Excess Adsorption of Organic Eluent Components from Mobile Phases Containing Electrolytes

The excess adsorption isotherms of organic eluent components from solutions containing electrolytes on a C18-bonded stationary phase are investigated by frontal analysis in staircase mode. The excess adsorption of acetonitrile increases when NaHSO₄, NaH₂PO₄, NaCl, or NaOAc is added to the eluent, but decreases upon addition of NaBr or NaClO₄. The excess adsorption of acetonitrile increases in the order of NaCl, NaHSO₄, NaH₂PO₄?>?NaOAc?>?NaBr, NaClO₄. On the other hand, the effect of electrolyte addition on the excess adsorption of methanol is not significant. The effect of electrolytes on the retention of alkylbenzenes in reversed-phase liquid chromatography is discussed on the basis of the excess adsorption of organic eluent components. The retention of alkylbenzenes shows negative correlation with the excess adsorption of acetonitrile. This indicates that the acetonitrile layer on the stationary phase does not act as a part of the stationary phase. A developed acetonitrile layer reduces the retention of alkylbenzenes by the competitive adsorption at the interface between the organic layer and the stationary phase.