Ion chromatographic separation of alkali metal and ammonium cations on a C18 reversed-phase column

Abstract The separation of alkali metal (Li + , Na + , K + , Rb + and Cs + ) and ammonium cations on a C 18 reversed-phase column using three anionic surfactants [sodium 1-eicosyl sulphate, sodium dodecyl benzenesulphonate and sodium dodecyl sulphate (SDS)] is described. Two methods were examined: (a) “permanent” coating, with the use of a C 18 reversed-phase column previously coated with the surfactants; and (b) dynamic coating, with addition of the surfactants to the mobile phase. With method (a) the separation of the six cations was achieved with SDS. However, the retention times gradually decreased owing to dissolution of the SDS coating. Good separation was obtained with method (b), where 10 m M HNO 3 containing 0.1 m M SDS was used as the mobile phase with conductivity detection, and it was applied satisfactorily to real samples. The effect of system peaks on determination is also discussed.