailable. Concentrations of particular species of sulfur in solution have been determined with emission spectroscopy by separation prior to quantification. High-performance liquid chromatography (HPLC) with ICP-AES detection has been used to quantify various sulfur-bearing surfactants (4). Sulfur-containing biological compounds from rat organs have been determined by ICP-AES after extraction and separation on a TSK Gel G3000 S W column (5). The conversion of an analyte from a solution species to a gaseous product is the basis of cold vapor and hydride generation techniques. In both of these techniques, sensitivity and detection limits are significantly improved over more conventional nebulization modes (6). Gaseous analyte is also produced and quantified by using a heated graphite atomizer
(7). ICP-AES detection has been used to determine hydrogen sulfide from solution as a gaseous product (8). Samples of groundwater, preserved with 0.1% KOH, were acidified to contain 0.5% HCl to liberate the hydrogen sulfide. Argon was used to flush the hydrogen sulfide from the top of a gas-liquid separator into the torch. The sensitivity of determination of osmium was enhanced by a factor of approximately 100 over conventional nebulization by converting the analyte to osmium tetraoxide vapor (9). The improvement was attributed to avoiding the inefficiency of the nebulization process as well as the cooling of the plasma by the solvent. The plasma served as an ion source for a mass spectrometer used to determine osmium isotopic ratios in nanogram quantities. HPLC with ICP-AES detection is significantly less sensitive than other detection methods used in liquid chromatography because of the loss of from 80% to 99% of the analyte upon nebulization to an aerosol (IO). Results from the investigation herein reported indicate, however, that the detection of sulfite by this method is far more sensitive than detection of sulfite under other conditions. It is proposed that a second mech-
0003-2700/89/0361-0272$01.50/00 1989 American Chemical Society
