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Achievement of the Analytically Ideal Steady-State Response at a Microelectrode-Based Scanning Electrochemical Detector under Flow Injection Analysis and Normal-Phase Chromatography Conditions Russell J. Tait,+Peter C. Bury,?Barrie C. Finnin? Barry L. Reed,+ and Alan M. Bond'*$ Department of Pharmaceutics, Victorian College of Pharmacy Ltd., 381 Royal Pde, Parkville 3052, Victoria, Australia, and Department of Chemistry, La Trobe University, Bundoora 3083, Victoria, Australia
A study of the one-electron reversible ferrocene oxidation process has been made under both flow injection analysis and normal-phase chromatography conditions in a thin-layer flow cell with a rapid-scanningmicroelectrode-based detector. A flow rate of 1.0 mL min-l, scan rates in the range of 2-10 V s-l, and platinum disk microelectrodes of radii in the range 5-60 pm enable voltammograms with the theoretically predicted steady-state characteristics to be obtained. Ready access to the analytically ideal steady-state response and to electrode process characterization coupled with other advantages available with microelectrodes provides the rapid-scanningmicroelectrode-based detector with substantial advantages over many of the more commonly used forms of electrochemical detection. INTRODUCTION Electrochemicaldetection in flow injection analysis (FIA) or high-performance liquid chromatography (HPLC) is usually undertaken in the amperometric mode using a fixed potential difference between the working and reference electrodes. When a flowing stream containing an electrochemically active analyte of interest passes over the working electrode, the current resulting from the oxidation or reduction process at the fixed potential is proportional to concentration. Recently, many detection systems capable of performing either square wave, differential, or normal pulse and cyclic voltammetry in flowing solutions have been developed for use as either FIA or HPLC detectors.'-29 As is the case with To whom correspondence should be addressed.
+ Victorian College of Pharmacy Ltd. 8 La Robe University.
(1)Goto, M.; Shimada, K. Chromatographia 1986,21,631-634. (2) Samuelason, R.;O'Dea, J.; Osteryoung, . - J. Anal. Chem. 1980, 52, 2215-2216. (3) Kounaves S. P.; Young, J. B. Anal. Chem. 1989,61,1469-1472. (4)Caudd, W. L.; Ewing, A. G.; Jones, S.; Wightman, R. M. Anal. Chem. 1983,55,1877-1881. (5)White,J. G.; St. Claire, R. L., III; Jorgenson, J. W. Anal. Chem. 1986,58,293-298. (6)Gunasingham, H.; Tay, B. T.; Ang, K. P. Anal. Chem. 1987,59, 262-266. (7) Matsue, T.; Aoki, A.; Ando, E.; Uchida, I. Anal. Chem. 1990,62, 409-411. (8)Wang, J.; Ouziel, E.; Yarnitzky,C. W.; Ariel, M. Anal. Chim. Acta. 1978.102.99-112. -,---, -~ (9)Brunt, K.; Bruins, C. H. P.; Doombos, D. A.; Ooaterhuie, B. Anal. Chim. Acta 1980,114,257-266. (10)Thogemen, N.; Janata, J.; R&i6ka, Anal. Chem. 1983,55,1986~~-
~~
1988. _.
(11)Last, T. A. Anal. Chem. 1983,55,1509-1512. 0003-2700/93/0365-3252$04.00/0
the diode array or scanningUV spectrophotometricdetectors relative to fiied-wavelength detectors, these more sophisticated forms of electrochemicaldetection introduce an extra domain for resolution of analytes. The extra domain in the spectrophotometry field is the absorbing wavelength; in the case of these new electrochemical detectors, it is the electrode potential. For a scanning electrochemical detector to be as useful with FIA or HPLC, as is the case when a diode array or scanning spectrophotometer is used, the voltammograms obtained should not only allow differentiation of analyte but the data obtained should also be of sufficiently high quality to characterize the electrochemical processes. When this latter feature can be achieved, all the checks against interference that are used in conventional stationary solution voltammetry become available. Thus ideally, it should be possible to produce a current response (peak height or peak area) which enables the concentration to be determined as well as to derive electrochemical parameters such as E I , ~ , number of electrons in the charge-transfer process, heterogeneous rate constant, etc., from the voltammogram. These two objectives are achievable when the voltammetry can be performed at scan rates that are fast enough to provide voltammograms unaffected by variation in analyte concentration as the analyte moves past the electrode and the repetition rate for each scan is frequent enough to provide a full representation of the FIA or HPLC peak (Le., at least 10 scans per peak are all required for quantitative determination). While many of the detectors developed in earlier studies have provided voltammograms which allow differentiation of analytes in the potential domain, none have been used to fully characterize the electrode processes in a manner equivalent to that expected in analytical applications undertaken in stationary solutions. The method of Goto and (12)Last, T.A. Anal. Chim. Acta 1983,155,287-291. (13)St'astnv. M.: Volf, R.: Beihdikov6.. H.:.Vlt.. I. J.Chromatom.Sci. 1983,2l,l8-2i. . . . (14)Wang, J.; Dewald, H. D. Anal. Chim. Acta 1983,153,325-330. (15)Scanlon, J. J.: Flaauer. P. A.: Robineon. G. W.: O'Brien. G. E.: St&&k, P. E : A d Chim. Acta 1984,158,169-177.. (16)Thomas, M.B.; Msimanga, H.; Sturrock, P. E. Anal. Chim. Acta 1985,174,287-291. (17)Ploegmakers, H. H. J. L.; Mertene, M. J. M.; Van Oort,W. J. Anal. Chim. Acta 1986,174,71-78. (18)St. Claire, R. L., III;Jorgenson, J. W. J. Chromatogr. Sci. 1985, 23,186-191. (19)Howell, J. 0.; Kuhr, W. G.; Eneman, R. E.; Wightman, R. M. J. Electroanal. Chem. 1986,209,77-90. (20)White, J. G.; Jorgenson, J. W. Anal. Chem. 1986,58,2992-2995. (21)CaBete, F.; Rlw, A.; Luque de Castro, M. D.; Valchcel, M. Anal. Chim. Acta 1988,211,287-292. (22)Kennedy, R. T.; Jorgenson, J. W. Anal. Chem.1989,61,436-441. (23)Tait, R.J.; Bond, A. M.; Finnii, B. C.; Raed, B. L. Collect. Czech. Chem. Commun. 1991,56,192-205. I
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Shimada,’ while permitting determination of peak potentials (Ep),provided only three to four scans per chromatographic peak and as such is of limited quantitative value. The detectors described by Samuelsson et aL2and Kounaves and Young3 use square wave voltammetry, which provides voltammograms that allow resolution of analytes with different half-wave potentials (El$, and which could be used for characterization of an electrochemical process, if the relevant theoretical models were developed and applied. The detectors described by Caudill et al.4 and White et al.6 use the more conventional forward sweep or cyclic voltammogram in the staircase format. The Caudill et al.‘detector uses a relatively large potential stepsize (60 mV), providing voltamograms of relatively low resolution. The results presented by White et al.6 demonstrate the practical use of a scanning electrochemical detector for differentiation of known analytes. However, voltammograms were obtained under conditions corresponding to a hybrid of steady-state and linear diffusion (transient voltammetry) as indicated by the basically sigmoidal shape, but with a small peak just beyond Ell2. This hybrid diffusion region precludes simple analysis of the voltammogram. The voltammograms presented by Gunasingham et al.6 are significantly distorted by ohmic potential (iR) drop, which is not surprising since these authors attempted the difficult task of perfoming cyclic voltammetry in a high-resistance solvent system with an electrode of conventional size. The presence of iR drop inhibits the theoretical analysis of voltammograms. Although not attempted by the authors, the hydrodynamic voltammograms reported by Mataue et al.7 would appear to be suitable for further data analysis. From the above overview of available data, it may be concluded that the simplest situation where scanning voltammetry would be used for complete characterization of the electrode process will arise when iR drop is negligible and either a completely steady-state or transient voltammogram is obtained rather than the usual complex case where hybrid forms of these two limiting cases are observed. The minimal iR drop-steady-state combination is likely to be achievable with microelectrodes. Furthermore, the smaller charging current and smallertime constants also are attractive features of microelectrode voltammetry that allow analytically sensitive and undistorted voltammograms to be obtained at relatively fast potential scan rates in solvents of high resistance.Since sufficiently small microelectrodes also permit steadystate voltammetryto be performed at relatively fast potential scan rates, steady-state experiments can be achieved over a reduced time interval, which minimizes instability from factors likely to vary over the course of the experiment (e.g., natural convection). The ability to be able to perform voltammetry under steady-state conditions in flowing solutions therefore offers a range of significant advantages over conventionalvoltammetry, which include a simplified sigmoidal-shaped voltammogram, low iR drop,
reduced double-layer capacitance, and enhanced mass transport to the electrode.In view of the reduced susceptibility to iR drop, steadystate voltammetry at microelectrodes can be performed in solvents of high resistance even with little or no supporting e l e ~ t r o l y t e . * ~ ~ ~This * 3 ~ feature extends the range of the electrochemicaldetector. A HPLC detector based upon the use of microelectrodes and steady-state measurementsshould therefore be suitable for use with normal-phase chromatography. That is, compounds which cannot be studied in an aqueous solvent system because of solubility restrictions, instability, etc., may be detected in a completely nonaqueous solvent system. In order to characterize an electrode process, a determination of the shape and the absolute position of the voltammogram is required and data must be free from iR distortion. The parameters of interest in steady-state voltammetry are the limiting current i b , El/2, and details of the wave shape, and should be independent of the potential scan rate. For a reversible process, the shape of a steady-state voltammogram is sigmoidalwith a symmetrical and characteristic shape about Elp, the inflection point of the voltammogram. A measure of both the symmetry and the shape of the voltammogram is provided by the “log plot” of E versus log[(& - i)/il. For a reversible process under steady-state conditions, the log plot is linear with a slope of 2.303RTlnF. Another measure of the slope of the voltammogram is provided by the Tomes% criterion, which is the difference between the wave potentials corresponding to one-quarter and three-quarters of ih(E114 -E3/4). Forareversibleprocess,El/4-E3/4should be 2.197RTl nF.25132938 Deviation from these values or a lack of symmetry in the voltammogram is indicative of kinetic or other complications in the electrode process. Kinetic determinations from voltammograms obtained under steady-state conditions with disk microelectrodes can be made by studying changes in the shape and/or El12 with electrode r a d i u s J ~ Q ~ ~ ~ In the present paper we describe the application of a rapidscanning, microelectrode-based, electrochemicaldetector that can be interfaced with FIA or HPLC to achieve steady-state voltammetry of analytes. The voltammograms obtained are of sufficient quality to allow identification of differences in electrochemicalcharacteristics of the analytes, in a fashion similar to conventional voltammetry in stationary solution, and achieve the theoretical response even in high-resistance organic solvents. The oxidation of ferrocene was used to evaluate the capability of the detector to achieve the theoretically expected response under conditions of FIA or normal-phase HPLC. The oxidation of ferrocene to the ferrocinium cation
(24) Wipf, D. 0.;Wightman, R. M.Anal. Chem. 1990,62,98-102. (26)Bond, A. M.;Henderson, T. L. E.; Mann, D. R.; Mann,T. F.; Thormann, W.; Zoski, C. G. Anal. Chem. 1988,60,1878-1882. (26) Ciszkowska, M.;Stojek, Z. J. Electroanal. Chem. 1986,213,189201. (27) Wehmeyer, K. R.; Wightman, R. M.J. Electroanal. Chem. 1986, 196,417-421. (28)Andrieux, C . P.; Garreau, D.; Hapiot, P.; Savbant, J. M. J . Electroanal. Chem. 1988,248,447-450. (29) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson, J.; S a v h t , J. M.J. Electroonal. Chem. 1988,243,321-335. (30) Bruckenetein, 5.Anal. Chem. 1987,69,2098-2101. (31) Wehmeyer, K. R.; De&, M. R.; Wightman, R. M. Anal. Chem. 1985,57, 1913-1916. (32) Bond, A. M.;Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1988,245, 71-104. (33) Oldham, K. B.; Zoski, C. G.; Bond, A. M.;Sweigart, D. A. J. Electroanal. Chem. 1988,248,487473. (34) Wightman, R. M.Anal. Chem. 1981,53, 1125A-1133A.
is recognized as a one-electron-transfer process with an extremely fast electron-transfer rate.24926939-41 The heterogeneous charge-transfer rate constant is so large that the oxidation of ferrocene at a microelectrode is considered an
(C6H6),Fee (C5H6)2Fe+ + e-
~
~~~
(35) Bard, A. J.; Faulkner, L. R., Eds. Electrochemical Methods. Fundamentals and Applications; Wiley: New York, 1980. (36) Bond,A. M.;Oldham, K. B.; Zoski, C. G. Anal. Chim. Acta 1989, 216,177-230. (37) Baer, C. D.; Stone, N. J.; Sweigart, D. A. Anal. Chem. 1988,60, 188-191. (38) Tome& J. Collect. Czech. Chem. Commun. 1937,9, 1W167. (39) Gang6, R. R.; Koval, G. A.; Lisensky, G. C. Znorg. Chem. 1980,19, 2865-2857. ~... __.
(40) Kuwana, T.;Bublitz, D. E.; Hoh, G. J. Am. Chem. SOC.1960,86, 5811-5817. (41) Howell, J. 0.;Wightman, R. M.Anal. Chem. 1984,66,524-529.
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example of a model reversible proce~s.~6*~2 The voltammetry of ferrocene is well established under stationary solution conditions in a variety of organic ~ o l v e n t a ,and 7 ~plots ~ ~ ~ ~ ~ ~ ~ ~ of E versus log[&, - i)/i]should afford a reciprocal slope very close to the theoretical value for a reversible one-electron process (58.2 mV at 20 0C)32*96,9B44 if the ideal steady-state response has been achieved in flowing solutions.
EXPERIMENTAL SECTION Analyticalreagentgradeferrocenewas purchasedfrom Aldrich. The supportingelectrolyteused was either tetraethylammonium perchlorate (EtrNC104) or tetrabutylammonium fluoroborate (BurNBF4) obtained in electrometric grade quality from South Western Analyticaland dried under vacuum at 60 OC before use. Solvents were obtained from Mallinkrodt or BDH Chemicals and were of HPLC grade. The detector systemfor microelectrodescanningvoltammetry is based on that described previously.as The detector was configured with a platinum disk microelectrode of radii 5-50 pm as the working electrode fitted to a BioAnalytical SystemsModelLC-17Athin-layer cell. The height of the solutionchannelwas approximately0.16 mm. The stainless steel top of the thin-layer cell served as the auxiliary electrode and a laboratory manufactured Ag/AgCl (0.1 M BurNBF4 in acetone, saturated AgCl) reference electrode was used. The platinum disk working electrodes were polished with a series of 5-pm, followed by 1- and 0.3-pm alumina slurries before each series of experiments. Unlike the case in the previous study,= all materials in the electrochemicaldetector cell which are exposed to the solvent are now inert toward common organic solvents. Consequently, 100% organic mobile phases, in addition to aqueous mobile phases, may be used indefinitely. All voltammetric data were obtained at 20 & 1 "C. Solvent was pumped with an LDC Constametric Model I11 HPLC pump (Laboratory Data Control). All samples were introducedinto the flowingstream by means of a Rheodyne Model 7125 injector fitted with a 100-pL loop. Retention of ferrocene in the normal-phase HPLC mode was achieved with a silica column (DuPont Si14.6 mm i.d. X 25 cm, 6 pm) with acetone aa the mobile phase. The mobile phase was maintained at a solution flow rate of 1.0 mL min-1. A second pump (Waters Model 510) introduced makeup solution containing the supporting electrolyte, 0.2 M BurNBF4 in acetone, immediately after the chromatography column. In this way, the mobile phase was optimized for chromatographic purposes and then altered by addition of electrolyte for voltammetric purposes. The makeup solution was maintained at a solutionflow rate equivalentto the solutionflow rate of the mobile phase. Standard solutions of ferrocene for the HPLC studies were prepared in acetone. Calibration curves of peak height versus concentration for ferrocene gave a straight line (Ra = 0.9991, n = 4) in the HPLC mode over the concentration range 50-0.25 nmol, confirming the required linearity of the response. For the flow-throughstudiea,a 1m X 0.5 mm i.d. Teflon mixing coil replaced the column, and an inverted test tube was placed between the pump and the cell to serve as both a bubble trap and pulse dampener. The system was analogous to FIA except that the analyte solution was introduced to the cell on a continuous basis rather thanas a bolus injection. A solution flow rate of 1.0 mL min-1 also waa used for these experiments. This system allowed evaluation of detector performance without any contribution from the chromatography column. First derivatives of voltammograms were calculated by the method of Savitzky-GolayU with a nine-point filter. Noisereduced voltammograms were were obtained by means of a Fourier transform step frequency filter accordingto the method of Cooley-Tukey.a All other voltammograms were processed (42) Wipf, D.0.;Kristensen, E. W.; Deakin, M. R.; Wightman, R. M. A d . Chem. 1988,60,306-310. (43) Koepp, H.-M.; Wendt, H.; Strehlow, H. Z . Elektrochem. 1960,134, 483-491. (44) Pendley, B. D.; Abruna, H. D. Anal. Chem. 1990, 62, 782-784. (46)Savitzky, A.; Golay, M. J. E. AMI. Chem. 1964,36,1627.
4.5 pA1
Figwo 1. Cyclic voltammogramfor the oxidation of 0.5 pM ferrocene in acetonitrHe (0.1 M E4NCQ) obtained with a 5-pm-radlus plathum dlsk mlcroekctrode at a potentla1 scan rate of 0.1 V s-1 and sdutlon Row rate of 1.0 mL min-l.
without noise reduction. The voltammograms were analyzed after background substraction, which was performed by fist averaginga representative set of background scans (usuallyfive) to give a background voltammogram and then subtracting this background voltammogram from the voltammogram of interest. After the initial five scans, the background current at microelectrodes in a thin-layer cell was found to be very reproducible under flowing solution conditions.
RESULTS AND DISCUSSION (a)Flow-ThroughStudies. (i) Sensitivity of Detector. Detection limits at a 5-pm-radius platinum disk microelectrode were determined by studying the reversible one-electron oxidation of ferrocene in acetonitrile (Figure 1)at a scan rate of 0.1 V 8-1 with a continuous solution of ferrocene flowing through the cell at a rate of 1.0 mL min-I. Under these conditions, a signal-to-noise ratio of 5:l is achieved for a single voltammogram of 5 X lo-' M ferrocene without any background subtraction, noise reduction, or data averaging. Voltammograms which have all noise frequencies above 30 Hz removed by means of a Fourier transform low-pass frequency filter enable a detection limit of 1X 10-7 M to be achieved with a signal to noise ratio of 5:l. (ii) Characterization of the Steady-State Response. Figure 2 shows a single-sweep voltammogram for oxidation of 0.91 m M ferrocene in acetonitrile (0.1 Et4NClOd) at a potential scan rate of 0.1 V 8-1 and solution flow rate of 1.0 mL min-1 with a 5-pm-radius platinum disk working electrode. Also shown in Figure 2 is a plot of the first derivative. The Ell2 value of +0.545 V versus Ag/AgCl is consistent with literature values,99*@and a plot of E versus log[(i - i ) / i ] gives a slope of 58.7 f 0.5 mV, which may be compared to the theoretical value of 58.2 mV expected under steady-state conditions at 20 OC. The similarity of the experimentally observed slope to the theoretical value suggests that nearsteady-state conditions have been achieved and that distortion due to iR drop, solution flow, or instrument artifacts is not significant. Closeinspection of the fiit-derivative plot reveals a symmetrical voltammogram about the E l p point. This would be expected for a reversible steady-state oxidation process under thermodynamic control. Furthermore, the derivative response is also essentially ideal, having a halfwidth of 90 mV and a peak potential of +0.55 V. The almost complete overlap of the forward and reverse scans of voltammograms obtained in flowing solution suggests (46)Press, W.H.; Flannery, B. P.;Te$toleky, S. A.; Vetterling, W. T. NumencalRecrpea. The Art of9ctentificComputing; Cambridge: New York, 1986.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993 8.0
80
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60
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4.0
10
2.0
0 480
0.2
0.4
0.6
0.8
1
E(V) Figure 2. Voltammogram for the oxidation of 0.91 mM ferrocene in acetonitrile (0.1 M Et4NCi04)obtained at a 5-pm-radius platinum microdisk electrode at a potential scan rate of 0.1 V s-' and a solution flow rate of 1.O mL min-l. Superimposedis a plot of the first derivative of the voltammogram. The left-hand scale is for the conventional voltammogram, and the right-hand scale is for the firstderivative plot. ~~~~
~~~
Table I. Scan Rate Dependence of the Limiting Current (L) Obtained at a Microelectrode in a Flowing Solution for the Oxidation of 1.0 mM Ferrocene in Acetonitrile (0.1 M EtdNC104)' scan rate (V 9-9 ili, (nA) scan rate (Vs-1) ili, (nA) 0.1 0.2 0.5 1.0
77 75 77 78
2.0 5.0 10.0
78
93 123
a At 50-pm-radius platinum disk microelectrode with a solution flow rate of 1.0 mL min-1.
that steady-state (or near-steady-state) conditions were achieved at the 5-pm-radius electrode, with a scan rate of 1 V s-l. Further evidence of the attainment of steady state comes from the lack of potential scan rate dependence of the limiting current over the range 0.1-2 V s-1 even for considerably larger 25-pm-radiusplatinum disk microelectrodes. A detectable increase in the value of ib,indicating deviation from steady state, was not observed until the potential scan rate was above 5 V s-1 (Table I). (b) Normal-Phase HPLC Studies. Chromatovoltammograms were obtained for 100-pL injections of 1.0 mM ferrocene under normal-phase HPLC conditionswith acetone as the mobile phase and postcolumn addition of electrolyte. Potential scan rates of 1and 10V s-1 were used with platinum microdisk working electrodes of 5-, 25-, and 50-pm radii. The elution of ferrocene from the chromatography column in the present studies is represented by a series of well-defined sigmoidal-shaped voltammograms presented in the threedimensional current, time, and potential format as opposed to the more common two-dimensional response and time format used for constant potential amperometry. However, the two-dimensional format can be obtained from a slice of the three-dimensionalchromatovoltammogram. Even though no separation of chemical species is achieved, the term chromatovoltammogramis introduced in this section of the work as a convenient means of distinguishing voltammetric data obtained under chromatographic-type conditions from data obtained in the flowing solutionstudies in section a above. Figure 3a presents a chromatovoltammogramfor ferrocene obtained with a 25-pm-radiusplatinum disk workingelectrode at a scan rate of 2 V s-l. The parallel nature of the limiting current and the background current regions during ferrocene oxidation suggest that the scan rate of 2 V s-1 was sufficiently
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15,
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Figure4. Chromatovottammogramof a 1Obnmolinjectionof ferrocene represented by a 3-D plot using a 45' viewpoint with a 15' tiit. A 50-pm-radius platinum disk microelectrodewas used with a 10 V s-' potential scan rate for the detection of ferrocene.
Ag/AgCl was found to be consistent with both l i t e r a t ~ r e ~ ~ 9 ~ ~ and the value contained in a conventional experiment with stationary solutions. The derivative plots contained in Figure 3b are completely symmetrical about the peak potential ( = E l / , )and also correspond exactly to the ideal shape for the derivative of a steady-state response. The symmetrical derivative plot is probably the analytically preferred format since the readily measured peak height is proportional to concentration and the peak potential also is evaluated readily. In contrast, measurement of the limiting current and E112 is more difficult from a sigmoidal-shaped curve. The contour plot (Figure 3c) often used in analytical chromatography is also essentially ideal. Figure 4 shows a chromatovoltammogram for the oxidation of ferroceneat a scan rate of 10V s-1 with a 50-pm-radius scan rate and, with a larger electrode radius than in Figure 3, deviation from steady-state behavior is indicated by the appearance of a peak-shaped response. The voltammograms 432 in Figure 4, while still well-defined, do not correspond to steady-state conditions and therefore do not readily allow determination of thermodynamic parameters because linear 384 diffusion contributes to the response. On the other hand, chromatovoltammogramsobtained with a smaller5-pm-radius platinum disk working electrode at potential scan rates of 10 336 V s-1 (Figure 5a) are similar in shape to those obtained for Figure 3a at a 25-pm-radius platinum disk working electrode with a 2 V s-1 scan rate and still correspond to the nearm usteady-state response because the linear diffusion terms are less important at the smaller electrodes. The E112 and slope of E versus log[(& - i ) / i ] values are 0.553 mV f 0.003 V I 240" 0.2 0.4 0.6 0.8 1.0 versus Ag/AgCl under the conditions of Figure 5 and 56.3 f E(v) 0.6 mV, respectively,again close to the theoretically expected Flgure5. Chromatovottammogramofa 100-nmolinjection of ferrocene response, as also is the case with the derivative (Figure 5b) represented by (a)a 3-0 plot, (b) a first derivative 3-D plot, and (c)a and contour (Figure 5c) plots. contour plot. A 5-pm-radius platinum disk microelectrode was used From the above data, it is clearly possible to achieve the with a 10 V s-l potential scan rate for the detection of ferrocene. Viewpoint projections for the 3-D plots are the same as for Figure 3. ideal steady-state and derivative steady-state theoretical A contour interval of 0.2 nA, starting at 0.2 nA, was used for the response in nonaqueous solventsused in normal-phase HPLC, contour plot. when postcolumn addition of electrolyteis implemented prior to electrochemical detection. Instrumental requirements, upon potential scan rate with 1-2 mV used to achieve adequate microelectrode size, and solution flow rate must be considered postprocessing of the voltammogram. White et aL5 also to achieve adequate data analysis. Complete definition of a achieved a potential step size of 2 mV with their system but peak eluting from a HPLC column requires that adequate other detectors described previously commonly used sizes of resolution should be available in both the potential and 10-50 mV, which may not always be adequate to accurately retention time domains. At scan rates consistent with characterize a steady-state voltammogram. The potential achieving steady state (1-10 V s-I),individualvoltammograms range available with the present system was +5.0 to -5.0 V, could take up to 2 s. With the computer system used,23a which is sufficiently wide to allow scanning to be performed minimum delay of 2 s was required between the end of one to the solvent limits of most of the commonly used HPLC voltammogram and the start of the next. The elution time organic solvents and may be used to take full advantage of for a peak in normal HPLC can range from about 30 s to the greater potential ranges possible with the use of micromany minutes so that at least eight steady-state voltammoelectrodes in many of the nonaqueous solvent grams were able to be used to represent the chromatographic The largest potential windows reported with other scanning peak. Resolution in the potential domain was dependant detectors appear to be -2.0 to +2.0 V.5 1 (SI
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
In this work, the only flow rate considered experimentally is 1.0 mL min-1, which is representative of flow rates typically used in HPLC. Theoretical and experimental studies show that the steady-state limiting current increases with flow rate as does the steady-state regime.23147 In principle, it therefore might be argued that higher flow rates should be used when electrochemical detection is employed. However, there are good chromatographic reasons for not increasing the flow rate to excessively high values. Fortunately, with microelectrodes the ideal steady-state regime can be achieved with the flow rates conventionally used in HPLC.
CONCLUSIONS The results described in this paper show that a rapid-scan microelectrode-based detector can be used to obtain the analytically desirable ideal steady-state response under flowing solution conditions encumbent in FIA and HPLC. The steady-state response is characterized by a completely sigmoidal shape, lack of separation between forward and reverse scans, and potential scan rate independence. The flowing solution conditions actually facilitate the achievement of the steady-state regime and therefore allow voltammograms to be analyzed in a simple manner to determine important physicochemical parameters of the electrochemical process.47 The flow of the solution reduces the size of the diffusion layer, which effectively increases the concentration gradient above the electrode surface. For a disk microelectrode in the (47) Tait, R. J.; Bury, P. C.; Finnin, B. C.; F b d , B. L.; Bond, A. M.
J. Electroanal. Chem., in press; and references cited therein.
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thin-layer cell design used in these studies, the flow profile about the electrode is well-defined and predictions concerning the effect of solution flow on the electrode response is possible.47 However, for other cell designs and electrode geometries, particularly those where turbulent regions exist at or near the electrode surface, the effect of solution flow may be less predictable. This study shows that, optimization of detector performance involves a delicate balance between the use of as small a working electrode as possible to assist the achievement of steady state yet large enough to provide a measurable electrolysis current, and the application of a potential scan rate as fast as possible to allow adequate representation of a chromatographic peak yet slow enough to achieve steadystate voltammetry. In the latter context it was found that steady-state voltammograms could be achieved with scan rates ashigh as 10V 9-1,with scan rates of 2-20 V 8-1being sufficient to allow voltammograms to be obtained with minimal effect from variation in analyte concentration as the solution flows past the electrode at flow rates of 1.0 mL min-l. This work shows that the use of microelectrodes under steady-state conditions provides a detection system whereby voltammogramswill be free from iR distortion in many solvent systems as evidenced by the chromatovoltammogramsobtained for the ferrocene system under normal-phase conditions with acetone as the mobile phase.
RECEIVEDfor review May 12, 1993. Accepted August 6, 1993." ~
0
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Abstract published in Advance ACS Abstracts, October 1, 1993.
