Mixed-Mode Liquid Chromatography of Aliphatic Anionic Surfactants with a Naphthalenedisulfonate Mobile Phase

Abstract
Separations of long chain aliphatic anionic surfactants (alkyl SO4- and alkane SO3-) are compared on three mixed-mode stationary phases (RP C8/-, RP C4/- and RP Phenyl/-anion exchange) using a naphthalenedisulfonate—acetonitrile mobile phase. The optimization study indicates that the RP Phenyl/-anion exchange column using 0.2mM NDS with about 90% ACN is the best combination for the separation of the C6—C18SO4- or C6—C18SO3- mixtures in about 30 min. Although hydrophobic effects are more pronounced in mixed-mode chromatography, equivalent-per-equivalent exchange of analyte and eluent ion is still required for sensitive indirect photometric or conductivity detection. A comparison of detection limits for the determination of these surfactants after mixed-mode separation shows photometric and conductivity detectors are better (picomole or nanogram range) than refractive index or fluorometry detectors by about a factor of 1000.

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