Abstract
Antibiotic residues in dairy products pose serious threats to food safety and affect human health. To accurately determine the level of sulfonamide and fluoroquinolone residues in milk， a deep eutectic solvent and improved needle filter-based liquid membrane microextraction （DES&INF-LMME） method combined with high performance liquid chromatography （HPLC） was developed for their determination in this study. Octanoic acid （hydrogen bond donor） and thymol （hydrogen bond acceptor） were selected to synthesize hydrophobic deep eutectic solvent， which acted as supporting liquid membrane （SLM） and extractant in the LMME process. Further investigations were conducted to examine the effects of various factors on extraction efficiency， including type and volume of DES， molar ratio of thymol to octanoic acid， type of extraction membrane， sample volume， salt type and addition amount， pH value， stirring speed and extraction time. The optimal extraction conditions were determined as follows： DES∶thymol-octanoic acid=2∶1 （n/n）， 100 μL； filter membrane， polytetrafluoroethylene （PTFE）； sample volume and pH， 30 mL （addition of 3 g ammonium sulfate） and pH=7； stirring speed， 400 r/min； extraction time， 35 min. Under the optimized conditions， the proposed DES&INF-LMME-HPLC exhibited good linearity in the range of 2.86-1 000 μg/L （r(2)>0.997 2）， with limits of detection （LODs） （S/N=3） and limits of quantification （LOQs） （S/N=10） ranging from 0.86-10.0 μg/L and 2.86-33.3 μg/L， respectively. The obtained enrichment factors （EFs） ranged from 32 to 84 with recoveries of 99.4%-109.2%. The intra-day and inter-day precisions at three spiked levels （low： 30 μg/L； medium： 50 μg/L； high： 100 μg/L）， expressed as relative standard deviations （RSDs）， were no more than 4.9% and 5.3%， respectively （n=6）. This method has been successfully applied to the determination of four antibiotics in commercially available milk products. The spiked recoveries for the four antibiotics at high， medium and low levels ranged from 88.9% to 113.4%. The proposed method is accurate， simple， sensitive and eco-friendly， and could be a good reference to the development of new analytical method for antibiotic residues in dairy products.