WATERS ASSOCIATES

ASSOCIATES. 137 Maple Street, Milford, Ma 01757. Telephone (61 7) 478- ... This degree of selectivity is rarely possible with a UV absorption detector...
1 downloads 0 Views 1MB Size
NEW required reading from Waters — the Liquid Chromatography People

Purification of Synthesis Reaction Mixtures Describes use of Waters' High-Perfor­ mance Liquid Chromatograph, in RECY­ CLE mode to prepare milligrams to grams of purified, individual posi­ tional isomers from a synthesis reaction mix­ ture. CIRCLE 236 ON READER SERVICE CARD

Know More About Your Polymer A 28 pg. primer on Gel Permeation Chromato­ graphy (GPC). This booklet explains how GPC rapidly determines molecular weight dis­ tributions of polymers, and how it can be applied to solving real problems. CIRCLE 237 ON READER SERVICE CARD

High-Performance Liquid C h r o m a t o g r a p h s Illustrated 8 page, brochure gives com­ plete description of Waters' High-Performance Liquid Chro­ matographs for all modes of liquid chro­ matography.

CIRCLE 238 ON READER SERVICE CARD

free

from

WATERS ASSOCIATES 137 M a p l e Street, Milford, Ma 0 1 7 5 7 Telephone (61 7) 4 7 8 - 2 0 0 0 The Liquid Chromatography

People

tered in trace analysis work, it is pos­ sible to operate an LCEC cell in the classical two-electrode mode. When three electrodes are used, the auxiliary electrode can be placed downstream as illustrated in Figure 2. This ar­ rangement, while convenient, does re­ sult in nonlinear behavior when large samples are injected (typ. >200 ng for compounds with small k') unless the ionic strength is very high or the gas­ ket very thick. Positioning the auxilia­ ry electrode opposite to the working electrode (i.e., across the flow stream) reduces the uncompensated resistance to a negligible value even when low ionic strength (6, indoleacetic acid, 5-hydroxytryptamine (26), tetrahydroisoquinoline alkaloids (27, 28), and the phenothiazines (11). Fig­ ure 4 illustrates the electrochemical detection of several aromatic amines and phenols. Once again, note the great selectivity which can be achieved by control of the electrode potential. Electronic Games Although constant-potential amperometry is the only LCEC approach which has been widely used in prac­ tice, it is possible to carry out more so­ phisticated experiments. Frequently, we are queried about the possibility of programming the potential during a chromatographic run using a triangu­ lar wave in the manner of cyclic voltammetry. There are several reasons why this idea is of no practical value. Steady-state hydrodynamic electroanalysis has the advantage of being relatively free of double-layer charg­ ing effects, and there is no point in