Green in situ ionic liquid-based microextraction for selective determination and operational speciation of Cr(iii) and Cr(vi) in environmental and biological samples: a sustainable analytical approach

Abstract

A pyridinium based-task-specific ionic liquid was successfully synthesized and characterized. Subsequently, an in situ solvent formation microextraction technique using the synthesized task-specific ionic liquid, coupled with atomic absorption spectrometry, was developed for the selective determination and operational speciation of Cr(III) and Cr(VI) species in various water and urine samples. The microextraction procedure for chromium ions was performed through the complexation of chromium ions with the ionic liquid 1-(2-bromoethyl)pyridinium bromide functionalized with 8-hydroxyquinoline to form a task-specific ionic liquid (1-(2-(quinolin-8-yloxy)ethyl)pyridinium bromide), abbreviated as [QPy][Br]. Subsequently, the complexation of [QPy][Br] with chromium ions in an aqueous medium was carried out, followed by the addition of hexafluorophosphate as a counter ion to the extraction system to induce an in situ metathesis reaction, producing the hydrophobic (water-immiscible) ionic liquid [QPy][PF₆], thereby enabling phase separation. Several experimental parameters affecting the microextraction system were optimized using multivariate response surface methodology. The resulting mathematical equations were employed to construct response surface plots for predicting the extraction behavior of the chromium ions and determining the optimum extraction conditions. Under the optimized conditions, the calibration curves exhibit linearity within the ranges of 100–500 and 10–1000 ng L⁻¹, limits of detection of 20.6 and 4.8 ng L⁻¹, and relative standard deviations for seven replicate determinations of 3.1% and 2.6% for Cr(III) and Cr(VI), respectively. Finally, the developed method was successfully applied to the microextraction and determination of chromium ions in several water and urine samples, yielding satisfactory recoveries. The term speciation in this work refers to an operational chemical discrimination based on selective extraction behavior rather than instrumental separation.