A study of a permanently coated polymeric column for simultaneous separation of inorganic anions and mono-carboxylic acids

A polymeric PRP-1 reversed-phase column was modified by coating it with hexadecyltrimethylammonium bromide and has been used as an analytical column for the separation of inorganic anions and mono-carboxylic acids. An aqueous solution of sodium salicylate was used as the mobile phase and the detection was performed by indirect UV detection. The exchange capacity of the permanently coated column was controlled by changing the concentration of the organic modifier, acetonitrile, in the coating solution. The coated column with a low ion-exchange capacity has a dual retention mechanism of ion-exchange and surface adsorption for carboxylic acids. The contribution of the two mechanisms in the separation was qualitatively evaluated under three chromatographic conditions, by means of equations for ion-exchange chromatography, an adsorption model of the electrical double layer and the separation factor. The extra-column factors that influence the retention of the surface adsorption of the carboxylic acids were discussed. Among the factors studied, the pH of the mobile phase plays a double role, i.e. adjusting the eluting strength and governing the contribution of each mechanism to the separation. Under the chromatographic conditions selected, simultaneous separation of inorganic anions and mono-carboxylic acids was achieved.