Reversed-phase ion-interaction chromatography of inorganic anions with tetraalkylammonium ions and divalent organic anions using indirect photometric detection

Abstract

An ion chromatographic method for the determination of anions using an octadecyl-bonded silica column with an ion-interaction reagent (IIR) and an ultraviolet (UV) absorbing eluent ion is described. Divalent naphthalenesulphonate and naphthalenecarboxylate eluent ions were examined in the presence of quaternary ammonium ions for use in the determination of inorganic anions by ion-interaction chromatography with indirect photometric detection. An eluent containing naphthalene-1,5-disulphonate (1,5-NDS) and tetrabutylammonium (TBA) ions was found to be the best in terms of sensitivity and versatility. Under conditions where the concentration of IIR on the stationary phase is relatively high, for example, the eluent contains 2 × 10^–4 mol dm^–3 TBA and 5% v/v methanol, the retention behaviour of hydrophilic anions was similar to that in ion chromatography with an anion-exchange column, whereas at low concentrations of IIR the distribution effect of the ion pairs contributed to the retention of anions. By using a short column (100 × 4.6 mm i.d.), the method can be applied to the analysis of water samples such as river, tap, sea- and rain water. The detection limits, corresponding to a signal-to-noise ratio of 3, were of the order of several ppb for inorganic anions commonly present in water samples. The use of 1,5-NDS as the eluent ion has several advantages over other eluents: (i) UV absorption is at longer wavelengths (near 300 nm) and is very strong; (ii) the eluent strength is not influenced by a change in the pH; (iii) the eluent ion does not form metal complexes; (iv) the eluent ion is readily soluble in a methanol–water mixture in the presence of the IIR; and (v) the reagent is commercially available in the acidic form and is relatively inexpensive.