Simultaneous Determination of Choline and L-Carnitine in Infant Formula Milk Powder by Hydrophilic Interaction High Performance Liquid Chromatography-Tandem Mass Spectrometry

The milk powder sample (1.000 g) was taken and 25 mL of 1.0 mol·L^-1 hydrochloric acid solution was added. After vortex mixing for 5 min, the mixture was hydrolyzed in 70℃ water bath for 3 h (shaking and mixing once every 30 min) and cooled. Then 20 mL of ether was added and the mixture was vortexed for 30 s, centrifuged at 6 000 r·min^-1 for 10 min at 4℃. 0.10 mL of the supernatant was taken and 50 μL of 1 mol·L^-1 ammonium acetate solution was added. The mixture was diluted to 10.0 mL with water. After vortex mixing, 1-2 mL of the supernatant was passed through a 0.22 μm water-based needle filter. The filtrate was separated on a Hypersil GOLD^TM AQ column (2.1 mm×100 mm, 1.9 μm), with a mixture of acetonitrile and ammonium acetate solution as mobile phase in gradient elution. Electrospray positive ion source and multiple reaction monitoring mode were selected in mass spectrometry analysis. Linear relationships were found between the peak area of quantitative ions and the mass concentration of choline and L-carnitine in the range of 0.05-1.0 mg·L^-1 and 0.01-0.20 mg·L^-1, respectively. The detection limits (3S/N) were 0.003, 0.005 mg·kg^-1, respectively. The proposed method was used to determine choline and L-carnitine in the quality control sample, giving the relative error between the measured values and the certified values within 10%. Choline and L-carnitine in infant formula milk powder samples were determined by the proposed method, the results were consistent with those obtained by the national standard method, and RSDs (n=6) were less than 9.0%.