ANION-EXCHANGE CHROMATOGRAPHY ON SELECTED SILICA-BASED REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC COLUMNS

An unexpected retention of nitrate was observed on a Zorbax Rx C-8 high-performance liquid chromatographic column suggesting an anion-exchange retention mechanism. The ability of the column to be an anion exchanger was investigated using aqueous phosphoric and acetic acid mobile phases. For comparison Nucleosil C-8 and Zorbax C-8 columns were also examined. Significant retention and separation of anions was achieved on the Zorbax Rx C-8, to some extent on the Nucleosil C-8, but not on the Zorbax C-8. Taking advantage of the unusual anion-exchange property, the Zorbax Rx C-8 column was used to simultaneously chromatograph the amine plus counter-anion components of some research pharmaceutical salts by a mixed-mode reversed-phase anion-exchange mechanism.