Determination of Cholesterol in Aquatic Products by UHPLC after Hydrolysis with Alkaline Ionic Liquid and Extraction with Aqueous Two-Phase Micellar System

Method for determination of cholesterol in aquatic products was proposed by UHPLC based on alkaline ionic liquid hydrolysis and aqueous two-phase micellar system (ATPMS) extraction, and the composition of ATPMS was optimised in terms of partition coefficient. Minced meat packed aquatic product of 0.10 g was taken and mixed with 0.36 mol·L^-1 hydroxide 1-butyl-3-methylimidazole salt (C₄mimOH) alkaline ionic liquid (1 mL), and the mixture was heated at 90 ℃ for 30 min. After cooling and centrifuging at 5 000 r·min^-1 for 5 min, the entire supernatant was taken, 3 mL of ATPMS citric acid-phosphate buffer (pH 7.0) as media consisting of 0.5% (mass fraction, the same below) 1-butyl-3-methyl imidazole acetate (C₄mimAc) and 5.0% (mass fraction, the same below) Triton X-114 was added, and the mixture was heated to the cloud point, keeping for 3 min. The lower phase was taken, diluted to 5 mL with methanol, vortexed for 30 s and sonicated for 5 min. An aliquot (1 mL) of the above solution was taken and passed through 0.22 μm filter membrane and the filtrate was used for UHPLC analysis. It was shown by results that the ATPMS consisting of 0.5% C₄mimIm or C₄mimAc and 5.0% Triton X-114 gave a high partition coefficient for extraction of cholesterol. Linear relationship between the mass concentration of cholesterol and respective peak area was kept in the range of 5-200 mg·L^-1, with detection limit (3S/N) of 7.5 μg·g^-1; the results (84.1%-89.9%) of recovery obtained from the ATPMS containing C₄mimAc were slightly higher than those (80.0%-83.9%) obtained from the ATPMS containing C₄mimIm, and were similar to those obtained from the control group with reference to GB 5009.128-2016 (KOH as catalyst and methanol as extractant). RSDs (n=5) of the determined values of the three groups were basically the same and less than 3.0%.