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

• Published in: International journal of environmental analytical chemistry 2016-01, Vol.96 (2), p.101-118
• Main Authors: Ayala-Cabrera, Juan F., Trujillo-Rodríguez, María J., Pino, Verónica, Hernández-Torres, Óscar M., Afonso, Ana M., Sirieix-Plénet, Juliette
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Dispersive liquid-liquid microextraction; environmental analysis; in situ microextraction; ionic liquid-based surfactants; ionic liquids; Ions; Laboratories; Liquids; metals; Scientific imaging; Surfactants
• ISSN: 1029-0397; 0306-7319; 1029-0397
• DOI: 10.1080/03067319.2015.1128538

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 ₄MIm-PF ₆) or the IL-based surfactant 1-hexadecyl-3-butylimidazolium bromide (C ₁₆C ₄Im-Br) as extractant solvents. For the water-insoluble C ₄MIm-PF ₆, the most conventional DLLME mode using acetonitrile as dispersive solvent was employed. For the water-soluble C ₁₆C ₄Im-Br, the in situ DLLME mode with lithium bis[(trifluoromethane)sulfonyl]imide (Li-NTf ₂) 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 ₄MIm-PF ₆ were: 100 μL of neat C ₄MIm-PF ₆, 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 C ₁₆C ₄Im-Br were: 0.8 mL of acetonitrile, 10 mL of water containing C ₁₆C ₄Im-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 C ₄MIm-PF ₆. The analytical performance of both methods was similar, being slightly better for the IL-DLLME method using C ₄MIm-PF ₆, with limits of detection (LOD) of 3.3 µg L ⁻¹ (versus 5.1 µg L ⁻¹ when using C ₁₆C ₄Im-Br), precision values as intraday relative standard deviation (RSD in %) lower than 8.8% (being of 10% for the C ₁₆C ₄Im-Br method) and an enrichment factor of 54 (being 27 when using C ₁₆C ₄Im-Br). The DLLME-FAAS method with C ₄MIm-PF ₆ 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 Cu ²⁺ in presence of foreign ions commonly found in natural waters.