Principle Of Pulsed Amperometric Detection

Amperometric detection is based on the measurement of a change in current due to oxidation, reduction, or complex formation of an analyte at the surface of an electrode (Figure 1).

In DC amperometry the electrode is maintained at a constant potential during the determination. This technique has been applied to the determination of a number of compounds and ions e.g., catecholamines, thiols, phenols, sulfite,

Complex Carbohydrates

Figure 1 Amperometric detection is contingent on a change in current due to oxidation, reduction or complex formation at the applied potential on specific electrode material.

Henshall iodide, cyanide (24). Carbohydrates are easily oxidized on gold or platinum electrodes at high pH and are therefore good candidates for electrochemical detection. However, DC amperometry is not useful for carbohydrates since the carbohydrate oxidation products foul the electrode causing a rapidly decreasing response on subsequent determinations (Figure 2). This problem is solved by pulsed ampero-metry which utilizes a triple pulse sequence rather than a constant potential on the working electrode (1-3). Figure 3 illustrates the basic principle of PAD. By using a repeating sequence of high positive (E2) and negative potentials (E3) following each measurement, a clean electrode surface is maintained which ensures consistency in response. The detector signal for a particular analyte is measured at a potential which is appropriate for the particular analyte (E1) by integrating the current for a fixed length of time (typically 200 ms) and storing the resulting charge in a sample-and-hold amplifier until the next measurement.

The potential settings for a particular analyte are best determined by using cyclic voltammetry or pulsed voltammetry (25). Cyclic voltammetry plots such as the one shown in Figure 4 for glucose on a gold electrode also throw light on the mechanism of the electrochemical processes in pulsed amperometry, and the reason for the high specificity of the technique. The solid line shows the variation in current as the potential is swept from -0.8V to +0.6V and back to 0.8V for glucose in 100 mM NaOH. The dashed line shows the current as a function of

Single Potential Amperometry

Figure 2 Decrease in detector response for repeat injections of a carbohydrate and an amino acid (concentration 10 |ig/L in both cases) using DC amp-erometric detection. Peaks: (1) leucine. (2) lactose. Column: CarboPac PA1 (10 * M, 250 X 4 mm i.d.) with guard, Eluent: gradient, 15 to 150 mM NaOH in 5 min. Detection: DC amperometry, Au electrode, range 1*A, applied potential 0.15V vs. Ag/AgCL.

Figure 2 Decrease in detector response for repeat injections of a carbohydrate and an amino acid (concentration 10 |ig/L in both cases) using DC amp-erometric detection. Peaks: (1) leucine. (2) lactose. Column: CarboPac PA1 (10 * M, 250 X 4 mm i.d.) with guard, Eluent: gradient, 15 to 150 mM NaOH in 5 min. Detection: DC amperometry, Au electrode, range 1*A, applied potential 0.15V vs. Ag/AgCL.

HPAE-PAD

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