Governing the separation selectivity of carbon acids on a novel poly(styrene-divinylbenzene)-based stationary phase in mixed-mode HPLC

Abstract

The ion-exchange features of a new multifunctional polymer column in HILIC mode were examined by varying the mobile phase pH and counter ion concentration. A significant decrease in the column anion-exchange capacity was demonstrated in the range of  2.85−5.76 which was attributed to the presence of secondary and tertiary amino groups in the functional layer. The k’-pH trends in the corresponding range were built for different organic acids ( 1−5) additionally the relative contributions of the ion-exchange to the overall retention were determined by eluting ion concentration altering. A decrease in the retention of anions and acids with  <4 and an increase for acids with >4 was shown with rising pH. The ion-exchange impact in the retention of the latter ones significantly decreased by concentration rise at   2.85, while the former being predominantly retained by the ion-exchange mechanism in the entire pH range. For such a high-capacity anion exchangers, the type of the k’−pH dependence was shown to be mostly determined by the analyte state, which in its turn depends on the  and  values. Mobile phase of рН  pK_a was found to be the most appropriate for separation of acids, providing retention by different mechanisms and allows practitioners to master electrostatic interactions by varying the concentration of the eluting ion. A mixture of 5 acids was entirely separated in five minutes with an efficiency of up to 32 000 plates/m in mixed-mode HPLC. A beneficial option of ionizable compounds separation on a novel polymer multifunctional column with no ion chromatography equipment being engaged has been shown in this work.