Determination of metal ions by on-line complexation and ion-pair chromatography

Abstract

An ion-pair chromatographic method utilizing on-line complexation and ion-pair formation in a post-column reactor was developed for the determination of copper, palladium, cobalt and iron in mixtures. The system features a reversed-phase column and a second eluent line feeding the ligand reagent, connected after the column via a T-piece, to a mixer and through that to a knitted tubing reactor. The ion-pair former was added to the eluent before the column and the ligand after it. The separation was studied using a binary eluent system containing cetyltrimethylammonium bromide (CTMABr) or tetradecyltrimethylammonium bromide (TDTMABr) in water-methanol (99:1, v/v) as ion-pair former and methanol. In addition, water-methanol (99:1, v/v) containing 1-nitroso-2-naphthol-6-sulphonate (126NNS) as ligand was added to the eluent, through the T-piece, after the column. Mixing of the two eluents took place in the mixer. Methanol was used both isocratically and in gradient addition. Selective UV-VIS detection of the metal-126NNS ion pairs was at wavelengths 230, 260, 310 and 400 nm and their identification was effected in wavelength range 190–600 nm. The metal complex formation in the aqueous methanol eluent evidently governed the retention of the ion pairs, while the selectivity of the method was provided by the different rates of reaction of the metal, the ligand and the ion-pair former in the mixer-reactor system. The detector response for copper, palladium, cobalt and iron was linear up to concentrations of 10 μ M. In spiked water-methanol samples the detection limits for these metals ranged from 1·10 −3 to 1 mg/l. When the on-line complexation and ion-pair formation method was tested with nickel, mercury and zinc, the results proved that these ion pairs were unstable. Because of the insufficient reproducibility of the absorption intensities of these metal ion pairs, their qualitative study could be performed only in the pH range 7–8. The method was successfully applied to real samples after removal of the organic material.