Anal. Chem. 1991, 63,1728-1732
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Ionically Modified Electrodes for Use in Nonpolar Fluids David E. Niehaus and R. Mark Wightman* Department of Chemistry, CB No. 3290, Venable Hall, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290 Paul A. Flowers Department of Physical Science, Pembroke State University, Pembroke, North Carolina 28372
The addltlon of water to the supercrltlcal fluld of carbon dloxkle or nitrous oxkle allows for voltammetry to be perfonned on analytes dlosolved In these solvents. Experimental data show that an amount of water In excess of Its sduMllty must be added to the electrochemlcal cell In order to mlnlmlze ohmlc dlstortlon. Thls, along wlth the observed shapes of voltammograms taken In these systems, lndlcates that water flhn formatbn Is taking place on the surface of the electrodes. Electrodes coated wlth tetrahexylammonium nllrate ((THA)NO,) are shown to be useful In detectlng specles dlssolved In supercrltkal CO, and N,O. (THA)NO, (mp69 "C) Is d e n at ambient temperatures under the supercrltkal flulds due to dlssolutlon of the flulds In the molten salt. Analytes dlssdved In supercrltlcal CO, or N,O and llquld heptane partltlon Into molten (THA)NO, flhns on electrodes lo glve voltammograms free from ohmlc dlstortlon. The voltammetrlc waves of ferrocene, anthracene, and 9,lOdlphenylanthracene are shown to be chemlcally Irreversible In the molten salt. The chemkal lrreverslblllty of these analytes Is not evident when (THA)NOS Is used as a conventlonal electrolyte In acetonltrlle t~Mlons. Post-SFC column detection of FeCp, at a molten (THA)NOS film electrode yields a detectlon llmlt of 0.1 ng.
INTRODUCTION The utility of microelectrodes to obtain undistorted voltammograms in solutions of high resistance has been well documented (1-4). However, in solvents of extremely low dielectric strength (e < 2), the ionic solubility is sufficiently low that conductivity is limited, and the ability to form a sufficiently compact double layer is precluded. One approach to overcome this problem is to use a conductive film that provides ionic communication between the working and counter electrodes. Voltammetry can then occur for molecules that partition into the conducting film. This approach has been used to perform electrochemistry on analytes dissolved in hydrocarbon solvents with little or no supporting electrolyte (5-7).
A major reason for our interest in this work involves the development of an electrochemical detector for supercritical fluid chromatography (SFC). SFC continues to be an area of active analytical research (8,9),and the supercritical fluids of C 0 2 (SC-COz) and NzO (SC-NzO) are two of the more popular mobile phases because of their moderate critical temperatures and pressures (31.1 OC, 1084 psi, and 36.4 OC, 1066 psi, respectively). However, these supercritical fluids have low dielectric constants and thus do not support the ionic environment necessary for conventional electrochemistry. In liquid chromatography, the problems associated with electrochemical detection in low conductivity media have been circumvented both by use of microelectrodes (10) and ionexchange membranes (11). Microelectrodes (12, 13) and
* To whom correspondence should be addressed. 0003-2700/91/0383-1726$02.50/0
polymer f i b electrodes (14) also have been found to be useful as gas chromatography detectors. Electrochemistry has been reported in the supercritical fluids of H20, NH,, SOz, and COz-methanol mixtures (15-19), but these fluids are relatively polar and will dissolve electrolytes. We have demonstrated that electrochemical measurements are not possible in unmodified SC-C02(20,21)and that the resistance problem may be overcome by coating the electrodes with a conducting polymer film (22, 23). In previous work (21),it was demonstrated that the addition of water to SC-C02permitted cyclic voltammograms for the oxidation of ferrocene to be recorded. Additionally, we showed that the addition of tetrahexylammonium hexafluorophosphate to SC-C02enabled electrochemistry to be observed by forming a molten salt film (at >60 "C below the salt's melting temperature) on the electrodes (21). We now present data that explore the underlying mechanisms of these two phenomena. The results indicate that added water forms a film on the electrode surface and that an amount of water in excess of its solubility must be present to obtain good voltammetric results. In order to more fully characterize the molten film results, cryoscopic resulta for a similar salt, tetrahexylammonium nitrate ((THA)NO,), are shown. In addition, the utility of molten (THA)N03 film electrodes for electroanalysis in SC-COz, SC-NzO, and heptane solutions is demonstrated. Finally, a preliminary demonstration will be given of molten (THA)N03film electrodes as detectors for SFC with COz as the mobile phase.
EXPERIMENTAL SECTION Reagents. Carbon dioxide and nitrous oxide (Scott Specialty
Gases, Plumstead, PA) were dried with a packed column of activated alumina. Analytical reagent grade solvents carbon tetrachloride, acetonitrile, and heptane (Mallinckrodt,St. Louis, MO) and cyclohexane (Fisher, Pittsburgh, PA) were used as received. Ferrocene (Johnson Matthey, Seabrook, NH) was purified by sublimation. Anthracene, 3,5-di-tert-butylcatechol,9,lO-diphenylanthracene, and methylhydrcquinone (Aldrich, Milwaukee, WI) were all 198% purity and used as received. Tetrahexylammonium nitrate was metathesized from silver nitrate and tetrahexylammonium iodide (Fisher) according to literature procedures (24). The crystals were recrystallized three times from ethyl acetate/ether solution and dried under vacuum for 24 h at 50 "C. The (THA)N03crystals melted at 68 "C, close to the literature value of 69 "C (24). Electrochemical Procedures. The microelectrodes used were described previously (21). Briefly, carbon fibers (r = 5 pm) or platinum wires (r = 5,50, or 125 pm) were sealed into capillaries either by using Epon 828 epoxy (Miller-Stephenson, Danbury, CT) with m-phenylenediamine curing agent (carbon electrodes) or by heat fusing in glass (Pt electrodes). A 26-gauge silver wire coiled around the tip of the capillary served as a quasi-reference electrode (QRE). The electrodes were polished with 5.0-, 1.0-, and 0.3-pm alumina (Buehler, Lake Bluff, IL). Shown in Figure 1 is the electrode assembly used for experiments with molten salt films. Microelectrodes were sealed into a ljI6-in.-i.d.glass tube with the Epon 828 epoxy. After curing the epoxy, the glass tube was cracked away. The polished surface of the electrode assembly (working electrode, reference, and in0 1991 American Chemical Society
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Flguro 1. Electrode wed for molten salt film electrodes. (A) electrode leads, (B) epoxy body, (C) Torr Seal, (D) '/,e-in. stinless steel fitting, (E) glass capillary, (F) 28-gauge Ag, (0)r = 5-pm C fiber. Bar equals '/le
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sulator) had an area of 2.0 mm2. The assembly was sealed into a stainless steel fitting with Torr Seal (Varian Associates, Palo Alto,CA). Salt films were applied to the electrcdw by evaporation of small volumes (pL) of carbon tetrachloride containing known amounts of (THA)NOS. From the mass of the salt applied, the area of the electrode assembly surface, and the measured density of the molten (THA)N03(0.88 g/cm3),f i i of a desired thickness could be formed. Electrodes constructed in this way were suitable for multiple uses in the supercritical fluids, indicating the materials used are not unstable in these applications. Electrochemical measurements under supercritical fluid conditions were conducted in a high-pressure stainless steel cell (21). Analytes and modifiers were added to the cell, which was then carefully purged with the gas to be used. The supercritical fluid was pumped into the cell with a constant-pressure syringe pump (Isco, Lincoln, NE), and the temperature was controlled with a resistive ribbon heater and monitored with a thermocouple. Electrochemid measurements were performed in a tweelectrode mode. The working electrode current was monitored by a Keithley (Cleveland, OH) 427 picoammeter connected to an AT-style personal computer via an interface board (Labmaster, Scientific Solutions, Solon, OH). The potential was computer controlled with locally written software. Voltammograms with larger platinum electrodes (0.25-mm diameter) were obtained with a PAR (Princeton, NJ) Model 174A polarographic analyzer and recorded on a Houston (Houston, TX) Model 2000 XY recorder. Experiments in acetonitrile solutions employed a cell with a Luggin capillary that contained a saturated sodium calomel electrode (SSCE) reference electrode and platinum wire auxiliary electrode. Solutions were deaerated with Nz, but no effort was made to exclude water from the solution. Cryoscopic Experiments. Cryoscopic experiments involving solid solutes were performed in a Pyrex test tube sealed with a rubber septum through which an iron-constantan thermocouple was inserted. Cooling curves were recorded by monitoring the analog output from the thermocouple gauge (Omega, Stanford, CA) with a strip-chart recorder (Houston Omniscribe, Houston, TX). Cryoscopic experiments involving gaseous solutes were performed in a similar manner by using the high-pressure stainless steel cell described above. Because of the hygroscopic nature of tetraalkylammonium salts, manipulations involving (THA)N03were performed in a drybox. Known quantities of the salt and solid solutes were added to the high-pressure cell by weighing the appropriate sealed containers outside of the drybox before and after material transfer inside the drybox. Use of the drybox proved to be impractical when using the salt for film electrode experiments. Chromatographic Procedures. Supercritical fluid chromatography experiments were performed with the constant-pressure
Figure 2. Upper: Cyclic voltammogram for the oxidation of 50 ph4 ferrocene in SC-N,O (P = 1300 psi, T = 80 "C) with 0.9 g/L added water. Electrode was a carbon fiber disk (r = 5 pm) sealed in epoxy. Scan rate was 100 mV/s. Lower: Cyclic voltammogram for the oxidation of 45 pM methylhydroquinone in SCCOp(1300 psi, 80 "C) with 1.5 g/L added water at a carbon fiber (r = 5 pm) electrode at a scan rate of 100 mVls.
pump and an SFC controller board (Isco) in conjunction with a personal computer. Separations were carried out on a RP-18 150" x 1-mmpacked column. A 0.5-m-long piece of 15pm-i.d. fused silica capillary served as the restrictor to give a flow rate of -40 pL/min. Samples were dissolved in cyclohexane, and the injected volume was 0.5 rL. The electrochemical detector was a locally constructed, sandwich-type flow cell. The working electrode (glassy carbon) and reference electrode (Ag foil) were sealed into an epoxy block. The area of the working electrode was -0.1 mm2. The electrodes were polished and coated with the salt as described above. A 0.005-in. polyethylene spacer with a 1.5-cm X 250-pm path was sealed between the epoxy block and a stainless steel block machined for connection to the SFC system. The cell potential was controlled with a locally constructed unit, and the current was measured with a picoammeter (Keithley)and recorded on a strip-chart recorder. Both the column and the electrochemical detector were maintained at a constant temperature inside a gas chromatographic oven. A UV detector (Isco) was placed in series after the electrochemical detector for signal confirmation.
RESULTS AND DISCUSSION Water Film Formation. Similar to our previous findings in SC-C02,voltammograms in pure SC-N20do not shown any current. When water is added to the cell, voltammograms can be obtained. Figure 2 (upper) shows a cyclic voltammogram obtained with a carbon microelectrode of ferrocene dissolved in SC-N20 containing 0.9 g/L water. The features of the voltammogram are identical with that reported previously in water-modified SC-C02 (21): considerable ohmic distortion, steady-state oxidation current, which is consistent in magnitude with diffusion from the supercritical fluid, and a reduction peak on the reverse scan. With lower amounts of added water, increased ohmic distortion is observed, while an increase of the amount of added water has little effect on the voltammogram. A cyclic voltammogram recorded a t a 5-pmradius electrode of methylhydroquinone in SC-C02containing 1.5 g/L is shown in the lower part of Figure 2. Many of the characteristics seen for ferrocene are also present in this voltammogram. However, the forward wave is peaked, suggesting adsorption, and no reverse wave is observed. We have postulated previously that the effect of added water on the voltammetry in SC-COzmay be either because of carbonic acid formation and dissociation, leading to increased conductivity of the fluid or to the formation of a
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Flgure 3. Varlatlon of ferrocene and methylhydroqulnone oxidation wave slope versus the amount of excess H20 In supercritical C02. [H20]is equal to the amount of water added per cell volume, and [HIO]was calculated from eq 1. (A,O) Ferrocene at platinum electrode, constant temperature (80 "C), pressure varied (1300-2000 psi). (0)Methylhydroqulnone at carbon electrode, constant temperature (80 "C), pressure varied (1700-2300 psi). (A) Methylhydroquinone at platinumelectrode, constant temperature (80 "C),pressure varied (1700-2300 psi). (0)Ferrocene at carbon electrode, constant temperature (84 "C), presswe varled (1300-2200 psi). (+) Ferrocene at carbon electrode, constant pressure (1500 psi), temperature varied (80-45 "C).
wetting layer of water on the electrode/insulator surface, which allows conductivity between the working and counter electrodes (21). Association of water with NzO has been inferred from solubility data (25),which, if the association products are ionizable, could lead to the observed conductivity. However, the possibility of formation of wetting layers exists in both fluids. Wetting layers form in two-phase immiscible systems (26, 2 3 , and the layers are thicker and more conductive if the surface is hydroxylated (27) or ionizable (281, such as the glass and epoxy materials used in the electrode assemblies. Evidence for a wetting layer requires knowledge of the solubility of water in the supercritical fluid. While such data are not available for NzO,the solubility (c) of water in SC-C02 can be calculated by the following semiempirical equation (29) c = p1*56exp(-2826/T - 0.81)
(1)
where c is concentration in g/L, p is the density of the gas in g/L, and T is the temperature in K. The density is a function of both temperature and pressure, and the values of the density were calculated from established equations (30, 31). The calculations show that water is only sparingly soluble in SC-C02 in the ranges typically employed. For example, a t 80 "C and 1300 psi, the calculated solubility is 0.5 g/L. While modifiers can alter the critical point, because water is only sparingly soluble in C02, the change is only a fraction of a degree (32). To find whether there exists a correlation between the observed voltammetry and the solubility of water in the supercritical fluid, the voltammograms of ferrocene and methylhydroquinone in SC-COPcontaining water were examined over a range of temperatures and pressures. With fixed amounts of added water, the pressure was varied (from 1300 to 2300 psi) at constant temperature (80or 84 "C). Increasing the pressure increases the density and leads to an increase in water solubility. In separate experiments, the temperature was varied (from 80 to 45 "C) at constant pressure (1500 psi). Lowering the temperature at constant pressure increases the density and thus affects two terms in eq 1. The voltammetric wave slope (measured as E3,4 was plotted versus the reduced amount of water present in the cell (Figure 3). Assuming the electrode kinetics are unchanged by the temperature and pressure, the wave slope should be a measure of ohmic distortion in the voltammogram (33). (Note the temperature effect on the wave shape over the temperature range studied would be small compared to the measured ef-
fects.) The plot clearly shows that the apparent resistance increases dramatically when the solubility of water in the fluid is greater than the added amount. This is consistent with the formation of a wetting layer and is opposite from the result expected if the conductivity were due to carbonic acid formation and dissociation in the supercritical fluid. The wave slope decreases to a limiting value with increased amounts of excess water, which suggests that an increase in wetting layer thickness results in greater conductivity. The voltammetric behavior in SC-N20/H20mixtures is similar, leading to the conclusion that the same mechanism is operant. Various other features of the observed voltammetry are also consistent with a wetting layer formed at the surface. Voltammograms recorded at glass-sealed platinum electrodes exhibit greater ohmic distortion than epoxy-sealed carbon electrodes. This behavior would not be expected for a mechanism that involves conduction through the bulk of the fluid but would occur with a wetting layer if the epoxy has more ionizable surface sites than the glass. The sharp peak for ferrocenium reduction seen in water-modified supercritical N20,which is also present in supercritical COz (211,suggests the eledrogenerated cation remains in a polar region adjacent to the electrode. The absence of a reverse wave for methylhydroquinone suggests that its electrogenerated product can leave the electrode. This result would be expected if the electrochemically generated product is the quinone as occurs in aqueous solutions. These experimental data all indicate that a wetting layer of water is formed on the electrodes and provides a route for conductivity between the electrodes. The presence of the water in this system greatly limits the observed potential window, particularly in the cathodic direction (- -0.5 V vs Ag QRE). Therefore, an alternative approach to solving the resistivity problem was examined. Cryoscopy of Tetrahexylammonium Nitrate. In a previous report, it was shown that the addition of tetrahexylammonium hexafluorophosphate ((THA)PF,) greatly improves the voltammetry in water-modified SC-C02 (21). It was shown that (THA)PF6 melts under H20/SC-CO2 at 60 "C below its melting point at atmospheric pressure and forms a conductive layer over the electrodes. Electroactive species can partition into the molten salt layer, and voltammetry takes place in this highly conductive environment. In order for films of this nature to be useful, the salt must be molten under the conditions used, and the salt must not be appreciably soluble in the fluid. (THA)PF, is only one of a large number of tetraalkylammonium salts, many of which have low melting temperatures (24,34-36). (THA)NO,) was chosen for study in this work because of its low melting point and its ease of preparation. A series of cryoscopic experiments was performed in order to investigate the melting point behavior of (THA)NOB. The freezing point depression of a liquid, dT, is determined by the molal concentration of solute particles dissolved in the liquid, m, and the liquid's cryoscopic constant, Kf, according to
dT = Kfm (2) assuming ideal behavior and negligible solubility of the solute in the frozen solvent. The freezing point depression for (THA)N03mixed with different amounts of 3,5-di-tert-butylcatechol and methylhydroquinone was determined. The freezing point was linear with molality (r = 0.999, n = 7) and yields a value of 99.7 "C kg/mol for Kf.This large value of the cryoscopic constant may in fact explain the tetrabutylammonium tetrafluoroborate/toluene solvent observed by Pickett (37)to be molten at temperatures 140 "C below the fusion temperature of the salt. This value of the cryoscopic constant is important because both COPand N20 are soluble in (THA)N03. In the simple
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Flgure 5. Cyclic voltammogram for 25 pM perylene in SCN,O (50 OC, 2100 psi) at a (THA)NO, molten salt film electrode (C electrode, r = 5 pm). Film thickness was -10 pm, and scan rate was 100 mV/s.
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Flgure 4. Upper: Cyclic voltammogram for the oxidation of 6.1 mM ferrocene in molten (THA)NO, (75 OC) at a Pt (r = 50 pm) electrode, w = 50 mV/s (solid line). Simulated voltammogram (dashed iine) for reversible couple. Diffusion coefficient for simulation was taken from steady-state voltammograms at small (r = 5 pm) electrodes. Lower: Cyclic voltammogram for the reduction of 8.0 mM 9,lOdiphenylanthracene In molten (THA)N03 (75 OC) at a Pt (r = 125 pm) electrode at a scan rate of 20 mVls.
case of physical dissolution, i.e., no chemical reaction between solute and solvent, the concentration of a gaseous solute in a liquid solvent is governed by Henry's law,
Pg = mgkg
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where k, is Henry's law constant in units of pressure/molality,
P, is the pressure of the gas above the liquid, and the dissolved gas concentration is expressed as molality, m,. Substitution of eq 3 into eq 2 yields
dT = (&/kg)Pg
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The freezing point depressions were determined for molten (THA)N03 under C02, NzO, and Ar as a function of gas pressure. The data again show a linear relationship (correlation coefficients >0.99), with slopes of 4.8 "C/lOO psi for COP,2.8 OC/lOO psi for N20, and 0.1 oC/lOO psi for Ar. (The low value for Ar suggests a limited solubility in the molten salt.) Thus, a t all conditions where C02 is supercritical, (THA)N03exists only in the molten state. Likewise, nearly the entire range of SC-N20 is accessible with molten (THA)NO3.
Voltammetry in Molten Tetrahexylammonium Nitrate. Cyclic voltammetry in molten (THAINO, (75 OC) under 1a h of Nz reveals an essentially featureless potential window of over 4 V (+1.5 to -2.5 V vs Ag QRE) at both platinum and carbon, in agreement with a previous report on molten tetrabutylammonium nitrate (38). This range corresponds to a similar window observed for acetonitrile solutions of (THA)N03of +1.5 to -2.5 V vs SSCE. Figure 4 (upper) shows a voltammogram of ferrocene dissolved in molten (THA)N03 (75 "C) recorded at a 50-pm-radius Pt electrode, and it is compared to a simulated voltammogram that accounts for radial diffusion of a chemically reversible species (39). The absence of a reverse wave and the poor fit on .the anodic scan suggest the presence of a chemical reaction following electron transfer. (Even for a 10-fold increase in the diffusion coefficient of the simulated voltammogram, a reverse-wave peak is still present.) Voltammograms recorded at scan rates up to 500 mV/s do not show a reverse wave of the expected magnitude. Voltammograms taken in solutions of ferrocene
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Flgure 6. Cyclic voltammogram for the oxidation of 5 mM ferrocene in nheptane at a (THA)N03-coated (thickness -50 pm) Pt (r = 5 pm) electrode. Temperatwe was 70 OC, and the scan rate was 100 mV/s.
dissolved in acetonitrile with (THA)N03 as the supporting electrolyte (with a ratio of [(THA)N03]/[FeCp2]similar to that in the melt) exhibit the normally found reversible behavior. Ferrocenium instability in molten salts also has been observed by Osteryoung and co-workers (40). Figure 4 (lower) shows a cyclic voltammogram for the reduction of 9,10-diphenylanthracene in dry, molten (THA)N03. The forward cathodic wave shows the peak shape expected with the larger electrode. However, the voltammograms are totally irreversible a t scan rates up to 500 mV/s. Similar results were obtained with anthracene under identical conditions. In contrast, solutions of 9,lO-diphenylanthracene in acetonitrile with (THA)N03 show reversible, one-electron behavior, as typically observed in aprotic solvents (41). The observation of irreversibility in the molten salt is consistent with a previous electrochemical study of several polycyclic aromatic hydrocarbons in molten tetrabutylammonium nitrate in which butylated reduction products were formed (38).
Voltammetry at Molten (THA)N03Film Electrodes. The use of molten tetraalkylammonium salt films in solventa of very low dielectric strength has several attractive characteristics. First, the salts are highly conductive in the molten state, having conductivities on the order of 10-3-10-zC1cm-', which are similar to solutions of the salts in organic solvents (42). Although these molten salts poasess high conductivities, they are relatively nonpolar due to the presence of the alkyl groups. Thus, solutes in the nonpolar solvent should partition into the salt film. Additionally, the high viscosity of the molten salts combined with the low viscosity of SC-C02should allow for the use of a two-fluid-phase system even under flowing conditions. Thin film electrodes of known thickness are conveniently constructed by deposition of the salt onto the electrode surface from solution (see Experimental Section). Finally, these molten salts have large potential windows, and thus, a large number of compounds could be oxidized or reduced in the melts. Cyclic voltammograms recorded with molten (THA)N03 film electrodes in N20 are similar to that obtained in pure molten (THA)NOB. Under COz, however, the cathodic limit
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Figure 7. Postcolumn detection of a 0.5-pL injection of ferrocene (1 mM In cyclohexane). Initial column pressure 2000 psi COP;temperature 44 "C. (A) Electrochemicel detection at (lHA)N03 Rlm electrode: E = +1.2 V, m = 5 nA. (6) UV detection at 245 nm: m = 0.005 A.
is less negative by approximately 500 mV. This may be due to the reduction of C02 dissolved in the salt film. Figure 5 shows a cyclic voltammogram of two anal*, O2and prylene, dissolved in SC-N20 at a (THA)N03 film electrode. The oxygen has Er12= -0.9 V vs the Ag QRE, while the perylene is reduced with EIl2= -1.5 V vs the Ag QRE. The large current observed for perylene may reflect efficient partitioning into the salt as well as efficient mass transport from the supercritical fluid. Similar results are obtained with these electrodes in SC-C02. It has been shown that SC-C02 behaves very much like a hydrocarbon solvent (43). In fact, a peculiarity of the tetraalkylammonium salts is their negligible solubility in aliphatic hydrocarbons much like that in SC-C02. In addition, heptane causes a freezing point depression for (THA)N03 to 55 "C. Shown in Figure 6 is a cyclic voltammogram for the oxidation of ferrocene dissolved in heptane at an electrode coated with (THA)N03. The results in heptane are similar to those in supercritical fluids, indicating a similar mechanism.
Molten (THA)N03Film Electrodes as Detectors for SFC. Preliminary experiments indicate that these molten (THA)N03film electrodes may be of use as electrochemical detectors for SFC. Figure 7 shows the postcolumn detection of a sample of ferrocene injected onto an SFC column using both a molten salt film electrode and a UV detector. The first peak in each chromatogram is the detection of the cyclohexane solvent. The second peak is the ferrocene (93 ng) and shows good signal-to-noise characteristics when compared to a previous attempt to accomplish electrochemical detection for supercritical fluid chromatography (44). The detection limit for injected ferrocene at the molten (THA)N03film electrode was 0.1 ng. The peak tailing seen with the electrochemical detector is presumed to be due to ferrocene diffusion within the molten (THA)N03film. While thinner films should alleviate this problem, controlling the film thickness proved difficult with the current cell design. Work is currently underway on a new cell to overcome this problem. CONCLUSIONS The mechanism for voltammetric detection in SC-C02and SC-N20,both containing added water, has been shown to be the result of water film formation on the surface of the electrodes. The amount of water present in the cell needs to be in excess of the solubility limit for ohmic drop to be minimized in the SC-C02system. However, this should not present a problem in using this system for SFC electrochemistry. If a water saturator, injector, column, and detector are placed in series at the same temperature, then the C02 at the detector should be saturated with water. Water-saturated SC-C02 has in fact been shown to be an effective mobile phase for SFC (45-47). Molten (THA)NO, also has been shown to be an effective electrode modifier for analytes dissolved in SC-N20,SC-C02,and heptane. The melting point behavior, which facilitates the use of these electrodes, has been shown to be due to the fluids dissolving in the molten salt. The chemical irreversibility observed for ferrocene, anthracene,
and DPA electrochemistry in molten (THAINOS, while of fundamental interest, does not appear to present a problem for amprometric detection. Additional research is underway to more fully characterize these molten salt electrodes as detectors for SFC. Registry No. (THA)N03, 682-03-1; COz, 124-38-9; N20, 10024-97-2; water, 7732-18-5; ferrocene, 102-54-5; anthracene, 120-12-7; heptane, 142-82-5.
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RECEIVED for review April 15,1991. Accepted May 31,1991. This research was supported by NSF (CHE-899623). P.A.F. acknowledges support from the University of North Carolina at Chapel Hill Program for Minority Advancement in Biomolecular Science.
