Fxb

Analyte

Na+OH" Eluent

Cation Exchange Membrane

Analyte & H20 To Detector

Cathode ©

Cation Exchange Membrane

FIGURE 4.12 Anion self regenerating suppressor (ASRS) for detection in ion chromatography (by courtesy of Dionex).

where n is the number of electrons, F the Faraday constant, u the linear velocity of the mobile phase, C is the analyte concentration, D the diffusion coefficient of the solute, A the electrode surface, and ^ the geometrical constant of the cell.

The quality of the sample clean up procedure often determines the detection limits. The instability of the reference electrode is the source of voltage noise.

Parallel dual electrode may be used for a number of reasons:

- With one electrode at a positive and one electrode at a negative potential, oxidizable and reducible compounds can be detected in one single chromatographic run.

- When two solutes with different redox potentials coelute from the column, the potential of one electrode can be selected such that only the most easily oxidized (or reduced) compound is detected while on other electrode both compounds are converted. The concentration of the second compound is evaluated by substraction of the signal.

Series dual electrodes are set up such that one electrode is in oxidative and the other is in reductive mode. The downstream electrode measures the products of the upstream electrode. The second electrode only responds to compounds which are converted reversibly. The redox product is more selectively detected.

Voltammetric analysis is performed by scanning the potential or by applying triangular potential wave form to the electrode. Coeluting peaks are distinguished if their voltagrams are significantly different.

Coulometry permits determination of chemical substances by measuring the quantity of electricity required for their conversion to a different oxidation state. The quantity of electricity or charge is measured in Coulombs. (The coulomb is the quantity of charge that is transported in 1 sec inlet

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