Quantitative trace analysis of surfactant mixtures by reversed-phase high-performance liquid chromatography with refractometric detection

Reversed-phase partition liquid chromatography on an octyl column allowed the separation of complex non-ionic poly(ethylene oxide)-type (PEO) surfactant mixtures resulting from the condensation of ethylene oxide with saturated fatty alcohols. As these compounds have no chromophoric group, they were detected by differential refractometry. Accurate quantitation of each oligomer (C_mE_n) allowed the main characteristics of each non-ionic surfactant, i.e., the nature and percentage of the different alkyl chains (with m = number of carbons) and the average number of ethylene oxide units ( ) to be obtained in one analysis. Preparative liquid chromatography was used to isolate pure oligomers with a higher degree of ethoxylation (n =10, 11, 12, 14 and more) than the commercially available standards, in order to determine a wide range of refractometric response factors. It appeared that they are constant as a function of alkyl chain length (C₁₀-C₁₆ range) but that they vary significantly and non-linearly as a function of the degree of ethoxylation, n. It was found that neglecting the variation of response factors can result in a distortion of the average ethoxylation number and in an unsatisfactory quantitative analysis. This chromatographic method, involving a quantitative and reproducible trace enrichment procedure with liquid-solid extraction, allowed the analysis of very dilute PEO mixtures in water. The components of complex PEO mixtures in water were determined at concentrations as low as 0.5 mg 1⁻¹, without any distortion of the distribution, the detection limit being 0.25 μg 1⁻¹ for the less abundant oligomers.