Abstract: Experiments were carried out to examine now each parameter of the standard pulsed amperometric detection (PAD) waveform affected the peak height and peak tailing in the FIA determination of penicillin G (I; benzylpenicillin). Sample (75 µL) was injected into a stream (0.7 ml/min) of 0.2 M acetate buffer of pH 4.7 and detection was carried out using a thin-layer cell with a Au electrode (0.25 cm diameter) a Ag/AgCl reference electrode and a stainless-steel counter electrode. The standard waveform was as follows: E1 1500 mV; E2 1600 mV; E3 -200 mV; T1 0.333 s; T2 0.353 s; and T3 0.167 s. E1 was varied from 1150-1550 mV, T1 and T2 were varied from 0.166-0.516 s, E2 was varied from 1600-1800 mV, T3 was varied from 0.016 to 0.466 s and E3 was varied from -300 to +50 mV. Results are presented. Alterations leading to improved analyte desorption efficiency decreased the peak tailing. It is well known that alterations of the times and potentials of each step within a PAD waveform can alter the sensitivity of the amperometric response, peak shape has also been found to vary with waveform adjustments. This work studied the variation in both peak heights and peak tailing as a function of waveform alterations for penicillin G oxidation in flow injection analysis. Large variations were found when the detection step and adsorption time were altered and smaller changes were observed during alterations of the other parameters, The major contribution to the tailing profile was inefficient removal of adsorbed analyte, which was subsequently retained until further PAD cycles. Alterations that improved the desorption efficiency led to reduced peak tailing, whereas alterations that hindered desorption caused an increase in tailing. The ability to minimize peak tailing will be advantageous for PAD usage with separation methods featuring ever-increasing resolution capability. (C) 1997 Elsevier Science B.V. 32 References

Abstract: The electrochemical oxidation of penicillins at Au electrodes was optimized; the best sensitivity was observed in strongly acidic solution, but the chemical stability was insufficient for use under these conditions. Use of a weakly acidic acetate buffer solution yielded sufficient stability at the cost of some sensitivity. By the application of pulsed amperometric detection, the penicillins could be monitored either directly or indirectly, depending on the specified waveforms. Cyclic voltammetry was performed with use of Ag/AgCl reference electrodes, solid metal working electrodes and a Pt-wire counter electrode. Direct detection by standard pulsed amperometry was the best detection method; when this was used in a flow injection system, the RSD for peak height was 0.1% and the detection limit for benzylpencillin was 0.4 µM.

Abstract: The suppression of the dissolved oxygen reduction signal at Pt and Au electrodes by the adsorption of organic and other non-metal species was used as an indirect measure of adsorbate concentration. This indirect approach, indirect adsorption detection (IAD), was used to detect amines, halides and sulfur compounds following FIA or HPLC. The measurements were carried out using a three electrode system consisting of a circular Pt or Au working electrode (2 mm diameter), a Ag/AgCl reference electrode and a stainless steel counter electrode. A pulsed voltage was applied to the working electrode and O2 reduction was monitored at negative potentials. The detection limits achieved by IAD were similar to those of pulsed electrochemical detection. The detection limit was 50 nM for the methylthiohydantoin derivative of glycine with a detection potential of -200 mV. The negative detection potentials required by IAD for the highest sensitivity also promoted gas production. A detection cell designed to be tolerant to gas production is required to realize the full potential of IAD.