Adsorptive stripping voltammetry (AdSV) is a recent technique (initially
named by Lam, Kalvoda and Kopanica in 1983) in which there is generally a
nonelectrolytic accumulation (adsorption) of the determinand followed by a
cathodic reductive or anodic oxidative scan. In this work, a hanging mercury drop
electrode (HMDE) was used to accumulate several purine derivatives with and
without the addition of Cu(lI) in a suitable buffer (usually Britton-Robinson, pH
7.5). In the presence of copper(lI) the copper(l) purine complex was usually
accumulated at about -0.1 V. Some of the compounds studied (i.e
dimenhydrinate, theophylline, guanine and azathioprine) can be determined as
free compounds (i.e with no Cu(lI) added) after accumulation at potentials
ranging from 0 to -0.2 V versus Ag/AgCI reference electrode. The determination
of these compounds was made after optimising pH, buffer constituents,
accumulation potential and accumulation time. The addition of Cu(lI) to these
purines makes possible accumulation at shorter accumulation times , and
improves the detection limits. For example, the detection limit of azathioprine in
the presence of Cu(lI) is as low as 2x10-10 M. Both differential-pulse polarography
(DPP) and DP-AdCSV (differential pulse adsorptive cathodic stripping
voltammetry) were used in the determination of theophylline and azathioprine in
their dosage forms (tablets). The relative standard deviations for the
determination of the pharmaceutical purines in their tablets were 3.0%. 4.2% and
1.3% for the measured concentrations of dimenhydrinate(1.5x10-7 M) ,
theophylline(2x10-7 M) by AdSV ; and azathioprine(2x10-6 M) by DPP,
respectively .
Other purines (eg xanthine,1,3-dimethyluric acid and some
methylguanines) did not give peak currents useful for their determination.
The addition of Cu(lI) was necessary to obtain satisfactory peak currents for
these purines. At low copper(lI) concentrations (Le.< 10-6 M), it is suggested that
the purines form Cu(l) complexes which are adsorbed at the HMDE surface:
Cu(lI) + Cu(O) + 2 L ~ 2 Cu(I)-L adsorbe
