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Anal. Chem. 1981, 53, 578-580
Suppression of Background Current in Differential Pulse Voltammetry with Solid Electrodes William F. Sokol' and Dennis H. Evans* Department of Chemistty, University of Wisconsin- Madison, Madison, Wisconsin 53706
A modification of differential pulse voltammetry has been proposed and tested. In the modlfled verslon, currents are measured at two dlfferent times during each pulse, and their difference is computed and dlsplayed. The resulting voltammogram has the same characteristics as those obtalned In the normal manner except that the magnitude of the peak Is somewhat smaller. More Importantly, background current of the type observed wlth platinum electrodes at positive potentials ls dramatically suppressed. The resull is an improved signal-to-background ratio for samples undergoing oxidation at potentials more positlve than about 4-1.5 V vs. SCE. The technique has been evaluated through the study of reverslble (9,lO-dlphenylanthracene) and irreverslble (2,4,6-trl-terl-butylphenol, m-anlsaldehyde, Isobutyraldehyde) oxidations at platlnum In acetonitrile spannlng potentials from t1.2 to 4-2.9 V vs. SCE. The modification resulted In about a 5-fold lncrease in signal-to-background ratio for m-anisaldehyde and lsobutyraldehyde.
The excellent sensitivity of differential pulse polarography has prompted a number of investigations of its applicability with solid electrodes which would permit the use of very positive potentials for which the dropping mercury electrode fails due to oxidation of mercury (1-11). In previous work (3,4)it was shown that excellent results for the determination of various organic species could be achieved by using platinum electrodes and acetonitrile as solvent. Very rapid recording of the voltammograms was achieved with relatively short rest periods between pulses and detection limits in the 100-ppb range were typical. When working at concentrations near the detection limit, it appeared (4) that the magnitude and slope of the background signal, rather than a poor signal-to-noiseratio, actually represented the practical limitation to detection of still lower levels. In aqueous media, the background appears to be an even more severe limitation (7). Thus it is important to find ways of suppressing the effects of the background in differential pulse voltammetry at solid electrodes. There are at least three types of processes which contribute to the background signal in voltammetric techniques: charging of the electrical double layer, faradaic reactions of impurities, and oxidation or reduction of the electrode surface. It is the last type which is often dominant with solid electrodes. In the case of platinum electrodes in acetonitrile (4),the current observed upon application of a potential pulse in the absence of electroactive solutes was found to be composed of two principal parts. The current during the first 10 ms was principally charging current which exhibited an exponential decay with a time constant of 1.8 ms. Beyond 15 ms the current decayed very slowly and its level was quite dependent on the electrode potential, becoming larger as the potential was made more positive. It was this latter component which Current address: Pfizer, Inc., Eastern Point Road, Groton, CT 06340.
was being measured at the end of the pulse and was causing the large and potential-dependent background observed in the differential pulse voltammograms. A complicating factor was the observation that the run-to-run reproducibility of the background was poor. A number of techniques have been used ( 3 , 4 , 12-15) to measure and correct for the background signal in voltammetry. A more direct approach is to suppress the background itself rather than attempting to measure and subtract the existing background. These attempts have been most successful with the dropping mercury electrode whose background current, due principally to double layer charging necessitated by drop growth, is rather well understood. One of the most successful is alternate drop differential pulse polarography (16) in which the two currents whose difference is recorded are measured at the same potential and drop age but on two successive drops rather than a single drop. A similar approach was developed by van Bennekom and co-workers (17-19) who adopted a suggestion of Klein and Yarnitzky (20)that the two currents whose difference is recorded be measured at two different times during the pulse. The faradaic signal arises from the fact that the diffusion-controlled current decays during the pulse. The background is almost completely eliminated because the two currents are measured at the same potential and the drop growth contribution can be removed by electronic application of a theoretical correction term. Other instrumental approaches have involved the application of reverse pulses to cancel double layer charging at stationary electrodes (21,22). Unfortunately these will not correct for any background due to irreversible electrode reactions. In view of the fact that the background observed in differential pulse voltammetry is due to a slowly decaying and highly potential dependent component of the pulse current, it seemed probable that an extension of the van Bennekom technique to differential pulse voltammetry at solid electrodes would be profitable. We have implemented this approach with our previously described computer-based instrumentation (41, and the results are presented in the present paper.
EXPERIMENTAL SECTION Instrumentation. The computer-based electrochemicalinstrumentation has been described elsewhere (4). The software developed for this work was named XPULSE and its features can be understood by reference to Figure 1. The modified technique will be cded XPULSE differential pulse voltammetry (XPDPV) to differentiate it from the conventional technique ( 4 ) , normal mode differentialpulse voltammetry (NMDPV). Potential-time programming for the two techniques is identical. The differences reside in the timing of the current measurements and the difference currents which are calculated and displayed. In NMDPV an average current prior to the pulse, i l , is substrated from an average current obtained near the end of the pulse, izb. In XPDPV two currents are measured during the pulse, izaand i2b, and their difference is displayed. These two currents are averages of a number of data points measured in each half of a data collection interval, t,. On the left side of Figure 1,the expected current-time response is plotted for a potential region where no reaction of the sample
0003-2700/81/0353-0578$01.25/00 1881 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981 579
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Potential-time programming and resulting current with definition of measured currents for XPULSE differential pulse voltammetry (XPDPV) and normal mode differential pulse voltammetry (NMDPV): potentiil giving background response (left);potential causing oxidation of sample (right). Flgure 1.
occurs and the currents are due to background processes. The background current in NMDPV, AiBNM, is large because the currents are measured at two different potentials and the slowly decaying component of the pulse current is strongly dependent on potential, On the other hand, in XPDFV the two currents to be substrated are measured at the same potential and the background current, AiBXP, is very small. The right side of Figure 1indicates the expected current-time response and the difference currents, A i T x p and AiTNM, for the two techniques when the potential has reached a value where the sample reacts. All potential and time parameters in Figure 1are independently variable by XPUISE. The software for NMDPV has been described (4). Cell and Electrodes. The cell was of conventional design (23) but was modified as follows: an additional ground glass joint was added for connection to the solvent reservoir, an 8-mm entry tube with septum was provided for addition of sample, a drain cock was added on the bottom of the cell, and the cell was not jacketed. The cell was purged with dry nitrogen which was vented through an oil-fdter bubbler. Oxygen did not appear to interfere so rigorous purging was not attempted. Construction and polishing of the platinum disk working electrode (0.053 cm2) has been described (24). The counterelectrode was a spiral of 16-gauge platinum wire. The silver reference electrode (AgRE) was composed of a silver wire in contact with 0.01 M AgN03/0.10 M tetra-n-butylammonium perchlorate/acetonitrile(25). Its potential was +0.29 V vs. aqueous
SCE. Reagents. Spectrograde acetonitrile (Burdick and Jackson, Muskegon, MI) was stored over Type 3A molecular sieves until needed. Supporting electrolytes were either tetra-n-butylammonium perchlorate or tetramethylammonium tetrafluoroborate both obtained from Aldrich Chemical Co. (Milwaukee, WI). Tetra-n-butylammonium perchlorate was recrystallizedfrom ethyl acetate-pentane and tetramethylammonium tetrafluoroborate from methanol-water, and both were dried for 24 h under vacuum at 100 "C. 9,lO-Diphenylanthracene and m-anisaldehyde (Aldrich) were used as received, 2,4,6-tri-tert-butylphenol(Aldrich) was recrystallized from ethanol-water and isobutyraldehyde was distilled under nitrogen at atmospheric pressure. Alumina (ICN Pharmaceuticals, Cleveland, OH,neutral W200, activity grade super I) was activated at 600 "C under dry nitrogen for 24 h immediately before use. Procedures. It was found that treatment of the solvent and electrolyte with alumina reduced the background signal for both NMDPV and XPDPV. Alumina is thought to remove traces of water and other nucleophilic impurities (26). The solvent and electrolyte were treated with alumina (about 10 g/50 cm3) by stirring in a separate reservoir, and the treated solution was
AgRE
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Comparison of background current for NMDPV (A) and XPDPV (8): (A) E, = 10 mV, E, = 100 mV, t d t, = 50 ms, t , = 100 ms; (6)E, = 10 mV, E, = 60 mV, t d = 16.67 ms, t , = 66.67 ms. Each measured current was the average of 187 data points; 0.10 M tetra-n-butylammonium perchlorate in CH3CN. Figure 2.
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Figure 3. (A) 9,lO-Diphenylanthracene (5.0 X M) in 0.10 M tetra-n-butylammonium perchlorate/CH,CN. E, = 10 mV, t, = 150 ms, t d = 8.34 ms, t, = 41.66 ms, E, = 25, 50, and 75 mV. Each measured current was the average of 204 data points. (6)2,4,6Trl-tee-butylphenol in 0.10 M tetra-n-butylammonium perchlorate/ CH,CN. E, = 10 mV, E, = 60 mV, t , = 100 ms, fd = 16.67 ms, t, = 66.67 ms. Concentrations: 8.0, 23.8, 41.2, and 58.3 NM. Each measured current was the average of 187 data points.
transferred to the cell through a transfer tube by application of nitrogen pressure. After background voltammograms were obtained, sample stock solution was added via microliter syringe. Before each day's experiments, the platinum working electrode was polished briefly with an aqueous suspension of 0.1 pm alumina Pretreatment was completed by obtaining several background voltammograms over the potential range to be used until no changes were observed. Then sample solution was added and the voltammograms were recorded. Between runs the working electrode was maintained at the initial potential. Up to 25 voltammograms could be obtained without observable changes in behavior. When rejuvenation was needed, the electrode was buffed lightly on a polishing cloth soaked with acetonitrile followed by scanning the potential as above. All experiments were performed at room temperature, 21 f 1 "C.
RESULTS AND DISCUSSION The backgrounds obtained by NMDPV and XPDPV are compared in Figure 2 over the range of 1.2-2.0 V vs. AgRE (+1.5 to 2.3 V vs. SCE). As expected, a dramatic suppression of the background has been achieved. The voltammetric peaks obtained by XPDPV are similar to those from NMDPV ( 3 , 4 ) . Figure 3A shows the effect of pulse amplitude, E,, on the peaks obtained for a reversible electrode reaction, the oxidation of 9,lO-diphenylanthracene
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981 A
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Flgure 4. Comparison of voltammograms of 2,4,6-trl-tert-butylphenol by NMDPV and XPDPV, 0.10 M tetra-n-butylamrnonlum perchlorate/ CH,CN: (A) NMDPV, E, = 10 mV, E, = 100 mV, t, = fa fd = 50 ms, 5.7 pM; (6)XPDPV, E, = 10 mV, E, = 60 mV, t, = 100 ms, t, = 16.67 ms, t, = 66.67 ms, 5.2 pM. Each measured current was the average of 187 data points.
Comparison of voltammograms of m-anisaldehyde by NMDPV and XPDPV, 0.10 M tetra-n-butylammonlum perchlorate/ CH,CN: (A) NMDPV, E, = 10 mV, E, = 100 mV, f, = 100 ms, t, t, = 50 ms, 1.4 pM; (6)XPDPV, E, = 10 mV, E, = 60 mV, fr
to its radical cation. Increasing E, causes an increase in peak width, a shift of the peak potential in the negative direction, and an increase in peak current. The peak current is close to a linear function of E, for E,
