Simultaneous determination of dihydroxybenzene and phenylenediamine positional isomers using capillary zone electrophoresis coupled with amperometric detection.

In general capillary zone electrophoresis (CZE) separation models, o-, m-, and p-phenylenediamine isomers can be separated in a weak acidic running buffer for their pK(a) values being 4.52, 5.64, 6.04, respectively, while o-, m-, and p-dihydroxybenzene isomers can be separated in a weak basic buffer for their pK(a) values being 9.40, 9.40 and 10.04, respectively. So, it is hard to find a suitable running buffer at a fixed pH in normal CZE for simultaneous separation of these two groups of positional isomers. In this paper, a novel method based on alternately running two different pH buffers in CZE coupled with amperometric detection (CZE-AD) was designed to simultaneously determine these two groups of positional isomers. It is found that when two different pH running buffers were employed alternately under appropriate order and time, the six analytes could be separated perfectly in less than 20 min and the detection limits were as low as 10(-7) mol/L. Furthermore, the effects of working electrode potential, pH and concentration of running buffer, separation voltage and injection time on CZE-AD were investigated. Experimental results demonstrated that the introduced method was practical to simultaneously determine two groups of positional isomers with different pK(a) and had some advantages of high sensitivity, good repeatability and small sample requirement.

Electrochemical behaviors of amino acids at multiwall carbon nanotubes and Cu2O modified carbon paste electrode.

A carbon paste electrode modified with multiwall carbon nanotubes and copper(I) oxide (MWCNT-Cu(2)O CPME) was fabricated, and the electrochemical behaviors of 19 kinds of natural amino acids at this modified electrode were studied. The experimental results showed that the various kinds of amino acids without any derivatization displayed obvious oxidation current responses at the modified electrode. It was also found that the current response values of amino acids were dependent mainly on pH values of buffer solutions. The phenomenon could be explained by the fact that the amino acids suffered complexation or electrocatalytic oxidation processes under different pH values. Six kinds of amino acids (arginine, tryptophan, histidine, threonine, serine, and tyrosine), which performed high-oxidation current responses in alkaline buffers, were selected to be detected simultaneously by capillary zone electrophoresis coupled with amperometric detection (CZE-AD). These amino acids could be perfectly separated within 20 min, and their detection limits were as low as 10(-7) or 10(-8)mol L(-1) magnitude (signal/noise ratio=3). The above results demonstrated that MWCNT-Cu(2)O CPME could be successfully employed as an electrochemical sensor for amino acids with some advantages of convenient preparation, high sensitivity, and good repeatability.

Simultaneous determination of phenylenediamine isomers and dihydroxybenzene isomers in hair dyes by capillary zone electrophoresis coupled with amperometric detection.

The simultaneous determination of three isomers of phenylenediamines (o, m, and p-phenylenediamine) and two isomers of dihydroxybenzenes (catechol and resorcinol) in hair dyes was performed by capillary zone electrophoresis coupled with amperometric detection (CZE-AD). The effects of working electrode potential, pH and concentration of running buffer, separation voltage, and injection time on CZE-AD were investigated. Under the optimum conditions the five analytes could be perfectly separated in 0.30 mol L(-1) borate-0.40 mol L(-1) phosphate buffer (pH 5.8) within 15 min. A 300 µm diameter platinum electrode had good responses at +0.85 V (versus SCE) for the five analytes. Their linear ranges were from 1.0 × 10(-6) to 1.0 × 10(-4) mol L(-1) and the detection limits were as low as 10(-7) mol L(-1) (S/N = 3). This working electrode was successfully used to analyze eight kinds of hair dye sample with recoveries in the range 91.0-108.0% and RSDs less than 5.0%. These results demonstrated that capillary zone electrophoresis coupled with electrochemical detection using a platinum working electrode as detector was convenient, highly sensitive, highly repeatable and could be used in the rapid determination of practical samples.

Simultaneous determination of antioxidants at a chemically modified electrode with vitamin B12 by capillary zone electrophoresis coupled with amperometric detection.

In the paper, a new kind of vitamin B(12) (acquo-cobalamine) chemically modified electrode was fabricated and applied in capillary zone electrophoresis coupled with amperometric detection (CZE-AD) for simultaneous determination of six antioxidants in fruits and vegetables. The catalytic electrochemical properties of the chemically modified electrode could obviously enhance oxidation peak heights responses by about five times to glutathione, ascorbic acid, vanillic acid, chlorogenic acid, salicylic acid, and caffeic acid compared with common carbon disk electrode. Furthermore, the effects of working electrode potential, pH and concentration of running buffer, separation voltage and injection time on CZE-AD were investigated. Under the optimum conditions, the six analytes could be completely separated and detected in a borate-phosphate buffer (pH 8.4) within 15 min. Their linear ranges were from 2.5x10(-7) to 1.0x10(-4) mol L(-1) and the detection limits were as low as 10(-8) mol L(-1) magnitude (S/N=3). The proposed method has been successfully employed to monitor the six analytes in practical samples with recoveries in the range 96.0-106.0% and RSDs less than 5.0%. Above results demonstrate that capillary zone electrophoresis coupled with electrochemical detection using vitamin B(12) modified electrode as detector is of convenient preparation, high sensitivity, good repeatability, and could be used in the rapid determination of practical samples.

Simultaneous determination of flavonoids in chrysanthemum by capillary zone electrophoresis with running buffer modifiers.

Despite the separation efficiency of capillary electrophoresis (CE) is much higher than other chromatographic methods, it is sometimes difficult to perfectly separate the complex ingredients in biological samples. One possible and simple way to develop the separation effect in CE is to add some modifiers in the running buffer. In this paper, the suitable running buffer modifiers were explored to simultaneously separate and detect six typical flavonoids (apigenin, luteolin, kaempferol, quercetin, (+)-catechin and (-)-epicatechin) which are the main active ingredients in chrysanthemum by capillary zone electrophoresis with amperometric detection (CZE-AD). It was found that when beta-cyclodextrin (beta-CD) and the mixture of methanol and ethanol were used as running buffer modifiers, a baseline separation of the six analytes could be accomplished in less than 20 min and the detection limits were as low as 10(-7) or 10(-8)gm l(-1). Other factors affecting the CZE separation, such as working potential, pH value and ionic strength of running buffer, separation voltage and sample injection time were extensively investigated. Under the optimum conditions, a successful practical application on the determination of chrysanthemum samples confirmed the validity and practicability of this method.