Determination of Benzoic Acid and Sorbic Acid in Soy Sauce and Vinegar by High Performance Liquid Chromatography After Vortex-Assisted Dispersive Liquid-Liquid Microextraction Based on Solidification of Floating Organic Drop

The soy sauce or vinegar sample （0.25 g） was placed in a 15 mL-polypropylene centrifuge tube, and dilute to 2.0 mL with water, and 1 mol·L^–1 hydrochloric acid solution was added to adjust pH to 2. 250 µL of 1-dodecanol was taken with a 1 mL-pipette gun, and quickly injected into the sample solution. The solution was vortexed for 60 s to form an emulsified turbid solution and centrifuged for 5 min. The centrifuge tube was placed in the refrigerator at −20 ℃ for 5 min and removed, and the 1-dodecanol layer was solidified into a block, which was carefully removed with a stainless steel medicine spoon, and placed in another clean 15 mL-centrifuge tube. 250 µL of 1-dodecanol was added into the sample solution, and the extraction was repeated in the same way. The 1-dodecanol layers were combined, heated, melt, and diluted to 5.0 mL with methanol, and the solution was passed through a 0.22 µm organic filter membrane. Benzoic acid and sorbic acid in the filtrate were determined by high performance liquid chromatography with Agilent Eclipse XDB-C₁₈ column as the stationary phase and a mixture of 20 mmol·L^–1 ammonium acetate solution and methanol at a volume ratio of 70∶30 as the mobile phase, using the external standard method at a detection wavelength of 230 nm. As shown by the results, linear relationships between the corresponding peak areas and mass concentrations of benzoic acid and sorbic acid were kept in the range of 1.0–50 mg·L^–1, with detection limits （3S/N） of 0.61 mg·kg^–1 and 0.35 mg·kg^–1, respectively. Test for recovery was made by standard addition method at three concentration levels on soy sauce and vinegar samples, giving results in the range of 95.6%−102% for benzoic acid and 95.3%−98.1% for sorbic acid, respectively, with RSDs （n=6） of the determined values less than 5.0%. This method was used for the analysis of actual samples, and benzoic acid and sorbic acid were detected to varying degrees.