SYNERGISTIC ELECTROCHEMICAL AND CHEMICAL MODIFICATION OF CARBON-PASTEELECTRODES FOR OPEN-CIRCUIT PRECONCENTRATION AND VOLTAMMETRIC DETERMINATION OF TRACE ADENINE

A new concept of electrode modification is presented, consisting of ch emically assisted electrochemical oxidative pretreatment of the electr ode 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) containi ng 0.1 mol/L NaNO3 for 60 s, they exhibit a unique ability to preconce ntrate adenine, an important biological substance, that served as a mo del compound. In this work, a thorough study has been performed to exp lore the new pretreatment procedure, but also its analytical applicati on for trace adenine determination was optimized. During the pretreatm ent of a CPE, a specific oxide layer is formed due to a synergistic ef fect 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 el ectrode surface. After medium exchange, the adsorbed oxidation product of adenine is reduced at -0.5 V (vs. Ag/AgCl) analytical purposes. Ap plying 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 sam ple solution was acquired. The voltammetric behavior of adenine al pla in and pretreated CPEs is also presented and discussed.