Effects of organic modifier and ion-pair reagent in liquid chromatography

Abstract

The capacity factors of phenylamine and naphthylamine sulphonic acids in reversed-phase, ion-pair liquid chromatography (RP-IPC) were measured. The combined effects of organic modifier (Cb) and ion-pair reagent concentration (Cp) on retention follow an equation based on the electrostatic model: $$\ln k' = a + b lnC_p + cC_b $$ the experimental value of b correlates well with the value of 0.5 z (z=charge on analyte ion) predicted for mono and divalent ions. The measured value of b is, however, lower than that predicted for trivalent ions, which may be due to the effective number of charges being less than the apparent number. The absolute values of a and c are much larger than those in RP-HPLC in the absence of an ion-pair reagent, quantitative correlation of a and c with retention values in RP-HPLC and solute charges has been observed and a good linear relationship between a and c has been obtained, strongly supporting the validity of the electrostatic retention model. A critical value, R, at which the negative effect of methanol on retention is equal to the positive effect of the ion-pair reagent (TBAI) has been proposed. The critical values obtained are related to the behaviour of the solute, the ion-pair reagent and the stationary and mobile phases, and lies between −0.04 and −0.065 in most cases, which means that the increase by exp times (=2.718×) the ion-pair concentration the original is equivalent to a decrease of 0.04–0.065 in volume fraction of methanol