Application of central composite designs for optimisation of the chromatographic separation of monomethylarsonate and dimethylarsinate and of selenomethionine and selenite by ion-pair chromatography coupled with plasma mass spectrometric detection

Abstract

Central composite designs (CCDs) were used in the study of the ion-pair chromatographic separation of arsenic and selenium species in tap water: monomethylarsonate, dimethylarsinate, selenomethionine and selenite. The ternary eluent consisted of tetrabutylammonium phosphate (TBA), sodium hydrogenphosphate (Na₂HPO₄) and 1% acetonitrile. CCD allowed the investigation of the influence of the eluent parameters, which varied from 0.5 to 4.2 mmol l⁻¹ Na₂HPO₄, 0.5 to 4.2 mmol l⁻¹ TBA and pH 4.9 to 8.2, on the capacity factors (k′) of arsenic and selenium compounds. Furthermore, another mathematical model that permitted the variation of the chromatographic selectivity of species, computed from their retention data to be followed, was investigated. This showed the ability to locate the optimum conditions within the experimental design, so that arsenic and selenium species could be simultaneously quantified with good efficiency and resolution. A comparison between the predicted and the experimental response values was made in order to assess the prediction quality of the model. Response surfaces and isoresponse curves obtained from the mathematical models allowed the determination of the optimum chromatographic conditions and the robustness of the method. The predicted optimum zone allowing satisfactory determination of both arsenic and selenium compounds was pH 5.5–6.5, 2.5 mmol l⁻¹ Na₂HPO₄ and 3.0–4.0 mmol l⁻¹ TBA.