Application of Diamond Microelectrodes for End-Column

Dec 31, 2002 - Highly boron-doped diamond microelectrodes were employed in an end-column electrochemical detector for capillary electrophoresis (CE). ...
4 downloads 7 Views 81KB Size
Download PDF
Anal. Chem. 2003, 75, 530-534

Application of Diamond Microelectrodes for End-Column Electrochemical Detection in Capillary Electrophoresis Dongchan Shin, Bulusu V. Sarada, Donald A. Tryk, and Akira Fujishima*

Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Joseph Wang

Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003

Highly boron-doped diamond microelectrodes were employed in an end-column electrochemical detector for capillary electrophoresis (CE). The diamond microline electrodes were fabricated from conducting diamond thin films (exposed surface area, 300 × 50 µm), and their analytical performance as CE detectors was evaluated in a laboratory-made CE installation. The CE-ED system exhibited high separation efficiency for the detection of several catecholamines, including dopamine (DA), norepinephrine (NE), and epinephrine (E), with excellent analytical performance, for example, 155 000 theoretical plates for DA. The diamond-based electrochemical detection system also displayed low detection limits (∼20 nM for E at S/N ) 3) and a highly reproducible current response with 10 repetitive injections of mixed analytes containing DA, NE, and E (each 50 µM), with relative standard deviations (RSD) of ∼5%. The performance of the diamond detector in CE was also evaluated in the detection of chlorinated phenols (CP). When compared to the carbon fiber microelectrode, the diamond electrode exhibited lower detection limits in an end-column CE detection resulting from very low noise levels and highly reproducible analyses without electrode polishing due to analyte fouling, which makes it possible to perform easier and more stable CE analysis. Capillary electrophoresis (CE) is a microcolumn separation technique that can separate target analytes on the basis of differences in electrophoretic mobilities via the application of high electric fields (several hundreds of V cm-1). It has been shown to be a very powerful instrumental technique, resulting in fast, highly efficient separation and analysis of complex liquid-phase mixtures.1 Electrochemical detection (ED) in CE has proven to be very useful in monitoring extremely small volumes of analytes * Corresponding author. Phone: 81 3 5841 7245. Fax: 81 3 3812 6227. E-mail: [email protected]. (1) Landers, J. P. Handbook of Capillary Electrophoresis, 2nd ed, CRC Press LLC: Boca Raton, FL, 1997.

530 Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

and has been shown to be a highly sensitive detection technique.2,3 One of the most commonly used detection methods adapted for CE is UV-visible absorbance detection. However, its sensitivity is limited as a result of the short light path length of the capillary. Although lower detection limits can be obtained with the use of laser-induced fluorescence (LIF) detection, this requires derivatization of the analytes as well as complex, expensive instrumentation. Electrochemical detection not only shows low detection limits approaching those of LIF detection, but also requires relatively simple, compact, low-cost instrumentation. Several types of electrochemical detection systems have been developed in order to address several points of concern in CE-ED, including the effect of high voltage (HV) on shifting baselines and the arrangement of electrode and capillary.4-11 Off-column detection with an incapillary electrode has been developed in order to reduce the effect of HV electric fields on the detection circuit.4,5 A fracture is made in the capillary, which is grounded prior to the detection cell. Another approach is end-column detection, which does not require the use of a decoupler.6-8 The third is on-capillary detection, in which the electrode is affixed to the end of the fusedsilica capillary perpendicular to the flow direction.9-11 However, up to now, several obstacles remain that have stood in the way of routine practical CE-ED measurements. In particular, the need for polishing and replacement of the electrode because of analyte fouling is an important concern, because this factor can limit the reproducibility of the analytical performance. Indeed, these timeconsuming user-dependent procedures prevent routine measurements in CE-ED analyses. One recent approach, which has been (2) Ewing, A. G.; Mesaros, J. M.; Gavin, P. F. Anal. Chem. 1994, 66, 527A537A. (3) Baldwin, R. P.; Voegel, P. D. Electrophoresis 1997, 18, 2267-2278. (4) Wallingford, R. A.; Ewing, A. G. Anal. Chem. 1987, 59, 1762-1766. (5) O’Shea, T. J.; Greenhagen, R. D.; Lunte, S. M.; Lunte, C. E.; Smyth, M. R.; Radzik, D. M.; Watanabe, N. J. Chromatogr. 1992, 593, 305-312. (6) Matysik, F. M. J. Chromatogr., A 1996, 742, 229-224. (7) Sloss, S.; Ewing, A. G. Anal. Chem. 1993, 65, 577-581. (8) Ye, J.; Baldwin, R. P. Anal. Chem. 1993, 65, 3525-3527. (9) Zhong, M.; Lunte, S. M. Anal. Chem. 1996, 68, 2488-2493. (10) Voegel, P. D.; Zhou, W.; Baldwin, R. P. Anal. Chem. 1997, 69, 951-957. (11) Hua, L.; Tan, S. N. Anal. Chem. 2000, 72, 4821-4825. 10.1021/ac020513j CCC: $25.00

© 2003 American Chemical Society Published on Web 12/31/2002

implemented for microchip devices, is the use of easily exchangeable screen-printed carbon ink electrodes.12-14 Here, we report on the attractive use of diamond electrodes in CE-ED. Highly boron-doped diamond films have recently received increasing attention for electrochemical applications.15-17 They exhibit very attractive electrochemical properties, such as low, stable background currents,18 a wide potential window in aqueous media,19,20 poor adsorption of most types of organic molecules,21,22 and long-term stability of the response.23,24 All of these properties make diamond a promising material, especially for electroanalytical applications. In our previous work, we investigated the electrochemical oxidation of several biologically and environmentally important chemicals, for which the diamond electrode showed excellent analytical performance, including both high sensitivity and stablity.22-28 Several recent reports have shown that the use of a diamond electrode has broadened the range of target chemicals to include those that exhibit high oxidation potentials as a result of its wide potential window and long-term stability.27,28 In addition, several outstanding properties of diamond itself can be very useful in extreme conditions, including its chemical and physical inertness and the highest known breakdown voltage strength.29 These are compatible with the HV operation of CE analysis. To the best of our knowledge, the diamond electrode has not been previously utilized with CE-ED systems, perhaps because of the difficulties of the fabrication of an appropriate CE-ED setup, even though the superior electrochemical properties over other electrode materials are well-known. We believe that the application of the diamond electrode in CE will make it possible to construct an easy-to-use, stable CE-ED system for routine, reproducible analysis. In the following sections, we report the first use of diamond microline electrodes in an end-column amperometric CE-ED system for the determination of several catecholamines: dopamine (DA), epinephrine (E), norepinephrine (NE), and also chlorinated phenols (CPs). The feasibility of (12) Wang, J.; Tian, B.; Sahlin, E. Anal. Chem. 1999, 71, 5436-5440. (13) Wang, J.; Chatrathi, M. P.; Tian, B.; Polsky, R. Anal. Chem. 2000, 72, 25142518. (14) Wang, J.; Chatrathi, M. P.; Tian, B. Anal. Chem. 2000, 72, 5774-5778. (15) Swain, G. M.; Anderson, A. B.; Angus, J. C. MRS Bull. 1998, 9, 56-60. (16) Pleskov, Y. V.; Evstefeeva, Y. E.; Kvotova, M. D.; Laptev, V. Electochim. Acta 1999, 44, 3361-3366. (17) Rao, T. N.; Fujishima, A. Diamond Relat. Mater. 2000, 9 (3-6), 384-389. (18) Yano, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electrochem. Soc. 1998, 145, 1870-1876. (19) Strojek, J. W.; Granger, M. C.; Dallas, T.; Holtz, M. W.; Swain, G. M. Anal. Chem. 1996, 68, 2031-2037. (20) Vinokur, N.; Miller, B.; Avyigal, Y.; Kalish, R. J. Electrochem. Soc. 1996, 143, L238-L240. (21) Xu, J.; Swain, G. M. Anal. Chem. 1998, 70, 1502-1510. (22) Fujishima, A.; Rao, T. N.; Popa, E.; Yagi, I.; Tryk, D. A. J. Electroanal. Chem. 1999, 473, 179-185. (23) Rao, T. N.; Yagi, I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Anal. Chem. 1999, 71, 2506-2511. (24) Popa, E.; Notsu, H.; Miwa, T.; Tryk, D. A.; Fujishima, A. Electrochem. SolidState Lett. 1999, 2 (1), 49-51. (25) Sarada, B. V.; Rao, T. N.; Tryk, D. A.; Fujishima, A. Anal. Chem. 2000, 72, 1632-1638. (26) Popa, E.; Kubota, Y.; Tryk, D. A.; Fujishima, A. Anal. Chem. 2000, 72, 17241727. (27) Terashima, C.; Rao, T. N.; Sarada, B. V.; Tryk, D. A.; Fujishima, A. Anal. Chem. 2002, 74, 895-902. (28) Rao, T. N.; Loo, B. H.; Sarada, B. V.; Terashima, C.; Fujishima, A. Anal. Chem. 2002, 74, 1578-1583. (29) Pan, L. S.; Kania, D. R. Diamond: Electronic Properties and Applications; Kluwer Academic: Norwell, MA, 1995.

Figure 1. Schematic diagram of the diamond microline electrode and its cross-sectional area (the exposed end of the diamond film is indicated in black).

diamond electrodes in a CE-ED system is evaluated by characterization of the analytical performance of diamond microelectrodes in CE operation. EXPERIMENTAL SECTION Diamond Microline Electrode. Highly boron-doped diamond thin films were deposited on Si (100) wafers in a high-pressure microwave plasma-assisted chemical vapor deposition (CVD) system (ASTeX Corp., Woburn, MA). The details of the preparation have been described previously.18 A mixture of acetone and methanol in a ratio of 9/1 (v/v) was used as the carbon source. B2O3, the boron source, was dissolved in the acetone-methanol solution at a B/C atomic ratio of 1:100. The deposition of the film was carried out at a microwave power of 5 kW. The film thickness was controlled by the deposition time (2.5 µm/h under optimized deposition conditions). The film quality was confirmed by Raman spectroscopy (Renishaw System 2000), using the sharp peak at 1332 cm-1, characteristic of crystalline diamond.30 Subsequently, the Si wafer substrate was removed by chemical etching with a mixed solution of HNO3 and HF (1:1) to produce a 50-µm-thick free-standing diamond thin film. To a piece of diamond film of appropriate dimensions (∼0.3-0.5 × 5 mm), a Cu wire was attached with Ag paint for electrical contact. The diamond microline electrode was prepared by sandwiching the free-standing film between two glass slides with UV adhesive (NOA 61, Norland Products). To obtain a structure with dimensions appropriate for the inner diameter of the fused silica electrophoresis capillaries, the cross section (50 × ∼300-500 µm) of the diamond thin film was exposed as an electrode surface area by polishing the glassdiamond-glass sandwich structure (Figure 1). Carbon Fiber Disk Electrode. For purposes of comparison, carbon fiber microelectrodes were prepared. Four or five carbon fibers of 7-µm diameter and 2-cm length were inserted into the narrow end of a pulled glass tube, and this structure was sealed with UV adhesive. A small amount of mercury was drawn into the glass tube with a Cu wire for ohmic contact, and the top end of the glass tube was completely sealed off with UV adhesive. After every CE run, the surface of the carbon disk electrode was polished with emery paper and alumina powder (0.05 µm) and, finally, was sonicated in deionized water. CE Apparatus. The laboratory-constructed CE-ED system was fabricated from polymethyl methacrylate (PMMA, Sanplatec Corp., Japan). The electrophoresis capillary, whose exit end was mounted on a plate, was inserted into the electrochemical detection system. The detector, which was a three-electrode system, was employed (30) Buckley, R. G.; Moustakas, T. D.; Ye, L.; Varon, J. J. Appl. Phys. 1989, 66, 3595.

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

531

in an end-column detection mode (see below). A Ag/AgCl reference electrode and a Pt wire auxiliary electrode were used. The diamond working electrode was mounted on a micromanipulator (Chuo Precision Industrial Co., Ltd., Japan). Uncoated fused-silica capillaries (25-µm i.d. × 370-µm o.d., GL Sciences Inc., Japan), 75 cm in length, were used for all measurements. The exposed face of the diamond microband electrode was oriented vertically with respect to the flat end of the capillary and was approximately centered, both horizontally and vertically. The distance between the capillary end and the electrode face was optimized at 25 µm. A separation capillary was flushed with 0.5 M NaOH for 1 h before use and each day and then flushed with 0.1 M NaOH, water, and operating buffer. The amperometric detection was performed with an Electrochemical Analyzer 800A (CHI instruments) connected to a Pentium II computer. The highvoltage power supply (model HCZE-30PN0.25, Matsusada Precision Inc., Tokyo, Japan) was used for the electrophoretic separations. Reagents. Morpholinoethanesulfonic acid (MES), epinephrine, catechol, and perchloric acid were purchased from Wako Chemicals (Osaka, Japan). Dopamine hydrochloride and norepinephrine were purchased from Aldrich. Phenol and chlorinated phenols were purchased from Tokyo Kasei Kogyo (Tokyo Japan). The electrophoresis buffers were MES for catecholamine detection and mixed borate/phosphate buffer for phenolic compounds. Buffer solutions were prepared with doubly distilled water (Milli-Q, Millipore) and were adjusted to the proper pH with NaOH or HCl. Stock solutions were prepared for the catecholamines (10 mM) with 0.1 M perchloric acid and for the chlorinated phenols (1 mM) with doubly distilled water. Both stock solutions were diluted to the desired concentration with running buffer. All buffer solutions and samples were finally filtered through a 0.22-µm syringe filter (Millex-GS, Millipore) before being used in the capillary electrophoretic measurements. RESULTS AND DISCUSSION Electrode Characterization. The diamond microline electrode was examined by use of cyclic voltammetry for the oxidation of ferrocyanide, Fe(CN)64- (Figure 2a). The voltammogram exhibits a steady-state limiting current plateau, which is characteristic of a microelectrode at low potential sweep rates.31 After fabrication of the diamond microline electrode, mechanical polishing was not necessary to activate the surface before each set of measurements. No detectable difference was found in the dayto-day electrochemical response for at least one week of CE analyses. The diamond electrode has already been found to exhibit little vulnerability to fouling problems.23-28 Moreover, we found that a diamond microdisk electrode yielded a perfectly reproducible electrochemical response during two months of storage in ambient air.32 In addition, the stability of the current response with diamond microfiber electrodes was maintained over a period of two months with daily testing of dopamine detection in the concentration range up to 100 µM.33 For purposes of comparison, a carbon fiber disk electrode, fabricated with four fibers, was (31) Forster, R. J. Chem. Soc. Rev. 1994, 289-297. (32) Sarada, B. V.; Rao, T. N.; Tryk, D. A.; Fujishima, A. J. Electrochem. Soc. 1999, 146, 1469-1471. (33) Olivia, H.; Shin, D.; Sarada, B. V.; Rao, T. N.; Fujishima, A. Manuscript in preparation.

532 Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

Figure 2. (a) Cyclic voltammogram at the diamond microline electrode for the oxidation of 1 mM K4Fe(CN)6 in 0.1 KCl (potential sweep rate, 10 mV s-1). (b) Cyclic voltammogram at a carbon fiber disk electrode for the oxidation of K4Fe(CN)6 (conditions same as for diamond electrode).

Figure 3. Hydrodynamic voltammograms for 3 catecholamines: DA (9), NE (b) and E (2) in CE. Conditions: capillary, 25 µm i.d. × 75 cm length; capillary-to-electrode distance, 25 µm; separation buffer solution, 30 mM MES pH 5.7; separation voltage, 23 kV; injection, 7 kV for 10 s; concentrations, DA, NE, and E, all 30 µM.

characterized by ferrocyanide oxidation (Figure 2b). The surface area of the diamond microline electrode (1.5 × 10-8 m2) was 25 times larger than that of the carbon fiber electrode (6.2 × 10-10 m2). Diamond Electrodes in CE Analysis. It is important to describe the arrangement of the diamond electrode with respect to the capillary end, because the electrochemical response depends critically on this arrangement (see Experimental Section). The most important parameter is the capillary-to-electrode distance, which was optimized to 25 µm on the basis of the current response. The whole electrode surface was not used equally for analyte detection because of the noncentrally symmetric nature of the band electrode. This fact is partially related to the presence of peak tailing, as discussed later. As a first step, hydrodynamic voltammograms (HDVs) were measured in order to determine the optimal detection potential for the three catecholamines (Figure 3). It is necessary to assess the relevant detection potential from the HDVs, because there can be detection potential shifts resulting from the influence of HV in end-column detection systems.34,35 In the case of the diamond microline electrodes used (34) Matysik, F. M. J. Chromatogr., A 1996, 742, 229-234.

Table 1. Separation Efficiency in CE and Analytical Performance of the Diamond CE Detectora

dopamine norepinephrine epinephrine

Nb

sensitivity pA µM-1

detection limit µM

RSD %

155 000 152 000 138 000

86.9 62.3 76.1

0.020 0.023 0.019

4.4 3.2 4.1

a The operating conditions were the same as those for the results in Figure 4. b N was calculated according to the following equation: N ) 5.54 (tm/W1/2)2, where tm is the migration time of each analyte and W1/2 is the peak width at half-peak height.

in the present work, the peak current response started at ∼0.3 V (vs Ag/AgCl), increased gradually to 0.8 V, and leveled off at higher values. Another factor to consider is the background current, which begins to increase at potentials above 0.8 V. Thus, we concluded that 0.8 V was the optimal detection potential for the subsequent CE-ED measurements. The separation efficiency and analytical performance of the diamond microelectrode in CE-ED was evaluated for the determination of a catecholamine mixture (see Table 1). The separation and detection of neurotransmitters has been a very important subject in bioanalytical chemistry, and the CE approach has been actively pursued in order to achieve high sensitivity and selectivity with small amounts of catecholamines.4,7,36,37 Figure 4a shows a representative electropherogram obtained for an equimolar (10 µM) mixture of DA, NE, and E. The peaks for these three compounds were well-resolved, and the separation efficiency obtained under these conditions was 155 000 theoretical plates (N, a measure of the separation efficiency) for DA on the basis of the half-peak width. There was slight tailing in the analyte peaks resulting from the detection of analytes diffusing from the capillary outlet to the extreme ends of the diamond band. Although we tried to reduce the length of the diamond microline electrode as much as possible, this length (∼300-500 µm) was still much larger than the capillary inner diameter. We confirmed that there was negligible tailing under the same conditions after exchange with a carbon fiber disk electrode. The relative standard deviation (RSD) of the peak currents indicates a reproducible current response for the repeated injection of a 10 µM mixture of DA, NE, and E (n ) 5). Linear calibration plots were observed over the 0.1-100 µM range for all compounds (correlation coefficients of g0.995; n ) 8). The sensitivity factors for the diamond detector were 87.6, 64.5, and 79.4 pA µM-1 for DA, NE, and E, respectively. The diamond microelectrodes exhibited greater stability in the electrophoretic response compared to that for the carbon fiber disk microelectrode. Under our CE conditions, the diamond microline electrode showed lower noise levels (∼0.5-1 pA) and a more stable background current than that of the carbon fiber disk electrode (whose minimum noise level was 2 pA; size as in Figure 1), even though its surface area was 25 times larger. In addition, variations in the background current (low-frequency noise) were much smaller and less irregular than those for the (35) Wallenborg, S. R.; Nyholm, L.; Lunte, C. E. Anal. Chem. 1999, 71, 544549. (36) Gavin, P. F.; Ewing, A. G. J. Am. Chem. Soc. 1996, 118, 8932-8936. (37) Liu, Z.; Niwa, O.; Kurita, R.; Horiuchi, T. J. Chromatogr., A 2000, 891, 149156.

Figure 4. (a) Electropherogram of 3 catecholamines: DA, NE, and E. Conditions: capillary, 25 µm i.d. × 75 cm length; capillary-toelectrode distance, 25 µm; separation buffer solution, 30 mM MES pH 5.7; detection potential, 0.8 V (vs Ag/AgCl); separation voltage, 25 kV; injection, 7 kV for 10 s; concentrations, DA, NE, and E, all 10 µM. (b) Response showing small irregular peak-to-peak noise in the background current with the diamond electrode in low concentration analysis (DA, NE, and E, all 0.1 µM; conditions same as in Figure 4a).

carbon fiber microelectrodes. It was even observed that the background current did not change greatly (less than a 100-pA range noise variation in background current at 25 kV), unlike carbon fiber electrodes (more than 24 nA under the same conditions), when the capillary-to-electrode distance was varied over the range ∼5-50 µm during HV operation. One of the important criteria for checking the analytical performance is the detector response at low analyte concentrations. The very low, stable noise levels in the background current of the diamond detector make it possible to achieve very low detection limits in connection to the end-column CE analysis. Figure 4b illustrates the determination of an equimolar (0.1 µM) mixture of DA, NE, and E with the diamond detector. This result shows that the diamond electrode enables sensitive analysis for low-concentration analytes, with relatively small peak-to-peak noise in the background current. These data indicate detection limits of around 0.020 µM for DA, 0.023 µM for NE, and 0.019 µM for E (based on S/N ) 3). To the best of our knowledge, these detection limits with the diamond detector are lower than any other catecholamine detections with end-column CE-ED using other electrodes.6,7,38 Another outstanding aspect of the performance of the diamond electrode in CE-ED is the reproducibility of the current response. As mentioned earlier, the diamond electrode has shown relatively (38) Jin, W.; Jin, L.; Shi, G.; Ye, J. Anal. Chim. Acta 1999, 382, 33-37.

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

533

Figure 5. Electropherogram of chlorinated phenols: (a) P, (b) 2-CP, (c) 2,4-diCP, (d) 2,3-diCP, (e) 2,4,5-triCP, and (f) 2,4,6-triCP (all 50 µM). Conditions: capillary, 25 µm i.d. × 75 cm length; separation buffer solution, 10 mM/10 mM mixed borate/phosphate (pH 7.8); detection potential, 1.0 V (vs Ag/AgCl); separation voltage, 25 kV; injection, 7 kV for 10 s.

high resistance to deactivation via fouling.23-27 We recently reported that an anodized diamond electrode exhibited excellent stability, with a response variability of 2.3% (n ) 100) in flow injection analysis, for the oxidation of a high concentration (5 mM) of chlorophenol, which is well-known for its electrode-fouling properties.27 It was also observed that diamond electrodes did not exhibit any fouling problems in the electrochemical oxidation of serotonin, the neurotransmitter which adsorbs strongly on ordinary carbon electrodes.25 Therefore, it is reasonable to expect longterm stability of response for the diamond microline electrode for low analyte concentrations. In addition to the reproducible response for 5 repetitive injections of a low-concentration (DA, NE, E each 10 µM) mixture, CE measurements were carried out to check the stability of the current response by making 10 successive injections of mixed solutions of catecholamines (each 50 µM), without any electrode treatment between injections. The good reproducibility of the peak current response was observed in this series of 10 successive injections of the DA/NE/E mixture (50 µM each), with ∼5% (RSD). Using the diamond CE detector, which is not easily deactivated by fouling, we can expect a stable analytical performance in CE analysis without the troublesome and time-consuming electrode arrangement steps that are necessary when the latter must be demounted and polished. The small response variation is attributed to partial adsorption of analytes on the capillary wall and slight differences in injection volume and not due to the fouling of the diamond detector. It was also observed that similar peak current responses were obtained in 10 successive injections of mixed solutions of DA, NE, and E (30 µM each) with ∼4-5% RSD values. Although we have not tested diamond electrodes for long-term stability under continuous CE operation, they exhibited highly reproducible response from day to day over a period of 1 week. Moreover, the response was reproducible even after the diamond microline electrode was exposed to dry conditions for several days. (39) Fermier, A. M.; Gostkowski, M. L.; Colon, L. A. Anal. Chem. 1996, 68, 1661-1664. (40) Durgbanshi, A.; Kok, W. T. J. Chromatogr., A 1998, 798, 289-296.

534

Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

On the basis of these advantages, the analysis of chlorinated phenols was examined in the same diamond-based CE-ED system. Chlorinated phenols are highly toxic pollutants, and their detection is one of great concern in environmental analysis. Figure 5 shows an electropherogram obtained for an equimolar (50 µM) mixture of phenol (P) and five chlorinated phenols (CPs). Because anionic CPs are directed against the cathodic electroosmotic flow, depending on different ionization effects, an electrokinetic injection of CPs results in relatively smaller peak currents for di- and tri-CPs than P and 2-CP (Figure 5). Compared to carbon fiber electrodes, in which there was an increase of irregular noise in the baseline current over the span of several repeated injections, diamond microelectrodes showed low, stable noise levels (1-1.5 pA) throughout 10 repeated injections without any electrode treatment. Although a slight decrease in the peak currents was observed with repeated injections for an as-deposited diamond electrode, there is expected to be better reproducibility with anodically oxidized diamond electrodes.27 More systematic experiments for the detection of CPs with diamond microelectrodes are in progress to obtain optimized analytical performance. CONCLUSIONS The results clearly demonstrate that the combination of borondoped diamond amperometric electrodes with CE systems results in a versatile analytical device with excellent analytical performance. Our data show that the use of diamond electrodes leads to improved electrochemical detection for CE operation, with low detection limits and a highly reproducible response for continuous injection as a result of its superior electrochemical stability. Recently, there has been considerable interest in the development of more practical CE-ED systems. The application of electrochemical detection in CE has thus far been limited to research with laboratory-made systems. To overcome the present limitations of CE-ED, it is necessary to avoid such problems as the need for time-consuming optimization of the placement of capillary and electrode and electrode deactivation via analyte fouling. A number of integrated CE-ED systems have recently been proposed to overcome those problems,9-13,39-40 and CE-ED system design has advanced for practical routine CE measurements. Because of its outstanding electrochemical properties, the diamond microelectrode appears to be a very promising electrode material with which to support these improvements. Current efforts in our laboratories are aimed at developing a diamond electrode detector for microchip CE applications. Preliminary results are encouraging and will be reported soon. ACKNOWLEDGMENT D.S. thanks the Rotary Yoneyama Memorial Foundation Inc. of Japan for a YD scholarship.

Received for review August 7, 2002. Accepted October 22, 2002. AC020513J