Electrosorptive detection in liquid-chromatography

Tore Ramstad, Purdue University

Abstract

We have investigated electrosorption (ES), or specific adsorption, as a basis for detection in liquid chromatography, and applied it (1) to the determination of selected inorganic anions in ion chromatography, and (2) to several simple classes of neutral, (sparingly) water-soluble organic compounds. We are able to detect inorganic ions at both mercury and solid (silver) surfaces, and the organics at mercury. For the determination of ions at mercury, differential double-layer capacitance (DDLC) measurements have proven most useful. At silver either differential DLC or integral CLS detection may be used to advantage. Detection limits realized at both mercury and silver are competitive with those usually quoted for conductometry. Calibration curves for mercury are linear over roughly two orders of magnitude; the corresponding curves for silver show slight curvature. An indirect scheme for ions, one which relies on a kinetic facilitation of the rate of electroreduction of a sluggish outersphere reactant (o.s.r.), was also shown to be workable. Application of LC-ES detection to organic substances was most successful for aqueous systems. Utilizing primarily DLC depression at mercury, we applied ES detection to alcohols, diols, mono- and dicarboxylic acids, amines, and alkanolamines; all were detected as the neutral (or associated) species near the potential of zero charge (pzc). Calibration curves for organic systems are generally sigmoidal. Amines, alkanolamines, and alkylsulfonates were further shown to be detectable as ions at potentials far removed from the pzc. In addition to aqueous ES detection, we examined the suitability of mixed aqueous-organic eluents for the determination of acids by ion suppression and of thiols and poly(ethylene glycol)s by reversed phase. While we made most use of capacitance depression for the determination of organics, we also investigated pulse/amperometric detection applied to the tensammetric (adsorption/desorption) peak. Another detection scheme investigated was based on changes in the drop time of mercury. Most studies were conducted using a large-volume wall-jet electrode/cell of our design. However, we also constructed a thin-layer cell, also of our design, incorporating a rapidly dropping mercury electrode. Additionally, we constructed a simple, inexpensive static mercury drop electrode for use in LC.

Degree

Ph.D.

Advisors

Weaver, Purdue University.

Subject Area

Analytical chemistry

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