pon^gg HANDBOOK a n d GENERAL CATALOG Contents Include: * N e w Products * Clinical Reagents * A m i n o Acid Analysis * Sequence D e t e r m i n a t i o n * Protein Modification * Peptide Synthesis * Laboratory Aids * Liquid Chromatography * Gas Chromatography Accessories * T h i n Layer Chromatography * Derivatization Procedures * Corning Biomaterial Supports For your PIERCE EUROCHEMIE B.V.
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ANALYTICAL CHEMISTRY, VOL. 4 9 , NO. 4, APRIL
1977
tivity of electrochemical detection is inversely related to the experimentally observed oxidation (or reduction) po tentials. Measured half-wave poten tials are often significantly greater than the corresponding thermody namic E ° ' values (particularly for or ganic molecules). Hydrodynamic voltammograms represent a rapidly achieved mass transport steady state. T h e magnitude of the current is di rectly proportional to the bulk con centration at every point along the voltammetric curve. Although it is often desirable to op erate an electrochemical detector on the limiting current plateau (E2 for compound A), in some cases it is ad vantageous to operate on the rising part of the curve (Ei for compound A). T h e lower the potential chosen, the better the selectivity. If, for example, A and Β are not well resolved by the column, it may be difficult to detect A at E2 in the presence of a thousand fold excess of B. Lowering the poten tial to Ei will decrease the sensitivity to A severalfold, but the sensitivity to Β may be decreased by many orders of magnitude. This principle has been used to detect trace amounts of p aminophenol (A) in the popular anal gesic drug iV-acetyl-p-aminophenol (acetaminophen) (B) (7). At E 3 , A and Β will be detected with comparable sensitivity, and at E 4 it would also be possible to detect C. Clearly, the selec tivity for C will not be good, and a greater reliance will have to be placed on the chromatography. T h e sensitivi ty for C can also be expected to be somewhat diminished when compared to more easily oxidized substances due to the greater background current (greater "baseline" in the chromatogram). At E5 and beyond it is likely t h a t the electrochemical detector will not be useful for trace analysis as the background oxidation (D) becomes ex orbitant. There are special experimental problems associated with the LCEC detection of trace amounts of reduc ible substances (E) because reduction of dissolved oxygen, trace metal ions, and hydrogen ions can be difficult to eliminate. Although these problems can certainly be overcome, most prac tical applications which have been published involve oxidizable compo nents. This trend may eventually be mitigated somewhat by recent im provements in electrode materials. Once the operating potential is cho sen, LCEC chromatograms are ob tained by plotting current as a func tion of time. In view of the fact t h a t excellent current-to-voltage convert ers can be constructed from rugged in tegrated circuit operational amplifiers with M O S / F E T inputs, it is not sur prising t h a t electrochemical detection
