Anal. Chem. 1986, 58,293-298
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Scanning On-Column Voltammetric Detector for Open-Tubular Liquid Chromatography Jackie G.White, Robert L. St. Claire 111, and James W, Jorgenson*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514
A scannlng mlcrovoltammetrlc detector for open-tubular llquld chromatography Is described. Thls on-column detector conslsts of a 9 pm 0.d. carbon fiber Inserted Into the end of a 15 pm 1.d. caplllary column. The electrode potentlal can be scanned at rates up to 1 V 8-' over a potentlal range of 0.0 to 4-1.5 V vs. Ag/AgCI. The electrochemlcal response Is consistent wlth the theory for thln-layer cells wlth modlflcatlons to account for the flowlng stream. Dtfferbnt methods for obtalnlng quantitative lnformatlon are dlscussed. Detection I h H s for hydroqulnone and catechol are less than IO-' M, and the response Is well-behaved over the tested range of 10-3-10-7 M. The two-dlmenslonal resolving power Is demonstrated In the separatlon of a human urlne sample.
Electrochemical detection for open-tubular liquid chromatography (LC) offers high sensitivity for electroactive compounds (1). Amperometric detection is the most popular mode because it generally provides the best sensitivity (1-10). The main drawbacks of fixed potential detection are poor selectivity and the lack of electrochemical information obtained with the technique. Differential pulse detection has been used to improve selectivity (10) but does not provide general electrochemical information about a sample. Techniques using potential scanning provide increased information content but usually involve a loss of sensitivity. Potential scan methods suffer from increased background due to charging current associated with the rapid scan rates necessary for real-time chromatographic detection. Square-wave voltammetry, a method in which the background current is discriminated against, has been used for detection in conventional high-performance LC (HPLC) (11,12). Last has demonstrated a multichannel detector based on the Coulostatic principle, which also discriminates against charging current (13, 14). In this paper a scanning microvoltammetric detector is used for on-column detection in open-tubular liquid chromatography. This detector has been previously demonstrated in amperometric and differential pulse modes (9, 10). The working electrode is a 9 pm 0.d. carbon fiber that is inserted into the outlet end of a 15 pm i.d. capillary column. An advantage of microvoltammetric electrodes, demonstrated by Howell and Wightman (15), is the reduced double layer capacity that permits rapid scanning of the electrode potential. The detection system described in this paper includes background subtraction capabilities to further reduce background currents. The method of background subtraction is discussed, and a qualitative analysis of the electrochemical response is given. Different methods of presenting chromatographic information are described, and the potential usefulness of the detector for the analysis of a complex biological sample is demonstrated. EXPERIMENTAL SECTION Apparatus. Wave form generation and data acquisition were controlled by an IBM XT personal computer (Boca Raton, FL) interfaced with a Tecmar Labmaster (Tecmar, Inc., Solon, OH). The Labmaster is equipped with 12-bitdigital-to-analog converters (D/A), 16-bitanalog-to-digital Converters (AID),and &bit timers. One timer was used to control the rate of data acquisition. The 0003-2700/86/0358-0293$01.50/0
D/A had a full output voltage range of -10 to +10 V. A voltage divider was used to reduce the range to -2 to +2 V, full scale. The D/A was connected via the voltage divider to a Ag/AgCl reference electrode (0.1 M KC1) and was used to directly control the cell potential. A Model 427 current amplifier (KeithleyInstruments, Inc., Cleveland, OH) with a 30-ms rise time (10-90%, corresponding to a time constant of approximately 15 ms) and variable gain connected the working electrode to the AID. A triangular wave form was used in which the voltage was stepped in 2-mV increments. The cycling of the potential was begun before injection and continued until the end of the chromatographicrun. Collection of data was initiated at the time of injection and continued until terminated by the user. Data were acquired every 10 mV, at the end of every fifth potential step. Data collection was controlled by an assembly language subroutine adapted from a previously published routine (16). Real-time CRT display of the current at two user-defined voltages was used to monitor the chromatogram. Data were stored in memory and could be transferred to the 10-Mbytefixed disk on the IBM XT after data collection was complete. All post-run processing was done in BASIC or compiled BASIC. An Amplot I1 digital plotter (Amdek Corp., Elk Grove Village, IL) was used to obtain hard copy plots. The chromatographicsystem and electrochemical cell have been previously described (9). Work reported here was done on 15 pm i.d., 190 cm long, octadecylsilane reversed-phase columns. The method of electrode pretreatment and cleaning was done as previously reported (IO). In the studies of electrochemical behavior as a function of mobile-phase flow rate, potential scan rate, electrode length, and buffer concentration, long injections of samples onto columns containing no stationary phase were used so that the electrochemicalphenomena could be studied free of the complicating effects of chromatographic band profiles. Injection times in the chromatographic studies were 5 s unless otherwise stated. By use of a scan rate of 1V s-l and a potential range of 0.0 to +0.8 V, approximatelysix scans are collected during the elution of a 10 s wide peak. Pretreatment of Urine. The urine sample was adjusted to pH 1.0 with HC1 and heated at 80 "C for 30 min. After the sample was cooled, it was readjusted to pH 3.0 with NaOH and filtered. Chemicals. The mobile phase used was a pH 4.5 buffer that was 1.0 M phosphate, unless specified otherwise. Sample solutions (except urine) were made by using the mobile-phasebuffer as the solvent. Phosphoric acid was obtained from Fisher Scientific Co. (Fair Lawn, NJ), and all other chemicals were obtained from Sigma Chemical Co. (St. Louis, MO). Fresh solutions were made for each series of experiments. RESULTS AND DISCUSSION Electrochemical Results. Background Subtraction. The voltammograms obtained during a chromatographic run are severely distorted by the background current, upon which the faradaic current is superimposed. This is seen in the top of Figure 1where the background current (solid line) and the current for M hydroquinone (dotted line) are plotted. (Note that oxidative current is plotted on the positive y axis. This convention will be used throughout this paper.) The shape of the background wave is very reproducible, but the magnitude changes, particularly during the first 20-30 scans. When scanning of the electrode potential is initiated, the current decreases rapidly for about 20 scans. After this initial period, the background stabilizes and the current decreases much more slowly, generally less than 1nA during a 15-min period. To ensure a stable background, potential scanning 0 1986 American Chemical Society
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Figure 2. Forward scan voltammograms of 0.1 mM hydroquinone as a function of flow rate. Scan rate was 1 V s-‘ and electrode length
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(top) Unsubtracted voltammograms of the background and M hydroquinone (---) and (bottom)the subtracted voltammogram. The scan rate was 1 V s-’, flow rate 0.39 cm s-’, and electrode length 1.6 mm.
Figure 3. Voltammograms of 0.1 mM hydroquinone as a functlon of scan rate. Flow rate was 0.39 cm s-‘ and electrode length 1.1 mm.
is begun about 60 s before injection of the sample. Because the shape of the background wave is reproducible and the magnitude changes are small, the background can be subtracted from the total current to give a voltammogram representing the faradaic current, as shown in the bottom of Figure 1. Generally a scan immediately prior to each peak is chosen for the background. (Because all data are collected and stored by computer, this can easily be done post-run.) If the peaks are not resolved or the chromatogram is quite complex, a scan is chosen before the first peak and used as the background for the entire chromatogram. This subtraction technique assumes that the background stays constant during the elution of a peak. Factors such as adsorption to the electrode, which could cause a change in the background, are currently being studied. Effect of Flow Rate. The electrode configuration may be regarded as a thin-layer cell in which the cell volume consists of a thin, annular layer of solution between the electrode surface and the column wall. Under zero flow conditions, voltammograms are symmetrical peaks, as expected for a thin-layer cell ( l a ,but the introduction of convective flow modifies the electrochemicalresponse. This can be seen in Figure 2, in which the forward scan of 0.1 mM hydroquinone is plotted at four different flow rates. The scan rate was held constant at 1 V s-l. A t potentials well below EO’, no oxidation occurs and the annular region becomes filled with solute molecules. As the potential approaches E O ’ , the current increases, causing the concentration of solute at the electrode surface to decrease and creating a flux to the electrode. When the surface concentration is essentially zero, flux (and thus current) reaches a maximum. Depletion begins in the annular region and current decreases. At this point the effect of the restricted diffusion layer becomes evident. The average distance from the electrode to the column wall is only 3 Fm, and diffusion can occur across that distance in approximately 4 ms (assumingsolute diffusion coefficient = cm2s-l). At scan rates of 1V s-l and less, this layer is smaller
than the diffusion layer under semiinfinite diffusion conditions. Therefore the entire annular region becomes depleted. When there is no flow through the column, the current drops back to zero. Under normal chromatographicconditions, flow delivers a constant flux to the electrode, causing the current to reach and maintain a “plateau” level greater than zero. As seen in Figure 2, the level at which the current plateau occurs is dependent upon the flow rate. As the flow rate increases, the waves become less peaked and steady-state behavior begins to predominant. These results indicate that two models must be combined to describe this system. At the zero flow extreme is the thin-layer model. It accounts for the depletion of the annular region and the peaked voltammetric wave. At the high flow extreme is the steady-state hydrodynamic model. This describes the role of convective mass transfer and the resulting steady-state voltammetric wave. At intermediate flow rates, a combination of the two models is observed, resulting in waves that show various degrees of peaking with the current reaching a steady state dependent on flow rate at high positive potentials. Flow rate is not the only factor that affects the shape of the voltammetric wave. Scan rate and the length of the electrode also play important roles in determining the current-potential response. Effect of Scan Rate. Figure 3 shows voltammograms of 0.1 mM hydroquinone at three different scan rates. The flow rate was kept constant at 0.39 cm s-l. The peak current of the forward scan is consistently larger than that of the reverse scan because products are swept away from the electrode surface by the flow. At high scan rates, some of the oxidized products are still in the vicinity of the electrode and are thus available for reduction. As the scan rate decreases, products have more time to escape the electrode and the reverse wave becomes smaller. As mentioned in the previous section, scan rate also affects the shape of the forward voltammetric wave. In a conventional
Figure 1. current (-)
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Forward scan voltammograms of 0.1 mM hydroquinone as a function of electrode length. Scan rate was 1 V s-’and flow rate 0.39 cm s-’.
Flgure 5. Forward scan voltammograms of 0.1 mM hydroquinone as a function of mobile-phase phosphate buffer concentration. Scan rate was 1 V s-’, flow rate 0.39 cm s-I, and electrode length 0.8 mm.
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Figure 4.
thin-layer cell, the peak current is directly proportional to scan rate (17). Under hydrodynamic conditions, current is independent of scan rate. It is expected, therefore, that this system would exhibit some intermediate dependence on scan rate. This is found to be true. The peak current shows a scan rate dependence, but it is not directly proportional as in a true thin-layer cell. At +0.8 V, where the current has reached a steady state, no scan rate dependence is observed. Furthermore, as the flow rate increases the peak current becomes less scan rate dependent. This is consistent with the combined thin-layer/hydrodynamic model previously described. At low flow rates, the system is approximated by a thin-layer cell modified by convection. The peak current exhibits a scan rate dependence, but that dependence is changed by the presence of the flowing stream. As the flow rate increases, conditions approach steady state and the peak current becomes less scan rate dependent. At high positive potentials, the system is under steady-state conditions a t all flow rates examined, and the current is not scan rate dependent. Effect of Electrode Length. The length of the electrode also is a contributing factor to the current-potential behavior. As seen in Figure 4, the peak current depends on electrode length, but at +0.8 V the current is the same for all electrode lengths. The peak current increases with increasing electrode length because the area of the electrode that becomes surrounded by solute molecules during the part of the scan when no oxidation occurs is greater, and thus the current is larger (increasing the importance of thin-layer-type behavior). After the annular region surrounding the electrode is depleted, current depends on the flux due to convection. Under these conditions, electrolysis is occurring principally near the electrode tip, therefore electrode length is no longer a contributing factor. The maximum peak current is limited by the distance along the electrode that can be replenished during the time when no oxidation occurs. The individual effects of flow rate, scan rate, and electrode length on the voltammetric wave shape have been discussed, but the interrelationship of these parameters must also be considered. Thin-layer behavior is enhanced by slow flow rates, fast scan rates, and long electrodes. At fast flow rates, slow scan rates, and short electrodes, hydrodynamic behavior dominates. Each parameter determines the effect of the other two. At slow scan rates, sigmoidal voltammograms are observed even at moderate flow rates, indicating that the hydrodynamic model predominates, whereas at high scan rates, fast flow rates are necessary to achieve a sigmoidal, steadystate voltammogram. Control of these three parameters can be used to favor either thin-layer behavior or the hydrodynamic steady state. Effect of iR Drop. One problem associated with thin-layer cells (13,and also encountered in this cell, is the occurrence of a potential gradient along the length of the electrode. The
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Flgure 8. Forward scan voltammograms of 0.1 mM ascorbic acid (AA), hydroquinone (HQ), catechol (CAT), and 4-methylcatechol (MCAT). Scan rate was 1 V s-I, flow rate 0.39 cm s-’, and electrode length 1.6 mm.
results of this are a shifting of the voltammetric wave to more positive potentials and lower peak currents. This can be seen in Figure 5, in which the concentration of phosphate buffer in the mobile phase was varied between 0.05 and 1.0 M. The higher ionic concentrationresulted in a steeper wave at a lower potential and a higher peak current. Chromatographic Results. Computerized data collection makes possible several different methods for obtaining post-run chromatographicinformation from the electrochemical data, depending on what type of information is desired. Figure 6 shows the forward scans for four compounds, in the order of their elution from the column: ascorbic acid (AA), hydroquinone (HQ), catechol (CAT) and 4-methylcatechol (MCAT). The simplest method for obtaining a chromatogram is to choose a single voltage and plot the current at that voltage vs. time. The chromatograms obtained at +0.8 V and +0.4 V are shown in parts A and B, respectively, of Figure 7. Because the relative peak intensities depend upon the choice of voltage, it is important to know the shape of the voltammetric waves. Choice of less positive potentials can be useful for the enhancement of a single peak, as seen in the chromatogram at +0.4 V where the HQ peak is predominant. This is only useful if the compound of interest is one of the most easily oxidized compounds in the mixture. Compounds with more positive E”’ values can be enhanced through a difference current technique, previously demonstrated by Last (13,14).In this method, the current at one potential is subtracted from the current at a second potential. Figure 7C shows the chromatogram resulting from the subtraction of the current at +0.44 V from the current at +0.48 V. These potentials correspond to the peaks of the AA,HQ, and MCAT waves, and thus the difference current is very small for these compounds. A large difference current is obtained for CAT, however, because +0.44 and +0.48 V bracket the rising part of the CAT wave. The result is a single
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Flgure 8. (Top)Voitammogram of M hydroquinone. Scan rate was 1 V s-’, flow rate 0.39 cm si, and electrode length 1.6 mm. (Bottom) Same voltammogram following a seven-point smooth. TIME (minutes)
Flgure 7. Chromatograms of an equimolar mixture (0.1 mM) of ascorbic acid (AA), hydroquinone (HQ), catechol (CAT) and 4-methylcatechol (MCAT), in order of elution: (A) single voltage chromatogram at +0.8 V vs. Ag/AgCI; (B) single voltage chromatogram at +0.4 V vs. Ag/AgCI; (C) dlfference current between +0.44 and 4-0.48 V vs. Ag/AgCI; (D) summed current between +0.3 and 4-0.8V vs. Ag/AgCI. Scan rate was 1 V sbi, flow rate 0.39 cm s-’, and electrode length 1.6 mm.
large peak for CAT and small peaks for the other three compounds. The difference current method can be used to improve the selectivity for a particular compound with a higher E O ’ . However, there is some loss in the signal-to-noise ratio. The signal-to-noiseratio and the “generality”of the detector can be improved by summing the currents between two voltages. This is seen in Figure 7D, where the currents were summed between +0.3 and +0.8 V and normalized for the number of points used. This improves the signal-to-noiseratio by using more of the available data. Because the data points are collected at even intervals of time, the result of summing the current is the same as integrating under the voltammetric wave. Integration of current results in charge, which is proportional to the number of moles electrolyzed. The chromatograms shown in Figure 7 are all the result of a single experimental run. One advantage of this detection system is that little experimental preplanning is necessary to obtain a particular type of chromatographic data. Data are collected by using a routine procedure and stored by the computer. Once the data are stored, many different data presentation techniques can be tried in order to determine how to obtain the most useful information. Detection Limits. The present detection limit is less than lo-’ M for HQ and CAT. Shown in the top of Figure 8 is the voltammogram for lo-’ M HQ. The forward and reverse waves are qualitatively the same shape as at high concentrations despite the noise. Based on a maximum peak-to-peak noise, the S I N is approximately 5. The noise level can be reduced through the use of a seven-point smooth (It?), increasing the
Table I. Comparison of Different Quantitative Methods method
slope
RASD”
I. voltammetric peak current 11. current a t +0.8 V 111. summed current IV. peak ”volume”
0.878 0.946 0.924 0.923
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0.018
corr coeff 0.9981 0.9972 0.9982 0.9983
“Relative average standard deviation of the slope (%).
S I N to about 9 (Figure 8, bottom). Linearity. One method for evaluating the linearity of a detector is the use of a log-log plot of response vs. concentration (19). Hydroquinone samples ranging from to M were used to assess the linearity of this detector. Four M injections at each concentration were made, except at where three injections were made. The nature of the detection system makes numerous methods of quantitation possible. Table I shows the slope, relative standard deviation of the slope, and correlation coefficient from log-log plots of concentration vs. current for four different cases. The current value used in each case is calculated differently. In cases I and I1 the current at a single voltage is taken from the voltammogram collected at the maximum of the chromatographic peak. In case I the voltammetric peak current is used, regardless of the potential at which the maximum occurs. Case I1 uses the current at +0.8 V, the most positive potential in the scan. Cases 111 and IV represent summed currents. In case 111the currents from the forward scan of the voltammogram collected at the chromatographicpeak maximum are summed. Case IV extends this by further adding all of the voltammograms collected during the elution of the entire peak, thus giving a “volume”under the peak. In all four cases, the background is subtracted out. The slope in each case is less than one, indicating that the current at higher concentrations is less than expected. This deviation from unity slope is attributable to the iR drop oc-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
TIME (minutes)
+
Figure 9. Single voltage chromatogram at 1.O V vs. Ag/AgCI of (in order of elution) 0.1 mM ascorbic acid, 0.1 mM olepinephrine, 0.2 mM tyrosine 0.1 mM dopamine, 0.1 mM hydroquinone. Scan rate was 1 V Si, flow rate 0.59 cm s-', and electrode length 1 mm.
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Flgure 12. Single voltage chromatogram of human urine at 4-1.0 V vs. Ag/AgCI. Potential range was 0.0to +1.5 V vs. Ag/AgCI, scan rate 1 V si, flow rate 0.39 cm si, and electrode length 0.8 mm. Enlarged region (- -) represents section of chromatogram displayed in Figures 13 and 14.
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Figure 10. Partial threedimensional chromatovoltammogramof the mixture shown in Figure 10. Background currents have been subtracted out.
curring in the system, causing the voltammograms to be distorted. This distortion results in a general shifting of the wave to more positive potentials and consequently lowers the current (17). This is particularly evident at the higher concentrations due to the increased current levels. The voltammetric peak current, used in case I, is the most likely to be affected by iR drop and therefore has the smallest slope. Cases 111and IV are similarly affected by the iR drop. The distortion of the voltammetric wave is more pronounced a t potentials close to Eo' than at higher potentials. Cases 111and IV sum over the entire potential range and are thus influenced by iR drop, although the effect is not as strong as in Case I. The highest slope is obtained for case 11, where the current at +0.8 V is used. At this potential, current is determined primarily by the delivery of solute to the electrode by the mobile-phase flow and thus is the least susceptible to iR drop. As indicated by the correlation coefficients,the log-log plots are linear, regardless of the method used. This means that by using a calibration curve, quantitative information is available. Applications. One distinct advantage of this detection system is its two-dimensional resolving power. Figure 9 shows the chromatogram of five compounds at +1.0 V (in order of elution): ascorbic acid, epinephrine, tyrosine, dopamine, and hydroquinone. The third peak contains both tyrosine and dopamine since they are not resolved chromatographically. This is apparent from the three-dimensional chromatovoltammogram shown in Figure 10. There is no single potential at which the presence of two peaks would be detected,
yet in the 3-D plot the two peaks are quite obvious. One point of interest is the stability of the current between and after the elution of peaks. For the chromatograms shown, the background scan used was a scan obtained immediately prior to the elution of ascorbic acid. It can been seen from the contour plot (Figure 11) that although a single background scan was used for the entire chromatogram, no large changes in current are seen, even after the elution of the phenolic compound, tyrosine. Some minor current variations are evident, particularly around the dead time and at low potentials, but no distortion of the faradaic current is observed. The 3-D and contour plots present the same data, but each method of presentation has its advantages. The 3-D plot resembles the more traditional chromatogram and is easily interpretable. Relative peak heights and shapes are readily apparent. In Figure 10, ascorbic acid shows an obvious difference in the voltammetric wave shape from the other components of the mixture. The 3-D plot is more continuous than the contour plot, and thus small variations in current are more apparent. However, sections of the chromatovoltammogram can be hidden in the 3-D plot by large peaks, a problem not encountered with contour plots. The contour plots are also particularly useful for studying peak symmetry. Both methods of data presentation are useful, and when combined, the three-dimensional and contour plots give a more complete view of the chromatovoltammogram. As a test of the usefulness of this detector, a sample of 24-h human urine was injected and the potential scanned between 0.0 and +1.50 V. The single voltage (+LO V) chromatogram is seen in Figure 12. The dotted insert represents the section used for the three-dimensional and contour plots shown in Figures 13 and 14. Figure 13 shows the existence of many peaks, only three of which are visible in the single voltage chromatogram at 1 V. Several large peaks occur at high potentials (+0.8 to +1.5 V) whereas some minor components
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\8.8 Flgure 13. Partial three-dimensional chromatovoltammogram of the human urine sample seen in Figure 12. Background currents have been subtracted out.
This detection system has been shown to be useful over the potential range of 0.0 to +1.5 V,and future work includes the extension of the potential range to more positive potentials. Detection at these potentials is made possible by the background subtraction technique, which minimizes the background current caused by solvent oxidation. All work done thus far has been oxidation at positive potentials. Investigations are planned to extend the potential range to negative potentials and thus make reductive electrochemical detection possible. The triangular wave form used in these electrochemical studies provides information about the electrochemical processes that occur. For general detection purposes, collection of the reverse scan is unnecessary. Furthermore, the reverse scan can have detrimental effects on the shape of the next forward scan, as seen in Figure 3. When the reverse wave is small, reduction is completed during the reverse scan and the current returns to zero when the next forward scan begins. When the reverse wave is large, as in the 1V s-l case, reduction is not completed during the reverse scan and overlaps into the next forward scan. This causes the current to be less than zero for the first part of the subsequent forward scan. Wave forms that eliminate this problem are to be investigated. One possibility is the combination of a forward ramp and a reverse step. The potential could be ramped slowly from the initial potential to the final potential, stepped quickly back to the initial potential, and held there for a fixed period of time. This would allow more time for reductions to occur and for the current to return to a constant initial state.
LITERATURE CITED Stulik, K.; Pacakova, V. CRC Crlf. Rev. Anal. Chem. 1984, 14, 297-35 1. Slais, K.; Krejci, M. J . Chromatogr. 1982, 235, 21-29. Hirata. Y.: Lin, P. T.; Novotnv, M.; Wiahtman, R. M. J . Chromatogr. 1980, 181, 287-294. Goto, M.; Koyangi, Y.; Ishli, D. J . Chromatogr. 1981, 206,261-268. Goto, M.; Nakamura, T.; Ishii, D. J . Chromatogr. 1981, 226, 33-42. Slais, K.; Kourllova, D. J . Chromatogr. 1983, 256,57-63. Goto, M.; Sakurai. E.; Ishii, D. J . Chrornatogr. 1982, 238, 357-366. Slais, K.; Kourilova, D. Chromatographia 1982, 16, 265-266. Knecht, L. A.; Guthrie, E. J.; Jorgenson, J. W. Anal. Chem. 1984, 56, 479-482. St. Claire 111, R. L.; Jorgenson, J. W. J . Chromatogr. Sci. 1985, 23, 186-191. Samuelsson. R.: O'Dea. J.; Ostewoung, . - J. Anal. Chem. 1980, 52, 2215-2216. Wang, J.; Ouzlel, E.; Yarnltzky, CH.; Ariel, M. Anal. Chlm. Acta 1978, 102,99-112. Last, T. A. Anal. Chem. 1983, 55, 1509-1512. Last, T. A. Anal. Chlm. Acta 1983, 155, 287-291. Howell, J. 0.;Wightman, R. M. Anal. Chem. 1984, 56, 524-529. Gates, S. C. 6~481984, 9 , 366-378. Hubbard, A. T.; Anson, F. C. "Electroanalytical Chemlstry";Bard, A. J., Ed.; Marcel Dekker: New York, 1970; Vol. 4. Savitzky, A,; Golay, M. J. E. Anal. Chem. 1964, 36, 1627-1639. Scott, R. P. W. "Liquid Chromatography Detectors"; Elsevier: New York, 1977; p 9.
j 0 12.1 Figure 14.
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Contour map of Figure 13.
are observed at lower potentials. As in the previous example, the background scan used was a scan immediately prior to the dead time. In a complex sample, particularly one in which electrode fouling is a problem (such as urine), this choice of background scan may seem inappropriate. However, Figure 13 demonstrates that good quality voltammetric information can be obtained with this method. However, there is no evidence in the contour plot (Figure 14) of large changes in the current, as would be expected if adsorption or electrode fouling had occurred.
RECEIVED for review June 24,1985. Accepted October 1,1985. Support for this work was provided by a grant from the Alfred P. Sloan Foundation, the E. I. du Pont de Nemours and Go., and the University Research Council of the University of North Carolina. Portions of this work were presented at the 1985 Pittsburgh Conference on Anilytical Chemistry and Applied Spectroscopy, New Orleans, LA.
