Ionic liquid-based dispersive liquid–liquid microextraction combined with high performance liquid chromatography–UV detection for the simultaneous pre-concentration and determination of Ni, Co, Cu and Zn in water samples

Ionic liquid-based dispersive liquid–liquid microextraction (IL– –DLLME) coupled with high performance liquid chromatography (HPLC) with UV detection was developed for the simultaneous extraction and determination of nickel, cobalt, copper and zinc ions. In the proposed approach, salophen (N,N'-bis(salicylidene)-1,2-phenylenediamine) was used as a chelating agent, the ionic liquid, 1-hexyl-3-methylimidazolium hexafluorophosphate, and acetone were selected as extracting and dispersive solvents, respectively. After extraction, phase separation was performed by centrifugation and the sedimented phase (ionic liquid) was solubilized in acetonitrile and directly injected into the HPLC for subsequent analysis. Baseline separation of the metal ion complexes was achieved on a RP-C18 column using a gradient elution of mixtures of methanol–acetonitrile–water as the mobile phase at a flow rate of 1.0 mL min-1. The influence of variables such as sample pH, concentration of the chelating agent, amount of ionic liquid (extraction solvent), disperser solvent volume, extraction time, salt effect and centrifugation speed were studied and optimized. Under the optimum conditions, an enrichment factor of 222 was obtained. The detection limits for Ni, Co, Cu and Zn were 0.8, 1.6, 1.9 and 2.8 μg L-1, respectively. The relative standard deviation (RSD) was in the range 3.6–5.0 % for all of the investigated metal ions. The proposed procedure was successfully applied to the determination of the studied metal ions in water samples.