Measurement of aflatoxin M1 in milk by ultra-high-performance liquid chromatography/tandem mass spectrometry

A sensitive method was developed using ultra-high-performance liquid chromatography (UHPLC)/MS/MS with positive electrospray ionization for determining aflatoxin M1 (AFM1) in milk and milk powder. A 50 mL quantity of low-fat liquid milk containing 100 ng/L AFM1, was prepared using immunoaffinity columns with a mean recovery rate of 79% (n = 3). UHPLC columns (BEH C18, BEH HILIC, and HSS T3) greatly reduced the chromatographic time and lowered the instrumental detection limits (IDLs) 16 to 58 times compared to an HPLC column (Betabasic C18). The HSS T3 column was chosen because it provided a low IDL (0.11 pg) and the lowest ion suppression of signal intensity (63.4%) among the tested columns. Matrix-fortified calibration curves were used for quantification and showed good linearity (r > 0.997) at 0.05-500 ng/mL. The LOD was 0.18 ng/kg for milk and 2.08 nglkg for milk powder, based on the signal intensity of the confirmatory product ion (m/z 259.1), which was less abundant than the quantitative product ion (m/z 273.1). Certified reference materials of milk powder at three levels (<0.05, 0.111 +/- 0.018, and 0.44 +/- 0.06 microg/kg) were measured within a day and between days; the results were all close to the certified levels with low variations (RSDs < 15%), showing good precision and accuracy.