APPROACHES TO THE DETERMINATION OF PHENOLS AND OTHER UNSATURATED COMPOUNDS
Abstract
Electrochemical detection for liquid chromatography has been successfully applied to easily oxidized compounds such as catecholamines, hydroquinone, activated phenols, aromatic amines, ascorbic acid and uric acid. In addition to possessing low oxidation potentials, these compounds can be separated by ion-exchange or reverse phase ion-pair chromatography which are compatible with electrochemical detection. The goal of this work was to extend the applicability of electrochemical detection to compounds which do not possess the ideal characteristics of the above compounds. The first compounds studied were hydrophobic phenolic antioxidants. Voltammetry experiments showed antioxidants may be divided into two groups: those easily oxidized (< + 0.8V vs. Ag/AgCl) such as butylated-hydroxy anisole and t-butylhydroquinone, and those more difficult to oxidize (> + 1.0V) such as the esters of p-hydroxybenzoic acid. The first group was found to be easily determined in foods, oils, and related materials. Typical detection limits were below 1 x 10('-11) moles for these compounds at a signal to noise ratio of 5. The p-hydroxybenzoates had somewhat higher detection limits. Increased noise, electrode filming, and decreased electrode life were encountered. Nevertheless, the method was well suited for benzoate determinations in various cosmetics. The determination of environmentally important phenols was also studied. Reverse phase separation with electrochemical detection was found to be useful for total phenols, cresols, and xylenols. Carbamates derived from phenols could be determined indirectly by hydrolysis in base and determination of the free phenol. In the determination of pentachlorophenol and its metabolite tetrachlorohydroquinone in urine, good precision was obtained at part-per-million and part-per-billion concentrations, respectively. A "reaction detector" based on amperometric detection was developed to circumvent problems encountered in direct electrochemical detection. A suitable reagent is continuously electrogenerated for reaction with eluting bands. Residual reagent is monitored by a thin-layer amperometric detector. Reagent can be generated in a separate stream or, in some cases, directly in the chromatographic eluant. The latter approach was used to generate bromine in a reaction detector for unsaturated compounds. Many phenols, vitamins, fatty acids, and prostaglandins were found to be sufficiently reactive with bromine to be detected using this unique approach. Response was dependent on reagent concentration, analyte concentration, reaction time, flow rates, and detector potential. Several factors influencing sensitivity, reagent stability, and precision were explored. The bromine reaction detector could readily detect 10('-10) mole of C-18 unsaturated fatty acids separated by reverse-phase chromatography. Only doubly and triply unsaturated prostaglandins were detectable. PGE(,2), PGF(,2(alpha)), PGA(,2) and PGB(,2) could be detected in seminal fluid. The bromine reaction detector eliminated electrode filming and gave increased response for phenols compared to direct amperometric detection.
Degree
Ph.D.
Subject Area
Analytical chemistry
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