Determination of thiocyanate in milk and milk powder by derivatization with pentafluorobenzyl Bromide using n-hexadecyltrimethylammonium bromide as phase-transfer catalyst.Gas chromatography-mass spectrometry was used to study thiocyanate derivatization residues in milk and milk powder.Based on the derivatization of thiocyanate with pentafluorobenzyl bromide,the samples were extracted and precipitated protein with mixture solution of acetonitrile and water by oscillation,then pentafluorobenzyl bromide was added in the exaction solution,finally the solution was purified by high-speed refrigerated centrifuge.The derivative products were scanned in selected ion monitoring mass spectrometry (SIM),identified by retention times and mass ratio of the abundance of fragment ion,and quantified by external standard method.The results shows that in the quantitative range of this method is 0.05 mg / kg ～ 20.0 mg / kg,the method can be used to detect sodium thiocyanate in milk and milk powder with high recovery,good accuracy and reproducibility; the sample preparation procedure is simpleand easy; the testing cost is low and the turnaround time is short.