THE S INDEX IN THE RETENTION EQUATION IN REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

The empirical retention equation log k' = log k(w)' - S-phi in reversed-phase high-performance liquid chromatography (RP-HPLC) was investigated to evaluate the properties of the parameter S. The S index, which is defined as the slope of log k' versus volume fraction of the organic modifier (phi) was systematically examined as a function of bonded phase density, column type and temperature in RP-HPLC. The S index for a particular solute was observed to be nearly constant even when column systems with different C18 packing materials are used. The dependence of log k' on eluent composition was found to be represented by parallel lines for a given solute for a variety of different stationary phases. The S index remains constant for a given solute despite the prolonged use of C18 column. The results showed that the S index is determined mainly by the interaction between the solute and the mobile phase. It was observed to decrease with the increasing column temperature for non-ionic solutes. Other factors influencing the measurement of the S index are discussed.