Development of fast pulsed electrochemical detection of carbohydrates separated by liquid chromatography and capillary electrophoresis

摘要

Pulsed Electrochemical Detection (PED) has become well known for sensitive and reproducible detection of carbohydrates separated by anion-exchange chromatography. This detection technique maintains electrode activity by combining potential pulses of brief duration that clean the surface by oxide formation and reduction prior to the detection step. This three-step potential-time waveform is usually applied at a frequency of ca. 1 Hz. However, the small sample volumes and fast separation times associated with capillary electrophoresis have required the consideration of PED waveforms with frequencies greater than 1 Hz. Therefore, the processes that ultimately control the upper limit of waveform frequency were determined;Because electrode activity is dependent upon oxide formation and reduction, design of high-frequency PED waveforms depends upon an understanding of these processes. Chronocoulometry was used to study oxide formation and reduction at Au and Pt electrodes in 0.1 M sodium hydroxide solutions. Gold electrodes exhibited oxide formation and oxide reduction processes that are complete on time scales compatible with high-frequency PED waveforms. Hence, Au was used as a working electrode in subsequent studies;The design of PED waveform frequencies greater than 1 Hz, for the detection of carbohydrates separated by anion-exchange chromatography and capillary electrophoresis, was based upon data from the kinetic studies. Fixed cleaning parameters (times and potentials) were used, and the frequency was increased by decreasing the integration period. Consequently, the shorter period for current integration at higher waveform frequencies resulted in a corresponding increase in detection limits. Resolution of CE peaks less than 12 seconds in width required the use of PED waveforms with frequencies greater than 2 Hz to attain reproducible peaks. The compromise between waveform frequency, detection limit, and peak resolution for CE-PED were examined.