Magnetic ionic liquid-based microextraction and preconcentration technique coupled to electrothermal atomic absorption spectrometry for trace chromium determination in drinking water samples

Abstract

The interest for chromium (Cr) is originated in the widespread use of this metal in several industries, such as metallurgical, refractory and chemical. Due to these industrial processes, large quantities of Cr-containing residues reach the environment and can have significant adverse biological and ecological effects. The natural concentration of total Cr in surface water is usually between 0.5 and 2 μg L-1 1. However, these low concentrations are not compatible with the limits of detection (LOD) reached by some detectors based on atomic spectrometry. Therefore, emphasis has been placed on the development of sensitive analytical methodologies for preconcentration and determination of Cr at trace levels2. The ionic liquid (IL) trihexyl(tetradecyl)phosphonium chloride has been used as an extractant in liquid-liquid microextraction techniques (LLME) due to its ability to form ion pairs1. However, a more recent generation of ILs has emerged, called magnetic ionic liquids (MILs), which preserve the specific properties of ILs but also show magnetism. This novel property of MILs brings more benefits to LLME techniques as it allows the development of analytical methods that avoid time-consuming steps such as centrifugation3. In the present work, a novel LLME technique based on the formation of ion pairs between the MIL trihexyl(tetradecyl)phosphonium tetrachloroferrate ([P6,6,6,14]FeCl4) and the anion [Cr(H2O)2Cl4]- was developed for the preconcentration and determination of Cr by electrothermal atomic absorption spectrometry (ETAAS). The resulting method consisted in adjusting the acidity of water samples to a concentration of 1 mol L-1 HCl, followed by the extraction of Cr with ion pair formation with the MIL, the use of acetonitrile as a dispersant and vortexing. The MIL was collected with the use of a magnetic rod, diluted in CHCl3 and an aliquot of the eluate was injected into the graphite furnace of ETAAS for Cr determination. Under optimal experimental conditions, an extraction efficiency of 95% was obtained, an LOD of 41 ng L-1 of Cr and a relative standard deviation (RSD) of 3.2% (for a Cr solution of 1 μg L-1 and n=10). The proposed methodology was successfully applied to samples of drinking water taken from different points of the water supply network.