Detection of Hydrazine, Methylhydrazine, and Isoniazid by Capillary

electrophoresis for the simultaneous detection of hydra- zine, methylhydrazine, and isoniazid. The Pd-modified microdisk electrode had high catalytic ...
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Anal. Chem. 1996, 68, 3350-3353

Detection of Hydrazine, Methylhydrazine, and Isoniazid by Capillary Electrophoresis with a Palladium-Modified Microdisk Array Electrode Jun Liu, Weihong Zhou, Tianyan You, Fenglei Li, Erkang Wang,* and Shaojun Dong*

Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

A palladium particle-modified carbon fiber microdisk array electrode was designed and employed in capillary electrophoresis for the simultaneous detection of hydrazine, methylhydrazine, and isoniazid. The Pd-modified microdisk electrode had high catalytic ability for hydrazines and exhibited good reproducibility and stability. The response for hydrazine was linear over 3 orders of magnitude with a correlation coefficient of 0.993. The detection limits for hydrazine, methylhydrazine, and isoniazid were 1.2, 2.1, and 6.2 pg, respectively. Since its introduction over a decade ago, capillary electrophoresis (CE) has become a very important technique in the area of liquid-phase separations. Sensitive detection systems are required in order to make full use of this powerful tool for separation. Originally described by Wallingford and Ewing,1 electrochemical detection (EC) has an advantage over the widely used UV/visible detection in that the response is not dependent on pathlength, so both sensitivity and selectivity can be provided. Recently, chemically modified electrodes (CMEs) have been exploited in CEEC for the determination of biological compounds, including the detection of thiols using a Hg/Au amalgam microelectrode2 or cobalt phthalocyanine (CoPc)-containing carbon paste microelectrodes,3 CEEC of disulfides at a mixed-valence ruthenium cyanide (RuCN)-modified microelectrode,4 determination of amino acids using a D-amino acid oxidase-modified electrode,5 and so on. Hydrazines are compounds of interest in both the chemical industry and pharmaceutical processes. For example, drug regulatory authorities are becoming aware of the need to control the levels of hydrazine in isoniazid and other hydrazine drugs owing to the carcinogenic and hepatotoxic effects of hydrazine. But it is difficult to analyze hydrazines by amperometric detection because of their large overpotentials at ordinary solid electrodes. Previous efforts toward enhancing the amperometric detection for hydrazines included the application of a preanodized glassy carbon electrode and various chemically modified electrodes, such as CoPc-modified carbon paste electrode,6,7 inorganic mixed-valent Prussian Blue (PB) and its analogous ruthenium cyanide film(1) Wallingford, R. A.; Ewing, A. G. Anal. Chem. 1987, 59, 1762. (2) O’Shea, T. J.; Lunte, S. M. Anal. Chem. 1993, 65, 247. (3) O’Shea, T. J.; Lunte, S. M., Anal. Chem. 1994, 66, 307. (4) Zhou, J.; O’Shea, T. J.; Lunte, S. M., J. Chromatogr. 1994, 680, 271. (5) Zhou, J.; Lunte, S. M. Anal. Chem. 1995, 67, 13. (6) Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1983, 55, 1782. (7) Korfhage, K. M.; Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1984, 56, 1514. (8) Hou, W.; Wang, E. Anal. Chim. Acta 1992, 257, 275.

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Figure 1. Cyclic voltammograms of 2.5 × 10-3 M K2PdCl4 with 0.1 M K2SO4 on microdisk CFE. Scan rate, 100 mV/s.

coated electrode,8,9 oxymanganese film-modified electrode,10 and Nafion/ruthenium oxide pyrochlore-modified electrode.11 We reported on a Pt-modified carbon fiber electrode for the determination of hydrazines by CEEC.12 In this work, we have developed a new method for electrodepositing Pd particles on a carbon fiber microdisk array electrode. The Pd particle-modified electrode exhibited excellent stability and highly catalytic activity toward hydrazines. Hydrazine, methylhydrazine, and isoniazid were separated and detected by capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MECC) with the modified electrode, respectively. EXPERIMENTAL SECTION Chemicals and Solutions. Hydrazine sulfate (HZ) and sodium dodecyl sulfate (SDS) were obtained from Beijing Institute of Chemicals, K2PdCl4 and isoniazid (ISO) were purchased from the Shanghai Reagent Factory, and methylhydrazine (MHZ) was (9) Wang, J.; Lu, Z. Electroanalysis 1989, 1, 517. (10) Taha, Z.; Wang, J. Electroanalysis 1991, 3, 215. (11) Zen, J.; Tang, J. Anal. Chem. 1995, 67, 208. (12) Zhou, W.; Xu, L.; Wu, M.; Xu, L.; Wang, E. Anal. Chim. Acta 1994, 299, 189. S0003-2700(96)00469-6 CCC: $12.00

© 1996 American Chemical Society

Figure 3. Voltammetric behavior of 1 × 10-3 M hydrazine sulfate at (A) bare and (B) Pd-modified carbon fiber microdiskarray electrodes in pH 5.52 phosphate buffer. Scan rate, 100 mV/s.

(CH Instruments, Inc., Cordova, TN) in a three-electrode system cell with an Ag/AgCl reference electrode and a platinum wire auxiliary electrode.

obtained from Fluka (Buchs, Switzerland). All reagents were of analytical grade and used as received. Solutions were prepared with doubly distilled water (filtered through a 0.45-µm membrane filter) and stored at 4 °C before use. Apparatus. All experiments were performed with a laboratory-built CE system.12 Briefly, electrophoresis in the capillary was driven by a high-voltage power supply (Spellman CZE 1000R, Plainview, NY). A 52.2 cm × 50 µm i.d. and a 60 cm × 75 µm i.d. uncoated fused-silica capillary was used (Yongnian Optical Fiber Factory, Hebei, China). A laboratory-made bipotentiostat was employed as the amperometric detector. The detection end of the capillary column is isolated from the applied electrical field, accomplished with a Nafion joint as described by O’Shea et al.13 End column detection was employed by using a wall-jet configuration. A 0.3-mL polyethylene vial was used as the detection cell and was operated in two-electrode configuration, with a microdisk array electrode and an Ag/AgCl reference electrode. The microelectrode array was fabricated from carbon fibers embedded in an epoxy composite from Toray Industries, Inc., Tokyo, Japan. A bundle of approximately 500-1000 filaments of high-strength carbon fibers of approximately 0.9-µm diameter are pulled through a resin bath of polymeric binder material to obtain a needlelike array with a tip diameter of less than 1 mm. Since only the ends of the fibers are exposed, this array can be regarded as an ensemble of microdisk. Cyclic voltammetry was carried out with a laboratory-made potentiostat and a Model 610 computerized voltammetric analyzer

RESULTS AND DISCUSSION Cyclic Voltammetry of K2PdCl4 and Formation of Pd-CFE. After electrochemical pretreatment, the carbon fiber microdisk array electrode (CFE) was cycled in the solution of 2.5 × 10-3 M K2PdCl4 and 0.1 M K2SO4 in the potential range from +0.3 to +1.6 V. Irreversible anodic peak can be observed as shown in Figure 1. The anodic current corresponding to the oxidation of PdCl42decreased gradually cycle by cycle and finally reached a steady state. Considering the large oxide functional groups on an electrochemical pretreated CFE surface, we suggest that the octahedral Pd(IV) complex formed by the planar complex of Pd(II) coordinating with these functional groups may hinder the later electrooxidation of Pd(II) complex through occupying active sites in the compact double layer; as a result, the anodic current decreased gradually.14 The palladium complex on the surface of CFE can be transformed to Pd by scanning the electrode between -0.4 and ∼+1.0 V in 0.1 M H2SO4 solution as shown in Figure 2. The shape of the cyclic voltammogram changes from cycle to cycle. The initial cycle (A) was started from 0.5 V vs Ag/AgCl in the cathodic direction. The significant cathodic peak (a) around 0.0 V on the initial cycle is attributed to the reduction of the electrode surface complexes of palladium. The peaks at this position decrease gradually (B, C) and reaching a steady state (D), corresponding to that all the Pd complexes on the electrode surface, had transformed to Pd0. The peaks of c and c′ were caused by a surface oxide formation and reduction. The peaks of b and b′ were hydrogen adsorption-desorption. The strong hydrogen adsorption on the Pd-CFE is indicative of the high dispersion of the catalyst on the carbon support. With the subsequent potentially scanning, the cyclic voltammogram reached a steady state gradually. XPS was used to characterize the surface of the GC electrode14 and the HOPG electrode.15 Palladium was mainly in the form of

(13) O’Shea, T. J.; Greenhagen, R. D.; Lunte, S. M.; Lunte, C. E. J. Chromatogr. 1992, 593, 305.

(14) Dong, S.; Li, F. Electrochim. Acta, in press. (15) Li, F.; Zhang, B.; Dong, S. J. Electroanal. Chem., in press.

Figure 2. Cyclic voltammograms of a completely deactivated PdCFE in 0.1 M H2SO4 solution: (A) first and half-potential cycles, (B) fifth cycle, (C) tenth cycle, and (D) steady state after cycling for 30 min. Scan rate, 200 mV/s.

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Figure 4. Cyclic voltammograms of 1 mM methylhydrazine and 1 mM isoniazid at (A) bare and (B) Pd-modified microdisk electrodes. Conditions as in Figure 3.

Pd(IV) complexes on both electrodes after the electrodes had been subjected to potential cycling at the same conditions as in Figure 1. The data of XPS also show the electrode surface complexes of Pd(IV) had almost completely transformed in Pd0 after the electrode cycled in H2SO4. This can further be confirmed by SEM. The maximum resolving power of JOEL LXA-840 SEM is 5 nm; Pd particles can not be seen, which indicates the size of the particles is below 5 nm. Also for in situ STM, the sizes of the Pd particles are in the nanometer scale. It is difficult to handle when using carbon fiber array electrode to do XPS, SEM, and STM experiments because of the very small size of the CF electrode. Considering that CF, GC, and HOPG electrodes are of carbon material, the CF electrode should have results similar to those of the GC and HOPG electrodes. Voltammetric Behavior of Hydrazines. Figure 3 shows the cyclic voltammograms of hydrazine obtained on a bare (A) and a Pd-modified microdisk array electrode (B). No oxidation peak of hydrazine was observed on bare CFE in the potential range from 0.0 to 1.0 V, whereas a defined oxidation peak occurred at 0.24 V on the Pd-CFE. The low potential of electrooxidation and high electrocatalytic current indicated the high electrocatalytic activity of Pd-CFE. As illustrated in Figure 4, the Pd-CFE can also produce an electrocatalytic response for methylhydrazine as well as isoniazid. However, the catalytic ability of Pd-CFE for hydrazine is much higher than that of methylhydrazine or isoniazid, due to the presence of the readily electrooxidized amine group. The Pd-modified carbon fiber microdisk array electrode exhibits highly stable catalytic capability toward hydrazines in batch. For example, after the electrode scanned between 0.0 and 1.0 V at 100 mV/s for 5 min, there were virtually no changes in the peak potential and current on CVs for 1 mM hydrazine. The Pd-CFE made this way can even resist ultrasonic cleaning for 1-2 min. CEEC Using the Pd-Modified Carbon Fiber Microdisk Array Electrode. Compared to the single carbon fiber electrode, the use of a normal size (>100-µm diameter) disk-type electrode offers several practical advantages including easier electrode/ 3352

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Figure 5. Hydrodynamic voltammograms of 5 × 10-3 M each of HZ (A), MHZ (B), and ISO (C) at the Pd-CFE. Buffer, 10 mM sodium phosphate, pH 5.52. Operating voltage, 15 kV. Sample injection, 5 s at 15 kV. Detection potential, 0.5 V vs Ag/AgCl.

capillary alignment and greater stability and reproducibility.16 Moremover, it is possible to obtain the higher current responses of a macroelectrode with the low background current by using an array electrode.4,17 Therefore, a Pd-modified carbon fiber array microelectrode was used in our work. Figure 5 shows the hydrodynamic voltammagrams (HDVs) of hydrazine, methylhydrazine, and isoniazid on Pd-CFE. Hydrazine exhibited plateau-shaped HDVs on the modified surface, reaching its maximal level at ∼0.3 V. The other two compounds, however, presented an increasing current HDVs behavior to 0.7 V. As a compromise of high sensitivity and low background current, a 0.5-V value was selected for simultaneous detection of hydrazines and isoniazid. Electropherograms of hydrazine, methylhydrazine, and isoniazid in phosphate buffer at pH 5.52 are shown in Figure 6B. (16) Ye, J.; Baldwin, R. P. Anal. Chem. 1993, 65, 3525. (17) Koley, S. D.; Simons, J. H. M.; Linden, W. E. Anal. Chim. Acta 1993, 273, 71.

Figure 6. Electropherograms of (a) HZ (0.5 mM), (b) MHZ (1 mM), and (c) ISO (1 mM) at (A) bare and (B, C) Pd-modified electrodes. Buffer (B), 10 mM sodium phosphate, pH 5.52. Buffer (A,C), 10 mM sodium phosphate + 10 mM SDS, pH 5.52. Sample injection, 3 s at 15 kV. Other conditions as in Figure 5.

The peak of HZ was very close to that of MHZ, so these two species cannot be base-line separated by CZE. Figure 6 C shows the well-defined and -resolved electropherograms obtained on the same column used in Figure 6B employing the same phosphate buffer with 0.01 M SDS added. Separation selectivity was enhanced dramatically by adding surfactant to the buffer due to the difference partitioning of solutes between the aqueous and micellar phases. In order to make a comparison, electrodetection of the three species on the bare microdisk array electrode at the same potential (0.5 V) is also given in Figure 6A. The detection limits were 1.0, 5.0, and 5.0 µM for hydrazine, methylhydrazine, and isoniazid, respectively (S/N ) 3). Based on an injection volume of 9.1 nL, the mass detection limits correspond to 1.2, 2.1, and 6.2 pg. The response of hydrazine was examined over the concentration range of 1 × 10-6-5 × 10-3 M. Linear regression analysis yielded a slope of 8.08 nA/mM with a correlation coefficient of 0.993. The Pd-modified carbon fiber microdisk array electrode exhibits highly stable catalytic capability toward hydrazines in a flow system. All the above experiments were made in the same Pd-modified microdisk array electrode in a period of 3 weeks with more than 100 runs. If the response was found decreased after the electrode was used for several days, a pretreatment of holding the electrode at +0.9 V for 10 s followed by -0.4 V for 5 s could be used to refresh the surface of the electrode, which might be polluted by the product of hydrazines. Reproducibility was improved by rinsing the capillary with 0.1 M sodium hydroxide solution, doubly distilled water, and running buffer for 3, 3, and 5 min, respectively, between each run. Last, this electrode has been used to determine hydrazines in real samples. Urine samples from a healthy female volunteer were injected and evaluated. Figure 7 shows the electropherogram of a 2-fold dilution of human urine assayed as such (Figure 7A) and after its spiking with hydrazine (Figure 7B), with hydrazine, and isoniazio (Figure 7C). The electrode was not sensitive for other constituents in urine. Hydrazine seems to be well resolved from the unknown species. CONCLUSION The palladium particle-modified carbon fiber microdisk array electrode has been evaluated for the simultaneous detection of

Figure 7. Electropherograms of (A) urine diluted 1:2, (B) the same urine sample to which 0.5 mM hydrazine (a) was added, and (C) the same as (B) but 1 mM isoniazid (c) was added before injection at a Pd-modified electrode; 75-µm-i.d. capillary, 60-cm-long separation capillary, and 2-cm-long detection capillary. Other conditions as in Figure 5.

hydrazine, methylhydrazine, and isoniazid by end-column CEEC system. The method has been proved to have a wider linear response range and lower detection limits at a more negative detecting potential compared with our previous reports by using a Pt-modified electrode in CEEC.12 ACKNOWLEDGMENT The authors acknowledge Prof. T. Kuwana of the University of Kansas for supplying us the carbon fiber microdisk array electrode used in this work. This project was supported by the National Natural Science Foundation of China.

Received for review May 9, 1996. 1996.X

Accepted July 22,

AC9604696 X

Abstract published in Advance ACS Abstracts, August 15, 1996.

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