Adsorptive stripping voltammetry of trimethoprim: Mechanistic studies and application to the fast determination in pharmaceutical suspensions

The adsorptive stripping voltammetric behaviour of trimethoprim (TMP) was studied at pH 3.8 and 7.0 by linear-sweep (LS) and cyclic voltammetry at the hanging mercury drop electrode. The charges and surface concentrations of the protonated TMP species were determined at both pH values. Taking advantage of the adsorption features of TMP fast voltammetric techniques (LS and square-wave (SW) voltammetry) were applied to the determination of TMP at the 10-7 mol dm-3 concentration level (pH 3.8). For these concentrations the relative standard deviations were <2% (N = 8) and the detection limit was 10 nM (3 ng/mL) for the SW-AdCSV (3 s; accumulation time 10 s, frequency 100 Hz). The use of SW-adsorptive cathodic stripping voltammetry originated a very fast and sensitive method for the direct analysis of TMP in pharmaceutical suspensions without any matrix effects or interference from sulfamethoxazole. No sample pre-treatments or solvent extraction procedures were needed. The quantitative results were in agreement with the data supplied by the manufacturer.