HPLC-MS/MS Determination of N-Dimethyl Nitrosamine in Food with Separation by Steam Distillation

A portion (200.00 g) of the smashed food sample was taken and mixed thoroughly with 100 mL of water, 10 mL of 88% (φ, the same below) formic acid solution and 50 g of NaCl in a 1 L distillation flask, which was then placed in connection with the steam distillation apparatus, and the distillation was started. By taking the course of steam distillation, N-dimethyl nitrosamine (NDMA) in food sample would be carried over with the steam and passed through the condenser and recieved in the recieving flask. As the surface of the solution in the flask attained to the mark of 400 mL, the distillation was stopped, and 30 mL of the distillate were taken and after adding 2 g of NaCl, purified by extraction with CH₂Cl₂ for 4 times with 20 mL of CH₂Cl₂ and shaking for 5 min in each extraction. The 4 extracts were combined and after adding 0.4 mL of 3.5% formic acid solution, the solution was evaporated to near dryness at 20℃ under reduced pressure. The residue was taken up with 1.6 mL of 3.5% formic acid solution. The solution was filtered through 0.22 μm filtering membrane, and the filtrate was used for HPLC-MS/MS analysis. Waters HSS T3 C₁₈ chromatographic column was used as stationary phase, and mixtures of methanol (A) and 0.6% formic acid solution (B) in various ratios were used as mobile phases for gradient elution. Modes of APCI⁺ and MRM were adopted in MS/MS determination. Linearity range found for NDMA was between 1.0 to 100.0 μg·L^-1. Detection limit (3S/N) found was 0.5 μg·L^-1. Using substantial samples of food as matrixes, test for recovery was performed by standard addition method, giving values of recovery in the range of 85.4% to 101%, and values of RSDs (n=5) less than 11%.