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

Central composite designs (CCDs) were used in the study of the ion-pair chr omatographic separation of arsenic and selenium species in tap water: monom ethylarsonate, dimethylarsinate, selenomethionine and selenite. The ternary eluent consisted of tetrabutylammonium phosphate (TBA), sodium hydrogenpho sphate (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 fact ors (k') of arsenic and selenium compounds. Furthermore, another mathematic al model that permitted the variation of the chromatographic selectivity of species, computed from their retention data to be followed, was investigat ed. This showed the ability to locate the optimum conditions within the exp erimental design, so that arsenic and selenium species could be simultaneou sly quantified with good efficiency and resolution. A comparison between th e predicted and the experimental response values was made in order to asses s the prediction quality of the model. Response surfaces and isoresponse cu rves obtained from the mathematical models allowed the determination of the optimum chromatographic conditions and the robustness of the method. The p redicted optimum zone allowing satisfactory determination of both arsenic a nd selenium compounds was pH 5.5-6.5, 2.5 mmol l(-1) Na2HPO4 and 3.0-4.0 mm ol l(-1) TBA.