Simultaneous quantitative trace analysis of anionic and nonionic surfactant mixtures by reversed-phase liquid chromatography

The aim of this work was to simultaneously analyse mixtures of a polydisper se polyethylene oxide (PEO) nonionic surfactant and an anionic surfactant ( sodium dodecylsulphate, SDS) in water containing sodium chloride in order t o quantify trace amounts of these mixtures after their adsorption at water- solid interfaces. A fractional factorial design was then used to optimise t he separation by ion-pair reversed-phase liquid chromatography as a functio n of six factors: the chain length of the tetraalkylammonium salt used as i on-pairing reagent which varied from methyl (C-1) to n-propyl (C-3); the co ncentration of this ion-pairing salt; the acetonitrile percentage in water used as organic modifier; the flow-rate; the temperature of analysis and al so the sodium chloride concentration. The factorial design enabled in a lim ited number of analyses, not only to determine which factors had significan t effects on retention times or on resolution between a pair of nonionic ol igomers, but also to modelize and then find the interesting and rugged area where this resolution was optimal as well as the conditions where time of analysis was not prohibitive, After optimisation of HPLC analysis, we used a trace enrichment procedure to quantify very low concentrations of SDS and C12E9 polydisperse PEO in water. A C-18 cartridge and a strong anionic exc hange cartridge were coupled and the conditions of elution were optimised i n order to obtain concentrated samples which were injected in the same elue nt than the HPLC mobile phase. Under such conditions, we were able to quant ify, in a single run, mixtures of anionic and nonionic surfactants at conce ntrations as low as 3.6 mu g l(-1) for SDS and 2.5 mu g l(-1) for each PEO oligomer in water. (C) 2000 Elsevier Science B.V. All rights reserved.