Mixed-mode Liquid Chromatography of Aliphatic Anionic Surfactants with a Naphthalenetrisulfonate Mobile Phase S. A. Shamsi / N. D. Danielson* Department of Chemistry, Miami University, Oxford, OH 45056, USA Key Words Column liquid chromatography Mixed mode stationary phases Indirect photometric and fluorometric detection Conductimetric detection Alkyl-sulphates, and sulphonat-phosphates 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 naphthalene- trisulfonate (NTS)-acetonitrile (ACN) mobile phase via indirect photometric, indirect fluorometric, direct or indirect conductivity detection. Mixtures of C6-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-equ!valent exchange of analyte 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. Introduction Long chain aliphatic anionic surfactants such as C6- C18SO4 and C6-C18SO3 are widely used in the deter- gent industry. The traditional method for the determi- nation of anionic surfactants is based on a color forming reaction with methylene blue [1, 2]. The method is nonspecific and is incapable of differentiat- ing among individual compounds within the homolo- gous series of anionic surfactants. Although prederiva- tization chemistry is required, gas chromatography (GC) [3] or GC-MS [4, 5] can analyze surfactant mixtures. Capillary electrophoresis (CE) [6, 7] has been successfully used to discriminate between homologues and isomers. However, separation of complex mixtures of long chain RSO3 and RSO4 (i.e.> C14 chain length) by CE is difficult to achieve mainly because of the very low mobility and poor solubility of these anionic compounds in aqueous CE electrolytes. Liquid chromatography (LC) has been the most widely studied method for surfactants since these charged compounds differ in terms of hydrophobicity. Al- though LC-MS has been used [8], the attendant complexity of the instrumentation may offset the utility of the approach somewhat. High performance liquid chromatography (HPLC) of anionic surfactants with no chromophore with direct spectroscopic detection must involve either a precolumn [9] or a postcolumn deriva- tization reaction [10]. Ion-interaction chromatography using a reversed phase column and indirect photomet- ric detection (IPD) is commonly used for the separa- tion and detection of anionic surfactants. The method- ology utilizes a light absorbing reagent that acts as both an/on-pairing reagent as well as a source of indirect detection [11-16]. A major shortcoming of the above technique is that it requires careful optimization of the ion-pairing reagent concentration as it has a major influence on both ion-exchange column capacity and detectability. Ion-exchange with suppressed conductivity detection [17] can involve a post-column reaction between an aqueous suspension of cation exchange material in the hydrogen form with the tetrabutylammonium borate eluent. Alkylammonium cations exchange with hydro- gen ions while borate is converted to boric acid. Although this reduces the detector background for direct conductivity detection of surfactants, the cation exchange material must be recycled. Reversed phase HPLC with suppressed conductivity detection of short chain [18-19] and more recently long chain surfactants [20] has been reported with sub nmole detection limits. Although the separation of long chain C6-C18SO ~ and C6-C18SO ~ was accomplished in 20 min, the procedure Chromatographia Vol. 40, No. 5/6, March 1995 Original ~3009-5893/95/03 0237-10 $ 3.00/0 9 1995 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH 237