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

<p>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 (C<sub>4</sub>MIm-PF<sub>6</sub>) or the IL-based surfactant 1-hexadecyl-3-butylimidazolium bromide (C<sub>16</sub>C<sub>4</sub>Im-Br) as extractant solvents. For the water-insoluble C<sub>4</sub>MIm-PF<sub>6</sub>, the most conventional DLLME mode using acetonitrile as dispersive solvent was employed. For the water-soluble C<sub>16</sub>C<sub>4</sub>Im-Br, the in situ DLLME mode with lithium bis[(trifluoromethane)sulfonyl]imide (Li-NTf<sub>2</sub>) 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 C<sub>4</sub>MIm-PF<sub>6</sub> were: 100 μL of neat C<sub>4</sub>MIm-PF<sub>6</sub>, 1 mL of acetonitrile, 10 mL of water, no control of pH for environmental waters, NaCl content of 23 g L<sup>−</sup><sup>1</sup>, diethyl dithiocarbamate (DDTC) as complexing agent at 10 mg L<sup>−1</sup> and final dilution of the micro-droplet with acetonitrile up to 70 µL. The optimum conditions for the in situ IL-DLLME method using C<sub>16</sub>C<sub>4</sub>Im-Br were: 0.8 mL of acetonitrile, 10 mL of water containing C<sub>16</sub>C<sub>4</sub>Im-Br at 25.2 mmol L<sup>−1</sup>, final dilution step of the micro-droplet with 200 µL of acetonitrile and remaining conditions as those of C<sub>4</sub>MIm-PF<sub>6</sub>. The analytical performance of both methods was similar, being slightly better for the IL-DLLME method using C<sub>4</sub>MIm-PF<sub>6</sub>, with limits of detection (LOD) of 3.3 µg L<sup>−1</sup> (versus 5.1 µg L<sup>−1</sup> when using C<sub>16</sub>C<sub>4</sub>Im-Br), precision values as intraday relative standard deviation (RSD in %) lower than 8.8% (being of 10% for the C<sub>16</sub>C<sub>4</sub>Im-Br method) and an enrichment factor of 54 (being 27 when using C<sub>16</sub>C<sub>4</sub>Im-Br). The DLLME-FAAS method with C<sub>4</sub>MIm-PF<sub>6</sub> 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<sup>−1</sup>, 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 Cu<sup>2+</sup> in presence of foreign ions commonly found in natural waters.</p>