SEPARATION OF SULFONATE AND SULFATE SURFACTANTS BY CAPILLARY ELECTROPHORESIS - EFFECT OF BUFFER CATION

The separation of alkanesulfonate (RSO3-) and alkyl sulfate (ROSO3-) surfactants by capillary electrophoresis (CE) is dependent on the buffer cation and its concentration. Divalent cations increased migration time and resolution to a greater extent than that of monovalent cations. As the cation additive concentration increases, anion surfactant migration time increases. The effect is due to a change in electroosmotic flow which occurs because of cation exchange between the cation additive and the weakly acidic silanol surface of the silica capillary wall. Resolution is markedly improved by using Mg2+ as the buffer additive. Examples of the CE separation of complex mixtures of RSO3-, ROSO3-, sulfosuccinate esters, benzenesulfonate derivatives, and linear alkylbenzenesulfonates (LAS) illustrate the scope of Mg2+ as a buffer additive. Nonaromatic anionic surfactants were detected by indirect detection at 230 nm using salicylate anion as a chromophoric buffer additive while aromatic anionic surfactants were detected by direct UV detection at 220 nm. Detection limits for direct detection were better because of lower background noise. For a C10SO3- standard and indirect detection, the detection limit was 0.6 pmol, while for a 2,4-dimethylbenzenesulfonate standard and direct detection, the limit was 0.05 pmol for a 3:1 signal-to-noise ratio and electrokinetic injection.