Determination of very low amounts of chromium(iii) and (vi) using dispersive liquid–liquid microextraction by in situ formation of an ionic liquid followed by electrothermal atomic absorption spectrometry

Abstract

A new procedure for the determination of very low levels of Cr(III) and Cr(VI) using ionic liquid dispersive liquid–liquid microextraction (IL-DLLME) followed by electrothermal atomic absorption spectrometry (ETAAS) is presented. The ionic liquid, 1-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₈MIm][NTf₂]), is formed in situ by mixing 1-octyl-3-methylimidazolium chloride ([C₈MIm]Cl) and lithium bis(trifluoromethanesulfonyl)imide ([NTf₂]Li) aqueous solutions, and is used to extract complexes originating from Cr(III) and Cr(VI) with ammonium pyrrolidinedithiocarbamate (APDC). No organic solvents are required, the dispersion being due to the very fine droplets of ([C₈MIm][NTf₂]) originated in the aqueous solution. After centrifugation, the concentration of chromium in the sedimented phase is determined by ETAAS. The total content of chromium is measured using 0.01 mol L⁻¹ APDC at pH = 1. In the presence of 0.001 mol L⁻¹ ethylenediaminetetraacetate, Cr(III) is not extracted, and the analytical signal corresponds to Cr(VI). The level of Cr(III) is obtained by difference. The enrichment factor of the procedure is 300 and a detection limit of 2 ng L⁻¹ is achieved. The relative standard deviation for 10 replicates at the 0.05 μg L⁻¹ level is close to 8%. The recoveries of spiked samples are in the 95–104% range. The method is applied to the analysis of water samples as well as lixiviates obtained from toys made of plastic materials.