Ionic liquids versus ionic liquid-based surfactants in dispersive liquid–liquid microextraction for determining copper in water by flame atomic absorption spectrometry

This work compares the performance of dispersive liquid–liquid method (DLLME) as a prior step for determining copper by flame atomic absorption spectrometry (FAAS), when using the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate (C4MIm-PF6) or the IL-based surfactant 1-hexadecyl-3-butylimidazolium bromide (C16C4Im-Br) as extractant solvents. For the water-insoluble C4MIm-PF6, the most conventional DLLME mode using acetonitrile as dispersive solvent was employed. For the water-soluble C16C4Im-Br, the in situ DLLME mode with lithium bis[(trifluoromethane)sulfonyl]imide (Li-NTf2) as metathesis reagent was employed. In both approaches, some effective parameters such as volumes of extractant and dispersive solvents, concentration of complexing agent, pH of sample solution, salting-out effect and final diluting solvent to ensure compatibility with FAAS, were properly optimised. The optimum conditions for the IL-DLLME method using C4MIm-PF6 were: 100 μL of neat C4MIm-PF6, 1 mL of acetonitrile, 10 mL of water, no control of pH for environmental waters, NaCl content of 23 g L1, diethyl dithiocarbamate (DDTC) as complexing agent at 10 mg L−1 and final dilution of the micro-droplet with acetonitrile up to 70 µL. The optimum conditions for the in situ IL-DLLME method using C16C4Im-Br were: 0.8 mL of acetonitrile, 10 mL of water containing C16C4Im-Br at 25.2 mmol L−1, final dilution step of the micro-droplet with 200 µL of acetonitrile and remaining conditions as those of C4MIm-PF6. The analytical performance of both methods was similar, being slightly better for the IL-DLLME method using C4MIm-PF6, with limits of detection (LOD) of 3.3 µg L−1 (versus 5.1 µg L−1 when using C16C4Im-Br), precision values as intraday relative standard deviation (RSD in %) lower than 8.8% (being of 10% for the C16C4Im-Br method) and an enrichment factor of 54 (being 27 when using C16C4Im-Br). The DLLME-FAAS method with C4MIm-PF6 was used in the analysis of environmental waters with successful performance, with relative recoveries of 110% and 105%, and interday precision with RSD values of 21% and 7.4% for spiked levels of 60 and 160 µg L−1, respectively. The results obtained when analysing an urban wastewater sample coming from an inter-laboratory exercise was comparable to those obtained for other 93 laboratories. The method was also valid for the determination of Cu2+ in presence of foreign ions commonly found in natural waters.