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

Juan F. Ayala-Cabreraa, María J. Trujillo-Rodrígueza, Verónica Pinoa*, Óscar M. Hernández-Torresa, Ana M. Afonsoa & Juliette Sirieix-Plénetba Departamento de Química, Unidad Departamental de Química Analítica, Universidad de La Laguna (ULL), La Laguna (Tenerife), Spainb CNRS, Laboratoire PHENIX, Sorbonne Universités, UPMC Univ Paris 06, Paris, FranceCONTACT Verónica Pino veropino@ull.edu.esABSTRACTThis 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 L−1, 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.