Determination of fatty alcohol ethoxylates by derivatisation with maleic anhydride followed by liquid chromatography with UV–vis detection

Abstract

The esterification of fatty alcohol ethoxylates (FAEs) by melting with maleic anhydride has been studied in the presence of urea. The reaction rate and yield of non-ethoxylated and ethoxylated alcohols increase largely when grinded urea is suspended in the reaction medium, a 100% yield being achieved at 80 °C in 15 min. The procedure is tolerant to the presence of large amounts of water. The UV–vis response factors of the derivatives of individual FAE oligomers vary little with the number of carbon atoms in the alkyl chain, n, and the number of ethylene oxide units, m. Derivatised samples of industrial FAE mixtures, cleaning products and river and sea waters were chromatographed on a C8 column of the fused-core type using gradient elution with acetonitrile/water mixtures plus 0.1% acetic acid. The hydrocarbon series with even values of n from 8 up to 20, were well resolved using column temperatures in the 15–30 °C range. Superior resolution between the consecutive oligomers within the series was also obtained at large values of m; however, reversion of the elution order for the derivatives with m = 1 and 0 hindered direct quantitation of a few oligomers at low m values. Full resolution of all the oligomers was achieved by deconvolution with a genetic algorithm assisted by the unconstrained local method of Powell. After preconcentration with C18 solid-phase extraction cartridges, both river and sea water samples also showed the signature of the FAE oligomers. The evaluation of total FAEs in these samples was demonstrated.