Retention behavior of o-phthalic, 3-nitrophthalic, and 4-nitrophthalic acids in ion-suppression reversed-phase high performance liquid chromatography using acids instead of buffers as ion-suppressors
In reversed-phase high performance liquid chromatography, the logarithm of the retention factor, log k, is usually correlated with the logarithm of the octanol-water partition coefficient, log K ow . The k and K ow of an ionizable analyte are greatly influenced by the mobile phase pH. In this paper, log k w of diprotic o-phthalic, 3-nitrophthalic, and 4-nitrophthalic acids, are obtained by extrapolation to pure aqueous fraction of mobile phase in ion-suppression reversed-phase high performance liquid chromatography with acetic acid and perchloric acid as the ion-suppressors. The K ow values of the three analytes are calibrated according to the apparent octanol-water partition coefficient, K 99 ow , under different pH conditions, and the log K 99 ow values show a much better correlation with log k w than do log K ow . The influences of two ion-suppressors, acetic and perchloric acids, on the retention behavior of these diprotic acids at different pH are contrasted. An abnormal trend is found in the k vs. pH w plot of the acetic acid system when the methanol content is low. A possible reason is that acetic acid is an even stronger organic modifier than methanol, besides being an ion-suppressor. The results make the selection of mobile phase for the separation of acidic compounds by ion-suppression reversed-phase high performance liquid chromatography direct, accurate, and practical.