Pulsed Electrochemical Detection: Waveform Evolution

Jul 01, 2011
Volume 29, Issue 7, pg 584–593

Pulsed electrochemical detection (PED) is entering its fourth decade of existence, and through the years a diversity of waveforms has been described in the literature. The rise of PED for the direct and sensitive detection of polar, aliphatic compounds following liquid chromatography (LC) or electrophoresis separations can be chronicled through the evolution of its waveforms. Along the way, some waveforms have flourished to gain favor and acceptance in chemical and biochemical analysis, while others have become extinct. The purpose of this column is to trace the developments of PED through the progression of its waveforms.

The primordial soup from which pulsed electrochemical detection (PED) arose is that of dc amperometry (1,2), whose waveform is a constant applied potential over time (Figure 1). Using dc amperometry as a means of electrochemical (EC) detection following chromatographic separations has proven to be a powerful analytical technique for the determination of compounds that undergo oxidation and, less commonly, reduction. The first commercial EC detector for high performance liquid chromatography (HPLC), whose popularity can be directly attributed to its high sensitivity, often high selectivity, and wide linear range, was introduced in 1974. In addition to EC detectors' relatively simple construction and ruggedness, their electrochemical cells easily adapt to miniaturization (for example, capillary electrophoresis and lab-on-a-chip devices) because the response is dependent on electrode area and not pathlength, as in optical absorbance methods.

The earliest applications of dc amperometry in liquid chromatography (LC) were for the determination of neurotransmitters in complex biological samples (for example, brain extracts), drugs and their metabolites in physiological fluids, and contaminants in environmental samples. These analytes tend to have aromatic organic structures (for instance, phenols, aminophenols, catecholamines, and other metabolic amines), and, as a consequence, they are readily oxidized at inert electrodes (gold, platinum, and carbon, for example) by applying a constant applied potential due to stabilization of free-radical intermediates via electronic resonance, or p-resonance. This stabilization is derived from the molecule's extended conjugation, which significantly lowers the activation energy barrier for the electrochemical reaction. Hence, these compounds are considered to be self-stabilized. In contrast, self-stabilization of reaction intermediates is not available to polar aliphatic molecules (for example, carbohydrates and biogenic amines), and the rate of their oxidation is either very slow or nil even though oxidation of the molecule may be favored thermodynamically (3).

In dc amperometry, the electrode only functions to accept or donate electrons in the detection mechanism, and the working electrode ideally should be inert. Any electrochemical involvement of the electrode (for example, adsorption of reactants or products) is detrimental to the detection process and leads to electrode deactivation. Thus, glassy carbon electrodes are popular in dc amperometry because of their resistance to fouling, which often occurs over a period of hours or days. As a result, the electrode surface must be mechanically polished or some other method of electrode reactivation must be undertaken on a daily basis to maintain response sensitivity and reproducibility.

In "electrocatalytic" detection, the electrode does play an active role in the Faradaic detection process by exploiting the adsorption of reaction intermediates to the electrode surface, which lowers the activation energy barrier and facilitates oxidation of the analyte. This process often leads to a dramatic increase in the rate of electrode deactivation, or fouling, which can lead to complete passivation of the "working" electrode within seconds. Carbohydrates have been detected under alkaline conditions using dc amperometry at some transition metal (for example, Cu, Ag, Co, and Ni) electrodes with little or no evidence of short-term fouling. A seminal paper by Fleischmann and colleagues (4) in 1972 discussed the mechanism of oxidation of carbohydrates and amines at oxide-covered transition metal electrodes. These electrodes have found limited commercial use due to their long-term instability and lack of reproducibility. Noble metal (such as Au and Pt) electrodes also exhibit high activity toward polar, aliphatic compounds, but signals are short-lived because of rapid fouling of the electrode surface. To perform analytical relevant detections following a separation, noble metal electrodes must be continuously reactivated, or "cleaned," on-line; and of all the possible cleaning pretreatments, only electrochemical reactivation can be performed effectively.

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