Optimal Settings for Pulsed Amperometric Detection of Carbohydrates Using the Dionex ED40 Electrochemical Detector

INTRODUCTION Pulsed amperometry is a powerful detection method for carbohydrates that requires no sample derivatization. Pulsed amperometric detection (PAD) is particularly useful in combination with high-performance anion-exchange chromatography (HPAE). For a full description of HPAEPAD, refer to Dionex Technical Note 20. For an introduction to amperometry refer to Conductivity and Amperometry, by Roy D. Rocklin, Ph. D. (P/N 34358), or Chapter 6 of the ED40 Manual. All are available from your Dionex representative. At high pH, carbohydrates are electrocatalytically oxidized at the surface of the gold electrode by application of a positive potential. The current generated is proportional to the carbohydrate concentration, and therefore carbohydrates can be detected and quantified. If only a single potential is applied to the electrode, oxidation products gradually poison the electrode surface. This electrode surface poisoning causes a loss of analyte signal. To prevent signal loss, the electrode surface is cleaned by a series of potentials that are applied for fixed time periods after the detection potential. A series of potentials applied for defined time periods is referred to as a waveform. Repeated application of a waveform is the basis of pulsed amperometry. The potentials of a waveform are designated E 1 , E 2 , E 3 , etc., where E 1 is the detection potential. The remaining potentials clean and restore the electrode for subsequent detection. Potentials are maintained for time periods t 1 , t 2 , t 3 , etc. The first time period (t 1 ) is subdivided into t del and t det . The delay period, t del , is the time that is allowed for the charging current (produced when changing potentials) to decay so that only current from analyte oxidation is measured during the detection period, t det (see Figure 1). Optimal values for all waveform parameters are determined by systematic variation of one parameter while holding the other parameters constant. An excellent discussion of the optimization of pulsed amperometry waveforms was published by LaCourse and Johnson . Dionex supports three waveforms for carbohydrate analysis. This Technical Note discusses each waveform with special emphasis on their benefits and liabilities. Figure 1 shows a schematic representation of each waveform. The ED40 electrochemical cell is equipped with a combination pH-Ag/AgCl reference electrode. The potentials presented in this Technical Note require use of the Ag/AgCl half of the reference electrode.