Multiple response optimization of sequential speciation of chromium in water samples by in situ solvent formation dispersive liquid–liquid microextraction prior to electrothermal atomic absorption spectrometry determination

Abstract

A sensitive and selective in situ solvent formation dispersive liquid–liquid microextraction (in situ DLLME) method followed by electrothermal atomic absorption spectrophotometry for sequential extraction and determination of trace level of chromium (Cr) species was developed. The preconcentration and extraction of Cr species were carried out using ammonium pyrrolidine dithiocarbamate as a chelating agent, and a water-immiscible ionic liquid based on in situ solvent formation as an extraction solvent. Various operating variables such as sample pH, the KPF₆ amount and the ionic liquid volume on the extraction efficiency responses (EE%) of Cr(III) and Cr(VI) ions were investigated. Response surface methodology using central composite design was employed for modeling of the responses. Multi-response optimization with desirability function (DF) approach has been used to maximize EE% one Cr species in the presence of another species. After this step, DF was employed to optimize the variables to define the best extraction conditions for Cr speciation. Under the optimal extraction conditions, the calibration curves were linear in the concentration ranges of 20–200 ng L⁻¹ for Cr(III) and 70–250 ng L⁻¹ for Cr(VI) with the limit of detections of 3.7 ng L⁻¹ for Cr(III) and 2.13 ng L⁻¹ for Cr(VI). The relative standard deviations of five microextractions of 100 ng L⁻¹ Cr(III) and Cr(VI) were 9.07 % and 2.39 %, respectively. Finally, the developed method was applied to the determination of Cr(III) and Cr(VI) ions in real water samples.