Liquid chromatographic method for determination of water in soils and the optimization of anion separations by capillary zone electrophoresis

Date
1994
Authors
Benz, Nancy
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James S. Fritz
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Chemistry
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Chemistry
Abstract

A liquid chromatographic method for the determination of water in soil and clay samples is presented. Methanol is used to extract water from the sample. The water is then separated from matrix components on a cation-exchange column via an ion-exclusion mechanism and determined using indirect spectrophotometric detection. The detection scheme is based on the acid-catalyzed equilibrium established in the eluent when trans-cinnamaldehyde reacts with methanol to produce trans-cinnamaldehyde dimethyl acetal and water. Initially, the equilibrium lies toward the acetal which has a low absorbance at the detection wavelength. The equilibrium is shifted toward the cinnamaldehyde when a sample containing water is introduced into the system. An increase in absorbance proportional to the shift in equilibrium and the amount of water present in the sample is observed. Conditions are optimized for the fast and accurate determination of water in soil and clay samples;Capillary zone electrophoresis (CZE) is an effective method for separating ionic species according to differences in their electrophoretic mobilities. CZE is used to separate inorganic and short-chain organic acid anions under conditions of reversed electroosmotic flow (EOF). These separations are usually achieved using a relatively high concentration (e.g. >0.25 mM) of a quaternary ammonium salt added to the electrolyte solution to reverse the EOF. It is now shown that excellent separations of these anions are obtained using low concentrations of a quaternary ammonium salt (e.g. 0.03 mM) and 1-butanol (e.g. 4%) to reverse the EOF;CZE separations of anions with similar electrophoretic mobilities are often difficult. It is usually necessary to modify the composition of the electrolyte solution in order to improve the separation resolution. The effect of pH and organic solvent on EOF and electrophoretic mobility is studied. Separations of several alkyl-substituted phenolate ions are optimized by changing the pH and the percentage of acetonitrile in the electrolyte solution. By studying electrophoretic mobility as a function of pH and percentage acetonitrile, optimum separation conditions are determined.

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