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The fabrication and application of a novel multichannel amperometric detection system (MADS) with chemically modified microelectrodes (CMEs) for liquid chromatography (LC) are described. These CMEs are based on the immobilization of copper tetraaminophthalocyanine(CuTAPc) on platinum (Pt) microelectrodes by cyclic voltammetry (CV). The CMEs show good catalytic activity oxidation of several analytes including thiocompounds. With the multichannel amperometric detection system (MADS) coupled to LC, a three-dimensional hydrodynamic voltammogram (3D HDV) (i.e. current as a function of time and potential) is obtained. The optimum detection potentials of thiocompounds and other analytes at the CMEs are easy to find. The novel multichannel amperometric detector with CMEs shows good stability and reproducibility in detecting the thiocompounds and other analytes in LC. The linear ranges cover over three orders of magnitude and the limits of detection are 10 pmol for cysteine, 16 pmol for glutathione, 16 pmol for N-acetylcysteine, 10 pmol for ascorbic acid and 16 pmol for uric acid. The application of this method with microdialysis sampling and normal sampling in human whole blood assay is successful. The method of LC-MADS with microdialysis sampling is more useful to assay the compounds in the biological matrix.

A new chemically modified electrode (CME) was found to be comparable to a glassy carbon electrode in terms of the eletrocatalytic effect of Cobalt Hexacyanoferrate (CoHCF) towards thiolcompounds (glutathione (GSH), cysteine (Cys), N -acetylcysteine (NAC)). Polynuclear CoHCF thin films are electrodeposited on a glassy carbon electrode by potential cycling between 1.2 V and −0.2 V versus Ag/AgCl in a fresh modification solution containing 0.5 mmol/l K 3 Fe(CN)6 and 1 mmol/l CoCl 2 in 0.1 mol/l KH 2 PO 4 (pH = 3.0). Combined with liquid chromatography, the CoHCF CME exhibits a good linear relationship between the concentration and the current responses of GSH (5.0 × 10 −6 –1.0 × 10 −4 mol/l), Cys (1.5 × 10 −6 –2.0 × 10 −4 mol/l), and NAC (3.0 × 10 −6 –1.0 × 10 −4 mol/l). The detection limits were 2.5 × 10 −6 mol/l for GSH, 1.0 × 10 −6 mol/l for Cys and 1.5 × 10 −6 mol/l for NAC. We coupled the microdialysis sampling technique with liquid chromatography-electrochemical detection (LC-ECD) and successfully determined glutathione in the brain of a freely moving rat.

A potassium salt of palladium(II)-substituted Keggin type heteropolytungstate, K 6 [(H 2 O)PdSiW 11 O 39 ] (abbreviated as PdSiW 11 in the following), was used to fabricate a new kind of chemically modified electrode (CME) by potential scanning on the surface of glassy carbon electrode. Characterization of this modified electrode was conducted by Fourier transform infrared reflection absorption spectroscopy (FTIRRAS), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). FTIRRAS and XPS showed that the species on the electrode surface sustained the same Keggin type structure as that of the heteropolytungstate in buffer solution. A couple of waves was observed on the PdSiW 11 CME, which was assigned to the redox process of the palladium center in the heteropolytungstate. This CME had high electrocatalytic activity towards nitrite reduction and exhibited good reproducibility and stability. The catalytic peak current was found to be linear with the nitrite concentration in the range 1–20 mmol/l with a correlation coefficient of 0.998. The PdSiW 11 CME is successfully applied as an electrochemical detector to monitor nitrite in flow injection analysis (FIA). The linear range is over three orders of magnitude and the detection limit is 24.0 pmol for nitrite determination. The relative standard deviation (RSD) for ten successive measurements of 1.0 mmol/l nitrite in FIA is found to be 3.3%.

Polynuclear indium(III) hexacyanoferrate(II, III) thin films are electrodeposited on a glassy carbon (GC) electrode through potential cycling between 1.1 and 0.0 V vs. SCE in a fresh modification solution containing potassium, indium(III) and hexacyanoferrate(III) ions. During the negative scans, the cationic In 3+ species interact with the simultaneously generated anionic Fe(CN) 4− 6 to yield a sparingly soluble mixed-valent coating on the electrode. Coated with Nafion film, the chemically modified electrode (CME) shows a better stability. The CME shows a single set of reversible, well-defined peaksc significantly separated from the solution responses. The apparent surface coverage is found to be 6.3×10 −9 mol/cm 2 . This CME exhibits an efficient catalytic activity towards the rapid oxidation of thiocompounds in cyclic voltammetry. Cysteine (Cys), glutathione (GSH), 6-thiopurine (6-TP) and methimazole (MMI) have good and stable current responses at the CME in liquid chromatography with electrochemical detection (LC–ECD). The linear ranges are over three orders of magnitude and the detection limits are 1.6 pmol for Cys, 20 pmol for GSH, 20 pmol for 6-TP and 1.6 pmol for MMI. The method has been successfully applied to assess the purity of methimazole, the contents of cysteine in human urine and other thiocompounds in artificial urines.

An electrochemical microsensor, constructed by chemical modification of Nafion, poly(vinyl pyridine) (PVP), palladium and iridium oxide onto a platinum microelectrode, has been developed for the determination of nitric oxide (NO). This microsensor exhibits excellent catalytic activity toward NO oxidation, which is confirmed by differential pulse voltammetry. Cyclic voltammetry and differential pulse amperometry are also performed to measure NO. The catalytic peak current is found to be linear with the NO concentration in the range 5.8×10 −8 –6.4×10 −6 mol/l, with a correlation coefficient of 0.9985. The detection limit of the microsensor is 1.5×10 −8 mol/l. In addition, the microsensor is also very selective with respect to interferents such as ascorbic acid, dopamine, and nitrite.