Vortex assisted-ionic liquid based dispersive liquid liquid microextraction of low levels of nickel and cobalt in chocolate-based samples and their determination by FAAS

In the study, a simple and green vortex assisted-ionic liquid based dispersive liquid liquid microextraction (VA-IL-DLLM) combined with flame atomic absorption spectrometry (FAAS) was developed for the extraction and determination of nickel (Ni) and cobalt (Co) in chocolate-based samples. An ultra-hydrophobic ionic liquid (IL), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [C(6)mim] [FAP], was chosen as extraction solvent. The method is selectively based on chelate formation between Ni(II)/Co(II) ions and ninhydrin (2,2-dihydroxyindane-1,3-dione) at pH 9.0, and then simultaneous extraction of the formed complexes into the micro-drops of IL, [C(6)mim] [FAP]. Extraction parameters (pH, the IL amount, chelating ligand concentration, vortex time, volume of dispersive solvent, and effect of interference ions) were evaluated and optimized. Under optimized conditions, the method was as follows; linear working ranges were within the 1.0-350 mu g L-1 and 0.7-400 mu g L-1 for Ni and Co, respectively. The limits of detections (3 x s(blank)/m, n:10) were 0.3 mu g L-1 for Ni (II) and 0.2 mu g L-1 for Co(II) with relative standard deviation < 3.1% for real samples. The method was validated by analysis of two standard reference materials (SRMs), and recovery tests based on spiking at two levels. Lastly, the method was reliably applied for the determination of trace Ni and Co in chocolate-based samples, with amounts in range of 1.5-192.7 mu g L-1 and in range of 3.8-152 mu g L-1, respectively.