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The application of MS in newborn screening is one of the most important branches of MS′s current clinical application.In this paper we first reported a new method of newborn screening for Phenylketonuria (PKU) by GC-CI-MS-SIM,which was relatively cheap,fast,sensitive and accurate.In this method Phenylalanine (Phe) and Tyrosine (Tyr) were extracted from dry blood spots with 0.1％ mol/L HCl-methanol.The protein in the extraction was gotten rid of by centrifugor.Phe and Tyr were reacted with 3.0 mol/L HCl-butanol,then the products Phe and Tyr butyl esters were reacted with trifluoroacetic anhydride.The final derivations from Phe and Tyr,alanine,3-phenyl-N(trifluoroacetyl)-,butyl este and L-tyrosine,N(trifluoroacetyl)-,butyl ester,trifl were analyzed by GCCI-MS-SIM.The contents of the two amino acids in blood samples were determined by calculating the areas of their final derivations with exterior standard.Finally,the molar ratios of Phe to Tyr in blood sample were calculated as the basis of PKU diagnosis.Because chemical ionization and selective ion measurement were used in this method,the limits of detection for Phe and Tyr were low and the sensitivity was excellent.About 30 blood samples were analyzed by this method,and the results prove that this method could successfully discriminate between normal infants and PKU positive ones.The price of instrument GC-MS is cheaper than that of MS-MS.Therefore it was capable for large-scale screening in developing countries.