EFFECTS OF ORGANIC MODIFIER AND ION-PAIR REAGENT IN LIQUID-CHROMATOGRAPHY

The capacity factors of phenylamine and naphthylamine sulphonic acids in reversed-phase, ion-pair liquid chromatography (RP-IPC) were measur ed. The combined effects of organic modifier (C(b)) and ion-pair reage nt concentration (C(p)) on retention follow an equation based on the e lectrostatic model: In k'= a + b In C(p) + cC(b) the experimental valu e 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 d ue to the effective number of charges being less than the apparent num ber. The absolute values of a and c are much larger than those in RP-H PLC 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 obtai ned, strongly supporting the validity of the electrostatic retention m odel. A critical value, R, at which the negative effect of methanol on retention is equal to the positive effect of the ion-pair reagent (TB AI) 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, whic h means that the increase by exp times ( = 2.718 x) the ion-pair conce ntration the original is equivalent to a decrease of 0.04-0.065 in vol ume fraction of methanol