Task-specific ionic liquid based in situ dispersive liquid–liquid microextraction for the sequential extraction and determination of chromium species: optimization by experimental design

Abstract

Task specific ionic liquid-based in situ dispersive liquid–liquid microextraction (in situ TSIL-DLLME) combined with flame atomic absorption spectrometry (FAAS) for the determination of Cr(III) and Cr(VI) species in aqueous samples was developed. The selective extraction of Cr species was carried out by the sequential complexation of Cr(III) and Cr(VI) with a 1-bromoethyl-3-methylimidazolium bromide ionic liquid functionalized with 8-hydroxyquinoline (8-HQ) and with ammonium pyrrolidine dithiocarbamate (APDC), respectively. Then, an ion exchange reagent of KPF₆ was added to the solution to perform an in situ metathesis reaction producing a water-immiscible TSIL. The factors affecting the extraction of the Cr species were optimized by using a multivariate response surface methodology (RSM) established by Box–Behnken design. The resulting quadratic equations were used to build response surface plots for the prediction of the responses of the Cr species and to find the optimal experimental conditions. Under the optimized experimental conditions, the calibration curves were linear over the concentration range 25–750 μg L⁻¹ for Cr(III) and 50–600 μg L⁻¹ for Cr(VI). Detection limits of 5.7 μg L⁻¹ and 11.3 μg L⁻¹ were obtained for Cr(III) and Cr(VI), respectively. The relative standard deviations for the six replicate determinations of 250 μg L⁻¹ of Cr(III) and Cr(VI) were 1.1% and 2.0%, respectively. Finally, the method was successfully applied to the extraction and determination of Cr species in water and urine samples.