Comparative Voltammetric Analysis of Adenine and Xanthine on a Pencil Graphite Electrode in the Presence of Copper Ions

This work compares voltammetric responses of adenine (Ade) and xanthine (Xan) on a pencil graphite electrode (PeGE) in the presence of copper ions. For electroanalytical monitoring linear sweep voltammetry (LSV) in connection with elimination procedure (EVLS - elimination voltammetry with linear scan), differential pulse voltammetry (DPV), and constant current potentiostatic stripping analysis (CPSA) were used. Both substances are oxidized on PeGE providing simple oxidation peaks in the absence of Cu(II) at about 0.8 V (Xan) and 1.1 V (Ade). Electrochemically produced Cu(I) ions give rise to new oxidation peaks corresponding to the oxidation of a sparingly soluble Cu(I)-purine complex at less positive potentials (similar to 0.45 V) while the former oxidation purine peaks are increased. Compared to Ade the oxidative signal of Xan is split into two peaks depending on scan rate. Peak-counterpeak EVLS signals reflect the charge transfer processes proceeding in an adsorbed state, and this fact confirms the suggested mechanism of both electrode processes. The splitting of xanthine oxidation signal was tested by means of a potential jump from -0.15 V to 0.65 V. The effect of scan rate, pH and concentration of Cu and ligands was studied in order to evaluate not only the nature of the additional Xan oxidation peak but also to optimize the best conditions for electroanalytical determination of Ade and Xan in mixtures.