Determination of Residues of 15 Glucocorticoids in Milk by Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry after Purification with QuEChERS Containing Titanium Dioxide

Considering that the existing pretreatment methods cannot effectively remove phospholipids in milk samples, and the ionization effect of glucocorticoids was poor, the title method was proposed. Milk sample (5.0 g) was extracted with 10 mL of acetonitrile for 2 min by vortex, and 1.0 g of sodium chloride was added for salting out. An aliquot (5 mL) of supernatant was mixed with commonly used QuEChERS adsorbents50 mg of octadecyl-bonded silica gel (C₁₈), 50 mg of N-propylethylenediamine (PSA) and 500 mg of anhydrous magnesium sulfate and 50 mg of titanium dioxide adsorbent. After whirling for 1 min, the mixture was centrifuged for 5 min, passed through a 0.22 μm filter membrane, and the filtrate was introduced into ultra-high performance liquid chromatography. The mixed solution of acetonitrile and 0.1% (volume fraction) formic acid solution at varous volume ratios was used for gradient elution as the mobile phase on Agilent ZORBAX SB-C₁₈ column, and the separated 15 glucocorticoids were detected by tandem mass spectrometer equipped with electrospray ion source in positive ion mode and multiple reaction monitoring (MRM) mode. In order to eliminate the effect of matrix inhibitory effect, the matrix-matched method was used for quantification. It was shown that the mass concentrations of the 15 glucocorticoids were linearly related to their corresponding peak areas in the range of 0.5-50 μg·L^-1, and the detection limits were 0.1-0.3 μg·kg^-1. Test for spiked recovery was made at the 3 concentration levels with negative milk samples as matrice, giving recoveries in the range of 89.7%-104%, and RSDs (n=6) of the determined values ranged from 3.2% to 12%. The proposed method was used for the analysis of 10 batches of milk samples, hydrocortisone was detected in one batch of the sample, with the detected amount of 0.37 μg·kg^-1.