Anal. Chem. 1989, 61, 584-589
584
Solid-state Voltammetry and Polymer Electrolyte Plasticization as a Basis for an Electrochemical Gas Chromatographic Detector Jon F. Parcher,' Carleton J. Barbour, and Royce W. Murray*
K e n a n Laboratories of Chemistry, University of North Carolina, Chapel Hill, N o r t h Carolina 27599-3290
A new type of electrochemlcal gas chromatographic (ECGC) detector is described. The detector conslsts of a fllm of polymer electrolyte, PEO,,/LiCF,SO,, coating a mlcroelectrode-based electrochemical cell and In contact with the effluent stream of a gas chromatograph. The PE0,8/LiCF,S03 polymer acts as an lonlcally conducting but physlcally rigid medium. Currents passed at the microelectrode/polymer interface depend on reactions of electroactlve solutes dkolved in and diffusing through the polymer phase. These currents respond to the presence of sample components In the gas phase through their sorptlon Into, and plastlclzatlon of, the PEO polymer phase. The polymer plasticizatlon results in larger dlffuslon coefflclents for the electroactlve solutes and, consequently, larger mlcroelectrode currents. Several forms of electrochemlcal potential control are examined as are the effects of chosen electroactive probe, probe concentration, polymer film thickness, and gaseous sample concentratlon and sorptlon. Faster detector responses are obtained with thin PEO,,/LiCF,SO, films and faster diffusing electroactlve probes. The detector responds linearly to small quantltles of sample but exponentially to large sample concentrations. The detector is unusual In that It Is most sensitive to later-elutlng components of a sample mixture: this effect results from the connectlon between the degree of sample component sorption or partltlon Into the polymer electrolyte and the resultlng degree of polymer plastlclzatlon and transport rate enhancement.
Electrochemical cells have been employed as detectors in liquid chromatography for some years ( 1 ) and recently by Jorgenson et al(2-5) for capillary electrophoresis (2-5). Owing to the perceived impracticality of the electrolysis of gaseous samples, no electrochemical detector for gas chromatography (GC) has until recently (6, 7) been described. Drawing upon the advantages (8)of the very small currents that flow at very small electrodes (microelectrodes), Pons et al. (6, 7) have designed an electrochemical GC detector that uses concentric microring and macroring electrodes separated by thin films of glass and epoxy sealant/insulators. Electrochemical reactions on the microring or at its edge contacting the insulator are coupled to ion migration across the surface of the insulator film separating the two electrodes. Linear concentration responses and sensitivities varying widely with sample type were reported. This paper describes a second, microelectrode-based electrochemical GC detector whose design is based on our observations of plasticization phenomena (9-13) in solid-state voltammetry research. This detector uses poly(ethy1ene oxide)/lithium triflate solid polymer electrolyte as ionically conducting solvent, and the electrochemical reaction observed On sabbatical leave from the Chemistry Department, University
of Mississippi, University, MS 38677.
is not that of the gaseous sample (6, 7)but is the electrolysis of an electroactive solute-probe dissolved in the polymer electrolyte and diffusing to the polymer/electrode interface. The response of this detector depends on partitioning of the gaseous sample into the thin film of polymer electrolyte and on the plasticizing effect that such partitioning has on the diffusion rate of the electroactive probe in the polymer solvent. The principles upon which this new GC detector operate thus appear to differ fundamentally from the earlier (6, 7) detector. The experiments presented aim at delineating the basic behavior of the polymer-plasticization-based electrochemical detector, which is novel in several respects, and illustrating several modes of electrochemical control of the detector microelectrode during elution and frontal chromatography experiments. This report will not attempt a quantitation or optimization of sensitivity parameters; this subject, which is important for practical applications of the ECGC detector, for establishing limits of detection, and for comparisons to other detectors, requires further (on-going) studies.
EXPERIMENTAL SECTION Chemicals. The poly(ethy1ene oxide) (Aldrich, molecular weight 600 000) was purified by ion exchange chromatography of a concentrated aqueous solution (14);the purified solution was frozen and lyophilized to yield a pure white polymer film. The polymer was dissolved along with LiCF3S03 (24)in 9:l (v:v) acetonitrile-methanol in a concentration ratio of 16 poly(ether) oxygens per lithium ion, which is designated PEOI6/LiCF3SO3. The electroactive probe, which was ferrocenecarboxylic acid, CpFeCpCO,H, or osmium tris(phenanthroline),[Os(phen)J(PF,),, was codissolved in this solution, microdroplets of which were evaporated on the electrode platform of the electrochemical microcell described below. The polymer concentration (0.011 g/mL) was such that evaporation of one 26-pL droplet gave a film about 13 pm thick. (The specific polymer concentration used in the casting solution is unimportant as long as it allows casting of polymer films of the desired thickness from conveniently sized droplets.) ElectrochemicalCell. The cell design is essentially the same as that pictured in our earlier solid-state voltammetry work (9, 10). Briefly, the tips of 10- or 25-pm Pt, 0.15-cm Pt, and 0.10-cm Ag wires serve as working microdisk, auxiliary, and pseudoreference electrode. These wires are sealed in glass and Teflon (9, 10) and polished in a common plane to form a 3-4 mm diameter, platform onto which a thin film of the polymer electrolyte is cast. A schematic cross section of the three electrodes covered with the polymer film is shown is Figure 1. Electrodes were refurbished by dissolving off the polymer film with water, sonicating in acetone, and gentle polishing on tissue paper. The cylinder of glass and Teflon in which the three electrodes are sealed is mounted in a 1/4 in. Swaglok tee fitting at the exit port of the thermal conductivity detector (TCD)of a HewlettPackard Model 5750 gas chromatograph. The exit port was arranged t o protrude into a Faraday cage for noise shielding for the low current electrochemical measurements. The electrochemical detector was operated at room temperature for all experiments excepting those in Figure 8. A typical test mixture for elution chromatography was composed of equal volumes of the four lower alcohols or of n-pentane, toluene, methylene chloride, acetonitrile, and pyridine. A packed SP2340 (cyanosilicone)
0003-2700/89/0361-0584$01.50/0G 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
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Figure 1. (A) Schematic diagram of polymer-coated three-electrode solid-state microelectrode cell in gas chromatograph effluent stream. The central electrode is the tip of a 10- or 25-hm wire sealed in a glass capillary; the auxiliary and reference electrode are tips of larger Pt and Ag wires sealed onto the glass capillary perimeter with shrinkage Tetlon tubing. The microcell is polished so that the three electrodes lie in a common plane, over which is cast the polymer electrolyte solution of the electroactive probe complex. (B) Schematic diagram of the partitioning of an organic vapor into the polymer electrolyte, changing the microfluidity of the polymer phase and thus the rate of diffusion of the electroactive probe complex through the polymer electrolyte.
chromatographic column was used in plug injection experiments. Experiments in a frontal mode used an open tubular column and a simple valved saturator. An IBM-PC-XT microcomputer was used to control the working electrode potential and to measure currents from the high sensitivity potentiostat (9) during chromatographic runs. Several forms of potential control were employed, constant potential, saw-tooth, cyclic potential sweeps, and square wave, as described in the text. Current-potential-time data were collected for each run, and regeneration of chromatograms, background subtraction, etc., were performed off-line.
RESULTS AND DISCUSSION A schematic of the electrochemical gas chromatographic (ECGC) detector design is given in Figure 1. Its electrochemical operation rests on contact of the Pt microdisk electrode with a film of PEOI6/LiCF3SO3polymer electrolyte that contains an electroactive solute (for example ferrocenecarboxylic acid). A microelectrode rather than macrosize electrode is used because the room temperature ionic conductivity of the PEOI6/LiCF3SO3polymer electrolyte is low. Currents at the microelectrode are small enough that ohmic potential losses in the electrolyte are not serious. Currents flowing at the microelectrode, when a potential sufficient to oxidize the ferrocenecarboxylic acid is applied, are governed by the ferrocene concentration and diffusion rate in the polymer solution (9, 10). The electrochemical situation is completely analogous to electrochemical voltammetry in conventional fluid solutions, except that the solvent is rigid and diffusion rates are much slower. The response of the microelectrode current to the gas contacting the solid-state cell occurs via changes in the ferrocene diffusion rate DFer(see Figure 1,lower). The gas stream
Figure 2. Response at an PEOle/LiCF,S03film containing no added electroactive probe: (A) saw-tooth potential excitation profile and (B) resulting current-potential (i.e., current-time) curves observed with the ECGC detector In a stream of He (+) or CH3CN-saturatedHe (0): (C) sampled currents (O),measured at +0.9 V during individual saw-tooth potential sweeps like (A), observed as successive fronts of saturated CH3CN vapor are introduced into and removed from the He stream passing the ECGC detector at ON and OFF. The corresponding TCD response is also shown (-).
at the GC column outlet passes over the polymer solvent film and, just as gaseous samples partitioned into the liquid stationary phase within the GC column, some of the gaseous sample will partition into the detector polymer electrolyte film. Dissolution of the sample in the polymer plasticizes it (a well-known effect in polymer chemistry), which increases the ferrocene diffusion coefficient and thus the observed current. When the band of gaseous sample passes by the electrode, the above process is reversed and the plasticization-enhanced current dies away. Exploring this plasticization-diffusion rate detector response mechanism requires inquiry into (a) the relation between the diffusion coefficient of the electroactive probe in the polymer and the vapor pressure of gaseous sample passing over the polymer/gas interface (i.e., sample concentration), (b) the rate a t which the gaseous sample partitions into the polymer electrolyte film and exerts its plasticization effect on the probe's diffusion coefficient, Le., detector response time and how to optimize it, (c) the reversibility and repeatability of the plasticization effect upon successive exposures of the polymer electrolyte film to a gaseous sample, (d) the appropriate manner of control of the microdisk electrode potential so as to detect changes in the electroactive probe's diffusion coefficient during the short periods of exposure to a band of sample vapor, (e) and the basis for differing sensitivities of the detector to different gaseous samples. We now report results bearing on these points. Background Currents. The ECGC detector was initially evaluated with no electroactive species present in the PE016/LiCF3S03film to determine the response of the electrochemical background currents to film plasticization. Figure 2B shows the currents observed when the film is bathed in steady streams of He and of CH3CN-saturated He and a saw-tooth potential excitation (Figure 2A) is applied to the microelectrode. The background currents are significantly larger in the CH3CN-saturatedgas and rise appreciably more
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Figure 3. Cyclic voltammetry of a mixture of 48 mM [Os(phen),12+ and 27 mM ferrocenecarboxylic acid [CpFeCp(COOH)]dissolved in a ca. 13 km thick film of PEO,,/LiCF,SO, on the miniature electrochemical cell of Figure l A , bathed in He ( - - - ) and in a stream of CH,CN-saturated (room temperature) He (-).
rapidly as the potential becomes more positive. The response of these currents to successive vapor fronts of CH3CN is quite reproducible (Figure 2C). The currents in Figure 2C were sampled at +0.90 V during repeated saw-tooth potential excitations. The plasticized background current (that above the response in He) for this electrode was ca. 50 PA, which is typical in the present set of experiments. The response time of the background current to an acetonitrile vapor front is quite fast, practically identical with that of the thermal conductivity detector. The background current sometimes relaxes more slowly when CH3CN is removed from the gas stream. The background current observations have value in three ways. First, they do respond to the composition of the gas stream and could, in principle, thus be employed as the basis for a detector. In practice, we believe it desirable to rely on currents occurring as the result of better-defined electrochemical processes. Secondly, the background current responses are reproducible. Third, since background currents respond to plasticization, background corrections made to currents observed for reactions of electroactive probes in the presence of plasticizing gases should be determined in blank experiments in the presence of the same level of plasticizer gas. Whenever practical, this was done throughout this paper. Current Responses Based on Redox Probes: Probe Concentration and Film Thickness. Cyclic voltammetry (200 mV/s) of a mixture of ferrocene carboxylic acid and [ O ~ ( p h e n ) ~in ] ~ PEO16/LiCF3S03 + (Figure 3) shows that currents for both electroactive solutes are enhanced in an CH3CN-saturated He stream (-) as opposed to He alone (- - -), The CH3CN-enhanced currents for the ferrocene oxidation are larger than those for [ O ~ ( p h e n ) ~because ] ~ + its diffusion coefficient in PEOl6/LicF3So3is larger. Voltammetry like Figure 3 shows how to select potentials appropriate for oxidation and reduction of the chosen probe species in other forms of potential control of the microelectrode. In most experiments, only one of the two probes is present at a time. The current observed from the ECGC detector should vary linearly with the concentration of electroactive probe added to the PEO16/LiCF3S03film and this is demonstrated in Figure 4. This experiment was based on a 1-Hz square wave potential excitation in which the potential was stepped to potentials that successively oxidize (+1.1 V) and rereduce (-0.2 V) the [ O ~ ( p h e n ) ~redox ] ~ + solute. Sampling the chronoamperometric-like current-time curve for [ O ~ ( p h e n ) ~oxidation ]~+ near the end of each cycle (see vertical marker line) gave the most reproducible behavior. Currents thus measured vary linearly with [0s(phen),l2+ concentration as shown in the figure inset. This is expected if the ECGC detector current is controlled by mass transport of the probe through the polymer in the manner explained above. Clearly, greater ECGC detector currents, and presumably sensitivity, should be obtained at larger electroactive probe
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1
concentrations. Solubility of the electroactive probe becomes important in this connection. We have not attempted to optimize the electroactive probe in terms of solubility, although there are several obvious ways that solubility could be increased. The response time of the ECGC detector to a change in gas stream composition may be determined either by the diffusion coefficient of the electroactive probe or the thickness of the PEO16/LiCF3S03film or by some combination of the two. The former dependency results from the time required for the diffusion profile of electroactive probe that develops within the electrochemical sampling time to adjust to a change in probe diffusion coefficient as the polymer is plasticized. The dependency on film thickness results from the time required for the PEOI6/LiCF3SO3film to equilibrate with the gaseous sample, which includes the rates of sample partition into and diffusion within the polymer film (Figure l),and potentially on the time scale of the molecular events leading to polymer plasticization and a change in the diffusion coefficient of electroactive probe. Effects of film thickness and probe diffusion coefficient are furthermore interrelated if film thickness and diffusion profile have comparable dimensions. Figures 5 and show the film thickness dependency of the responses of PE0,,/LiCF3S03 films containing CpFeCp-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
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Figure 6. Response at PEO, /LiCF,SO, films of indicated thickness containing 50 mM [Os(phen),]'+ electroactie probe to a square wave potential excitation where, like Figure 4, the current is measured near the end of the oxidative potential step. The electrode is exposed to successive fronts of He and acetonitrile-He, shown (-) as measured
by the TCD detector.
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successive fronts were passed by the ECGC detector. We suspect this is caused by gradual and irreversible precipitation of the less soluble electrode reaction product, ferrocenium triflate, which slowly depletes the polymer around the electrode of reactive probe. The point is that while high probe concentrations may yield higher detector sensitivity, concentrations exceeding solubility limits can yield instability in detector response. Response to Sample Concentration and Type. For a given sample, the relation between microelectrode current and sample concentration (or size) is jointly governed by the isotherm for sample sorption into the polymer film and the relation of electroactive probe diffusion coefficient to the sorbed sample concentration. For the specific case of CH3CN sorption into PEO16/LiCF3S03,we have shown (11)that free volume theory can be used to represent how the diffusion + changes coefficient D ( 0 s ) of [ O ~ ( p h e n ) ~in] ~PEol6/LiCF3SO3 with sorbed concentration CCHsCN.To a first approximation, this amounts to the exponential relationship
(COOH) and [0s(phen),l2+,respectively, to fronts of CH&N vapor. The data were collected near the end of the positive potential step in a repetitive square wave potential excitation D(OS)a ~XPIPCCH~CNI (1) like that in Figure 4. (The background current in this experiment is that observed when only dry He flows over the where fi is a constant. detector and is the current at the left of each figure before Two important predictions can be made from this relathe onset of the front of CH3CN.) Several aspects of these tionship. First, since the microelectrode current is linearly results require comment. First, the thinnest films respond proportional to D(Os),in the simplest (hemispherical) diffumost rapidly to both appearance of the CH3CN front and its sion geometry (8), the microelectrode current will change cessation as measured by the TCD detector (-). This is most exponentially with sorbed concentration at high concentration clearly seen in Figure 6. Second, the faster diffusing ferrocene but will respond linearly at concentrations that are low enough yields the more rapidly responding films irrespective of film to linearize eq 1. Second, among a series of samples, the thickness. The response time difference between the thinnest detector will be most sensitive to those with the largest films (10 pm for [ O ~ ( p h e n ) ~and ] ~ +8 pm for CpFeCp(CO0H)) partition coefficients into the polymer film, since these yield is not very great but the response times for [ O ~ ( p h e n ) ~ ] ~ + -the largest concentrations within the polymer. containing films slow more with increasing film thickness than The ECGC detector response at large CH3CN vapor presdo those of CpFeCp(CO0H)-containing films. Previous sures, between 30% and 100% saturated (RT), was examined cmz/s and 2 X lo* measurements (9, 10) give D = 3 X by exposing a PEOl6/LiCF3SO3 film containing both cm2/s for CpFeCp and [ O ~ ( p h e n ) ~ ]respectively, ~+, in CpFeCp(CO0H) and [ O ~ ( p h e n ) ~to ] ~a+ series of CH&N PEOl6/LiCF3SO3equilibrated with room-temperature-satuvapor fronts while scanning the microelectrode potential over both redox waves as was done in Figure 3. The log (oxidation rated CH3CN vapor. The preceding observations, taken together, suggest that the diffusion rate of the electroactive current) values for CpFeCp(CO0H) and for [ O ~ ( p h e n ) ~ ] ~ + probe probably influences response time more than does the (corrected for the underlying ferrocene current) are plotted rate a t which the CH3CN sample permeates into and plasvs CH3CN partial pressure (as measured by the TCD response) ticizes the polymer film. Thus, detectors that are designed in Figure 7A. The rough linearity of the plots shows that the for fastest response should involve choice of very thin polymer responses of the diffusion coefficients of both redox probes electrolyte films and of rapidly diffusing electroactive probes. to sorbed plasticizer follow eq 1. The result confirms our earlier study (11). A third aspect of Figures 5 and 6 is that the currents observed for the CpFeCp(CO0H)-containing polymer film differ The Figure 7A currents for CpFeCp(CO0H) and [Osfor the 8- and 15-pm films whereas those for the three hen)^]^+, normalized for concentration and diffusion coefthicknesses of [O~(phen)~]~+-containing films, within experficient, are similar, which suggests that the plasticization is nonspecific in its effect on polymer solute mobility. This is imental uncertainly, do not. In this connection it is important a fundamentally important point, but we believe careful obto remember the geometry of the microdisk electrode and polymer film. For short electrolysis times, slow diffusing servations are needed over a broader range of solutes before electroactive species, or thick films, the diffusion profile geany general conclusion is drawn. ometry in the polymer is either linear or hemispherical whereas Response of the ECGC detector to low sample concentrafor long time electrolysis, fast diffusing electroactive species, tions is shown in Figure 7B where currents passed a t a conor thin films, the diffusion profile geometry becomes a very stant potential on the plateau for [ O ~ ( p h e n ) ~oxidation ]~+ are short cylinder. We have analyzed this somewhat complex measured as a series of acetonitrile samples are successively injected onto a packed column. (The large current transient situation elsewhere (IO). While we have not attempted to theoretically treat the present data, the behavior seen leads near zero time is due to the detector voltage being stepped us to believe that the film-thickness dependency in Figure 5 to the oxidizing potential just shortly before the sample was simply reflects a (probably partial) switch from the latter to injected in these experiments; the [ O ~ ( p h e n ) ~diffusion ]~+ layer the former diffusion geometry as the film is made thicker. The has reorganized and a background current level established main point to remember is that interpreting the absolute at about 4 min in the example shown.) The small sample sizes magnitudes of observed currents requires attention to diffusion used, and the plug injection, provide a more typical gas geometry as well as previously mentioned factors. chromatographic circumstance than the frontal experiments above, and we see that the eluted peaks are well shaped, with We should mention a t this point that while the responses in Figures 5 and 6 are reproducible with successively repeated a small degree of band tailing. The currents are linearly on-off CH3CN fronts, we did not find this to be the case using related to the amount of acetonitrile injected as seen by the (unsubstituted) ferrocene as the electroactive probe. The comparison with the concurrently obtained TCD response in ferrocene oxidation currents drifted toward smaller values as Figure 7C. That is, we are on the linear part of the isotherm
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Figure 7. (A) Currents i, measured on the CpFeCpCoCm (55 mM) and [O~(phen)~]*+ (102 mM) oxidation plateaus of cyclic voltammograms lke Figure 3. as the ECGC electrode is exposed to fronts of CH3CN-!ie gas containing high partial pressure of acetonitrile, plotted according to eq 1. Currents are normalized for concentration and diffusion and 3 X lo-’ cm2/s for coefficient, using the D equals 2 X CpFeCpCOOH and [Os(phenb12+at PIPo = 1.0. (B) Chromatographic peaks detected with an PE0,e/LiCF,S03 ECGC containing 48 mM [Os(phen),12+ by applying a constant potential (+1.0 V vs Ag) on the [~s(pt~en),]~+ oxidation plateau, for the successive injection of various small volumes (0.5-5 pL) of acetonitrile onto a 2-m SP-2340 column at 52 OC. (C) Currents detected in (B) compared to the injection volume (as measured with the TCD response).
in Figure 7A. This is an important result inasmuch as a linear detector response is in general more analytically desirable. Application of the ECGC to detection of a simple chromatographic resolution is shown in Figures 8 and 9. In Figure 8, a 4-gL sample of an equivolume mixture of the lower alcohols methyl, ethyl, l-propyl, and l-butyl alcohol, is injected onto an SP-2340 column and its elution detected by the currents passed a t a constant potential on the plateau for [O~(phen)~]*+ oxidation. In Figure 9 , 5 p L of an equivolume mixture of methylene chloride, toluene, acetonitrile, and pyridine is injected onto an SP-2340column and detected with the ECGC using a constant applied potential. No electroactive probe is introduced into the polymer electrolyte in Figure 9; recall that Figure 2 showed plasticization effects on background currents. In both experiments, well-formed chromatographic responses are obtained. (In Figures 8 and 9, potential was applied and background current level established prior to injection; the background currenta are subtracted from the ECGC responses shown.) The band shapes are similar to the TCD responses except that a slight tailing is again evident. Since the ECGC detector is downstream from the instrument TCD detector, slightly larger band broadening for the former is expected. In any chromatographic separation, the retention times and order of elution of the components of the mixture from the column are governed by their individual capacity factors. The capacity factor for an individual sample component is proportional to the fraction of the sample that becomes sorbed into the stationary polymer phase in the column packing as the component’s band passes through the column. The sensitivity of the ECGC detector response is similarly determined by the amount of the sample sorption, in this case the sorption
Flgure 8. (A) TCDdetected chromatogram of a 4-pL injection of an equivoiume mixture of methanol, ethanol, 1-propanol, and 1-butanol onto a 4-m SP-2340 column at 52 ‘C. (B) ECGC currents detecting the same chromatogram as (A), using an PE0,e/LiCF3S0, ECGC containing 48 mM [ O s ( ~ h e n ) ~ and r a constant applied potential (4-1.0 oxidation plateau. Column and ECGC V vs Ag) on the [&(phen),] detector temperature both 52 OC. +
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being into the polymer electrolyte of the detector. We consequently expect a correlation between chromatographic capacity factor and ECGC detector sensitivity; these parameters should increase together. The relationship between chromatographic capacity factor and ECGC detector response is clearly evident in Figures 8 and 9. In the thermal conductivity chromatograms, the later-eluting peaks are more spread out because of the diluting effect of chromatographic bandbroadening phenomena; the current peaks in the ECGC chromatogram also exhibit more band broadening in the later
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
peaks as expected. However, while the amplitudes of the thermal conductivity peaks are diminished by the bandbroadening effect, the amplitudes of the ECGC peaks are little different from one another in Figure 8 and in Figure 9 are actually larger for the later eluting components. In fact, the ECGC detector is quite unresponsive to the early-eluting pentane in Figure 9. The larger intrinsic sensitivity of the ECGC detector to later eluting peaks is a very unusual, and potentially useful, gas chromatographic detector characteristic. We are currently exploring the capacity factor/detector response further to place it on a quantitative footing. The chromatogram in Figures 8 also emphasizes the most basic feature of the detector response, which is (Figure 1)that the electrochemical signal does not respond directly to the gas-phase composition but rather responds to the composition of the polymer phase and thus indirectly to the gas-phase composition. The present detector furthermore does not rely on any electroactivity of the gaseous sample but transfers the polymer partitioning characteristics of the gaseous sample to changes in electroactivity of an flexibly chosen redox probe. These characteristics are quite different from those of the previously reported electrochemical detector for gas chromatography (6, 7)which, according to the authors, responds by electrolysis of the gaseous sample. As noted in the Introduction, we do not attempt here to quantitatively assess, or to optimize, the ECGC detector sensitivity or to compare it to other detectors such as the TCD. It is clear from Figures 8 and 9 that a comparison of the ECGC sensitivity to that of other detectors is not straightforward, since the ECGC sensitivity is a function of capacity factor. It will not be surprising to find that the ECGC detector may have greater sensitivity than TCD and other detectors for samples with large capacity factors but much worse for samples with small capacity factors. Irrespective of the detector sensitivity issue in the analytical GC sense, we should note that the experiments described here may have a number of nonanalysis applications. These include investigating the dynamics of chemical reactions in polymer electrolyte phases and the molecular basis of polymer plasticization, topics of current interest in our laboratories. Finally, as part of demonstrating how the ECGC detector functions, we have employed several different electrochemical
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procedures for collecting data: saw-tooth, square wave, cyclical potential sweep, and constant potential. From the standpoint of sensitivity, we are not prepared a t this time to recommend one of these approaches over the other. Using a constant microelectrode potential is clearly attractive from the viewpoint of experimental simplicity. Also, the quantity of transient data collected in the more complex potential excitations can tax both the data collection rate and storage capacity of the microcomputer system employed (if the entire transient is saved). It is well-known on the other hand, that great increases in electrochemical sensitivity are available with appropriate waveforms (15). Additionally, sensitivity may be improved by choice of microelectrode geometry; for example microbands can yield larger measurable currents with relatively little expense in iR drop difficulty, as compared to microdisks.
LITERATURE CITED Kissinger, P. T. Anal. Chem. 1977. 49, 447A. Knecht, L. A.; Guthrie, E. D.; Jorgenson, J. W. Anal. Chem. 1984, 56, 479-482. St. Claire, R. L.; Jorgenson. J. W. J. Chromatogr. Sci. 1985, 23, 186-191.
White,J. G.; St. Claire, R. L.; Jorgenson, J. W. Anal. Chem. 1988, 58. 293-298. White, J. G.; Jorgenson, J. W. Anal. Chem. 1986, 58, 2992-2995. Ghoroghchian, J.: Safarari, F.; Dibble, T.; Cassidy, J.; Smith, J. J.; Russell, A.; Dunmore, G.; Fleischmann, M.; Pons, S. Anal. Chem. 1986, 58. 2278-2282. Brina, R. B.; Pons, S . ; Fleischmann J. Electfoanal. Chem. 1988, 244, 81-90, Wightman, R. M. Science 1988, 240, 415. Reed, R. A.; Geng, R. L.; Murray, R. W. J. Electfaanal. Chem. 1986, 208, 185-193. Geng, L.;Reed, R. A.; Longmire, M. L.; Murray, R. W. J. Phys. Chem. 1987, 9 7 , 2908-2914. Geng, L.; Longmire, M. L.; Reed, R. A.; Parcher, J. F.: Barbour, C. J.; Murray, R. W. Chem. Mater., in press. Geng, L.; Reed, R. A.; Oliver, B. N.; Egekeze, J.; Kim, M.-H.; Wooster, T.; Murray, R. W. J . Am. Chem. Soc ., in press. Murray, R. W., C. N. Reilley Award Symposium, Piftsburgh Conference, New Orleans, LA, Feb 1988. Papke, B. L.; Ratner, M. A,: Shriver, D. F. J. Phys. Chem. SolMs 1981, 4 2 , 493. Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications; John Why: New York, 1980.
RECEIVED for review August 30, 1988. Accepted December 9, 1988. This research was supported in part by grants from the National Science Foundation and from the Department of Energy (DE-FG05-87ER13675).
