Mixed-mode liquid chromatography of aliphatic anionic surfactants with a naphthalenetrisulfonate mobile phase

Summary Separation of three classes of anionic surfactants (alkyl sulfates, alkanesulfonates, and alkyl phosphates) are achieved on a mixed-mode reversed phase (RP) phenyl/-anion exchange column using a naphthalenetrisulfonate (NTS)-acetonitrile (ACN) mobile phase via indirect photometric, indirect fluorometric, direct or indirect conductivity detection. Mixtures of C₅–C₁₈ sulfates, C₆–C₁₈ sulfonates, and C₁–C₄ phosphates (mono- and di-ester) can be separated in less than 20, 24, and 20 min respectively. Although hydrophobic effects are more pronounced in mixed-mode chromatography, equivalent-per-equivalent exchange of analysis and eluent ion is still required for sensitive indirect photometric, fluorometric, or conductivity detection. The detection limits of alkyl sulfates and alkanesulfonates are in the range of 3–15 pmoles which are at least an order of magnitude better than suppressed conductivity detection. The determination of surfactants in a variety of real samples is also presented.