Determination of Glycidic Fatty Esters in Infant Formula Milk Powder by Isotopic Internal Standard-Gas Chromatography-Mass Spectrometry

Sample of milk powder (4.000 0 g) was extracted thrice with ether (25, 15, 15 mL) and petroluem ether (25, 15, 15 mL) in succession to have the fat separated into the ether upper layers, which were combined and evaporated to dryness and dried at (100±2)℃. The dried fat was kept at 4℃. Two portions of the fat were weighed (0.100 0 g) and placed separately into 2 plastic tubes, marked A and B. To both of the tubes, 500 μL of a mixture of tert-butyl methyl ether and ethyl acetate (8+2) and 100 μL of 2.00 mg·L^-1 d₅-3-MCPD dipalmitate standard solution were added and both of the solutions in Tube A and Tube B were hydrolyzed for exactly 5 min by adding 1.0 mL of 0.1 mol·L^-1 sodium methylate-methanol solution. At the end of 5 min, 200 μL of neutralizing solution C1 and 100 μL of neutralizing solution C2 were added immediately to the solutions in Tube A (pH 1.0) and Tube B (pH 4.0) respectively, to stop the hydrolytic reaction. The solutions in the 2 tubes were extracted separately with 3 mL of n-hexane to remove fatty impurities. The lower aqueous phases were taken and treated separately by passing through a micro-column packed with diatomite and eluting thrice with 3, 3, 15 mL of ethyl acetate successively. The eluates were combined and evaporated to dryness, and the residue was taken up with 2 mL of isooctane. 100 μL of HFBI were added to each of the 2 solutions from Tube A and B separately and derivatization was carried out for 30 min and stopped exactly at the end of 30 min by adding 2 mL of saturated NaCl solution. Supernatant from Tube A was used for determination of ester of 3-MCPM and 3-MCPD transformed from glycidic ester in total and supernatant from Tube B was used for determination of ester of 3-MCPD. Both of the determinations were carried out by GC-MS with isotopic internal standard. Amount of glycidic aftty ester (GE's) in the sample was found by substraction as shown by the formula given. The linearity range for GEs was found between 13.4 to 402 μg·L^-1, with detection limit (3S/N) of 0.015 mg·kg^-1. Values of recovery and RSD (n=6) found were ranged from 95.2% to 103% and less than 4.0% respectively.