"Optimization Of A Modified Electrode For The Sensitive And Selective Detection Of α-dipeptides"
J. Chromatogr. A
1995 Volume 705, Issue 2 Pages 171-184
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Jian-Ge Chen, Edward Vinski, Kevin Colizza and Stephen G. Weber*
Abstract:
The modified electrode was prepared by subjecting a vitreous C electrode to 1.2 V vs. Ag/AgCl for 4 h in a flowing electrolyte (1 ml/min) containing 0.1 mM Cu in phosphate buffer of pH 8. The selectivity of the modified electrode was altered by controlling the pH and composition of the detection eluent. The electrode was more selective for α-dipeptides over β- and γ-dipeptides and amino-acids at pH 9.8. At pH 8, the electrode was selective for all dipeptides over amino-acids. The detection limits for dipeptides were ~e;10 nM in a flow injection system employing a 20 µL sample loop, a detection potential of 1.1 V vs. Ag/AgCl and an eluent (1 ml/min) containing 0.1 mM Cu in phosphate buffer of pH 8. Surface analysis showed that the oxidation of the vitreous C electrode gave rise to its selectivity. The oxidation of dipeptides at the modified electrode was completely inhibited by 10 mM Mg2+ in the eluent. The modified electrode was applied to the detection of eight dipeptides and glutathione (four α-, two β- and two γ-dipeptides and γ-glutamyl tripeptide) following separation by anion-exchange chromatography (details given). Sensitive and selective detection of dipeptides is important in neurochemistry. We have developed a flexible detection scheme for dipeptides based on a modified carbon electrode. The modification arises from the anodic treatment of the carbon electrode in alkaline solution. The flexibility of the detection scheme arises from the different conditions used in both the modification and the detection. It is shown that the modification step requires the presence of cupric ion, while the detection step does not. On the other hand, it is shown that the presence of copper in the detection eluent, as well as the pH of the environment, can be used in controlling the selectivity of the modified electrode. For example, the modified electrode is more selective for α-dipeptides over β- and γ-dipeptides as well as amino acids at pH 9.8, whereas it is selective for all dipeptides over amino acids at pH 8.0. Detection limits of dipeptides on the order of 10 nM were achieved at pH 8.0 by flow injection analysis with a knotted Teflon tubing connecting the injector and the detector that gave a typical peak volume of about 0.50 mL at 1.0 ml/min. From surface analysis it is shown that the oxygenation of the glassy carbon electrode gives rise to the selectivity. The oxidation of dipeptides at the modified electrode is completely inhibited by 10 mM Mg2+ in the eluent.
Disaccharides
Glutathione
Interferences
Mixing coil
Knotted reactor
Optimization
