Determination of nifedipine, ampicillin and penicillin G and their electro-oxidation products by voltammetric techniques

Abstract

The study of the electrochemical behaviour of ampicillin (AMP), penicillin G (PG) and nifedipine (NFD) were carried out using three different working electrodes namely hanging mercury drop electrode (HMDE), mercury meniscus modified silver amalgam electrode (m-AgSAE) and boron-doped diamond electrode (BDDE). All measurements were made versus Ag/AgCl (3 M KCl)) using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and adsorptive stripping voltammetry (AdSV). Due to the toxicity of mercury, m-AgSAE was constructed as an alternative electrode for HMDE. The DPV determination of NFD were conducted using mixtures of Britton-Robinson (BR) buffer pH 8 with methanol in three different volume/volume (v/v) ratios (1:1, 9:1, and 99:1) in the concentration range of 0.2 to 20 μmol/L. The ratio 9:1 was chosen as the optimal ratio for NFD determination. The limit of quantification (LOQ) was 0.12 μmol/L for (HMDE) and 1.2 μmol/L (m-AgSAE). Attempts to increase the sensitivity using AdSV at both electrodes were not successful. For the determination of AMP and PG, the optimal conditions have been performed in Britton-Robinson (BR) buffer in the concentration range of 1 to 9 μmol/L (HMDE) and 10 to 90 μmol/L (m-AgSAE). The limit of detection (LOD) for AMP and PG at HMDE were 0.09 and 0.065 μmol/L, respectively and the LOD for AMP and PG at m-AgSAE were 3.8 and 2.5 μmol/L, respectively. PG also was successfully determined using the BDDE working electrode where PG showed well developed oxidation peak at potential between 800 to 1100 mV vs Ag/AgCl (3 M KCl) in BR buffer. The highest and best developed peak was obtained at pH 4 at a potential of +980 mV vs Ag/AgCl (3 M KCl). Attempts to increase the sensitivity using adsorptive stripping DPV at BDDE were not successful. The LOD and LOQ for PG were 0.23 μmol/L and 1.5 μmol/L, respectively. The practical application of the newly developed method was verified on the determination of NFD, AMP and PG in spiked samples of drinking and river water using optimum conditions. Voltammetry and LC-MS method was used for monitoring the electro-oxidation mechanism of AMP, PG and NFD. The electro-oxidation process was performed in three different mediums of electrolyte which were tap water (pH~6.5) and BR buffer of pH 4 and pH 10, while the electro-oxidation time were set to 15, 30 and 60 minutes. A series of mixed metal oxides (MMO) titanium-based electrodes (TiO2/Ti, IrO2-TiO2/Ti, RuO2-TiO2/Ti and two RuO2-IrO2-TiO2/Ti with different ratios of the metal oxide) were used as an anode. AMP, PG and NFD were successfully degraded using electro - oxidation process with the best MMO electrode for the degradation was RuO2-IrO2-TiO2/Ti in tap water (pH~6.5) medium. LC-MS analysis was performed to determine the by-product and mechanism of the degradation process of the drugs were proposed.