LC Determination of 4 Estrogens in Infant Formula with Column Switching

After ultrasonic dissolution with water, the sample of infant formula was extracted ultrasonically for 30 min with acetonitrile. At the speed of 15 000 r·min^-1, the solution was centrifugated for 10 min at 5℃. The residue was extracted with ethyl acetate and then the two extracts were combined. The upper organic phase was evaporated to near dryness by N₂ blowing at 40℃. The residue was dissolved with 50% ( φ) methanol solution. The solution was filtered on 0.22 μm filtering membrane. LC was applied to the determination of 4 estrogens in the filtrate with column switching. Thermo BioBasic SEC-120 chromatographic column and Waters XBridge BEH C₁₈ chromatographic column were used as purification column and analytical column respectively with mixtures of water and methanol with different mixing ratios as mobile phase in the gradient elution, and UV detector was adopted in the determination. Linear relationships between values of peak area and mass concentration of the 4 estrogens were kept in the same range of 1.0-20 μg·L^-1, with lower limits of determination (10S/N) in the range of 0.1-0.3 μg·kg^-1. On the base of blank sample, test for recovery was made by standard addition method; values of recovery found were in the range of 90.0%-105%, with intra-day RSDs (n=6) and inter-day RSDs (n=11) of determined values in the ranges of 1.6%-5.4% and 3.5%-6.2% respectively.