Retention Behavior of Inorganic Anions in Hydrophilic Interaction Chromatography

The retention behavior of inorganic anions was studied in hydrophilic interaction chromatography (HILIC). In this study, five kinds of HILIC stationary phases (amino, imidazole, amide, pyridine and zwitterionic) were investigated. It was found that only amino and imidazole columns exhibited the separation of inorganic anions under HILIC conditions. The retention mechanism was further investigated under both columns. A reversed elution order of inorganic anions was observed under the HILIC condition compared with those observed under the ion-exchange chromatography mode (IEC). The effect of salt species and their concentration in the eluent were investigated under constant acetonitrile (ACN) content. Sodium chloride and sodium perchlorate were chosen as the salt, and the salt (sodium perchlorate) concentration was varied from 10 to 40 mM to confirm the effect of the electrostatic interaction. The slope values of the plots of the log retention factor (k) versus the log eluent concentration were calculated to be between -0.43 and -0.45 for the amino column, while those obtained on the imidazole column were between -0.68 and -0.73. Various concentrations of ACN were also examined with 20 mM sodium perchlorate, and the typical HILIC retention behavior was observed on both amino and imidazole columns. Due to the obtained results, it is considered that the separation of inorganic anions under the HILIC condition was achieved by both electrostatic interaction and partition.