Magnetic ionic liquids as extraction solvents in vacuum headspace single-drop microextraction

Abstract

A vacuum headspace single-drop microextraction method based on the use of magnetic ionic liquids (vacuum MIL-HS-SDME) for the determination of short chain free fatty acids is described for the first time. The basis of the method involves the use of a rod magnet to aid in maintaining a small microdroplet of magnetic ionic liquid (MIL) during headspace single-drop microextraction (HS-SDME). The application favors reduced pressure conditions inside the sampling vial while maintaining the MIL droplet in the headspace. After extraction, the MIL microdroplet containing extracted FFAs is transferred to a headspace vial where static headspace desorption is performed, followed by gas chromatographic-mass spectrometry (GC-MS) analysis. A number of MILs were studied and the trihexyl(tetradecyl)phosphonium tris(hexafluoroacetylaceto)manganate(II) MIL was found to be the most suitable for the proposed method. A comparison with atmospheric pressure MIL-HS-SDME revealed that analytes reached equilibrium faster when reduced pressure conditions were applied and that an enhancement in the extraction efficiency of analytes under these vacuum conditions was observed at any extraction time. Under optimum conditions, the method requires only 20 µL of MIL placed at the end of a rod magnet and the evacuation of air using a modified extraction vial and a vacuum pump. Afterwards, 10 mL of sample containing 30% (w/v) of NaCl is injected in the vial and the vacuum MIL-HS-SDME is performed at 45 °C and 600 rpm for 60 min. The MIL microdroplet can easily be transferred to a 4.2 mL modified headspace vial for the headspace desorption and GC-MS analysis. The entire method is characterized by wide linearity ranges, low limits of detection for analytes (down to 14.5 µg L−1), good reproducibility (with relative standard deviation lower than 13%), and relative recoveries ranging from 79.5% to 111%. The proposed vacuum MIL-HS-SDME was applied towards the analysis of two different milk samples with the majority of analytes being detected and quantified.