Pulsed potentiometric detection in capillary electrophoresis using platinum electrodes

Abstract

A new potentiometric analytical technique, termed pulsed potentiometric detection (PPD) has been developed and introduced as an end-capillary detection technique in capillary electrophoresis (CE). In contrast to normal potentiometry where the potential is measured in a steady-state, PPD involves the application of one or more independent pre-pulses to the detection electrode prior to the measurement period. The potential of the detection electrode is then allowed to move towards a steady-state value during which the potential is measured. The pulsing cycle is then repeated and a further measurement made. PPD was investigated using one or two pre-pulses of up to ±1V for 10ms on a platinum or gold detection electrode for a range of common anions separated in neutral to acidic electrolytes. Both the signal and the baseline noise were found to depend strongly on the sequence, sign and magnitude of the pre-pulses with the best results for a positive +800 mV pulse followed by a negative −800 mV pulse, both for 10ms. Notably, the pulsed system was more rugged with significantly less baseline drift compared to non-pulsed potentiometric detection schemes using metal electrodes. Two types of detection response were observed: general, for electroinactive analytes with detection sensitivity and limits of detection comparable to those obtained using traditional potentiometric detection, and analyte specific detection response for the electroactive analytes bromide, bromate, iodide and iodate. For the latter group of analytes a substantial improvement in detection sensitivity and limits of detection (LODs) were observed under optimal pulsing conditions. Using a 25 μm capillary and an injected sample plug of 2mm, LODs in the sub-μM range were observed. Very high separation efficiencies ranging from 222000–480000 theoretical plates m⁻¹ and no apparent peak tailing were obtained. Reproducibility for peak heights was better than 5% RSD.