Synergistic electrochemical and chemical modification of carbon paste electrodes for open-circuit preconcentration and voltammetric determination of trace adenine

A new concept of electrode modification is presented, consisting of chemically assisted electrochemical oxidative pretreatment of the electrode surface. If carbon paste electrodes (CPEs) are polarized at +1.75 V (vs Ag/AgCl) in a dilute alkaline medium (1 x 10(-3) mol/L) containing 0.1 mol/L NaNO3 for 60 s, they exhibit a unique ability to preconcentrate adenine, an important biological substance, that served as a model compound. In this work, a thorough study has been performed to explore the new pretreatment procedure, but also its analytical application for trace adenine determination was optimized. During the pretreatment of a CPE, a specific oxide layer is formed due to a synergistic effect of a combined anodic and chemical oxidation process, in which the presence of sodium nitrate plays a crucial role. In a subsequent step under open-circuit conditions, adenine is oxidized by the oxide layer, and the resulting oxidation product is strongly adsorbed onto the electrode surface. After medium exchange, the adsorbed oxidation product of adenine is reduced at -0.5 V (vs. Ag/AgCl) analytical purposes. Applying 10 min accumulation and square-wave voltammetric operation, the detection limit of 20 mu g/L (1.5 x 10(-7) mol/L) of adenine in a sample solution was acquired. The voltammetric behavior of adenine al plain and pretreated CPEs is also presented and discussed.