A fully automated method for the detection of four tobacco-specific nitrosamines(TSNAs)in mainstream cigarette smoke(MSS)has been developed.The new developed method is based on two-dimensional online solid-phase extraction-liquid chromatography-tandem mass spectrometry(SPE/LC-MS/MS).The two dimensional SPE was performed in the method utilizing two cartridges with different extraction mechanisms to cleanup disturbances of different polarity to minimize sample matrix effects on each analyte.Chromatographic separation was achieved using a UPLC C18 reversed phase analytical column.Under the optimum online SPE/LC-MS/MS conditions,N′-nitrosonornicotine(NNN),N′-nitrosoanatabine(NAT),N′-nitrosoanabasine(NAB),and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone(NNK)were baseline separated with good peak shapes.This method appears to be the most sensitive method yet reported for determination of TSNAs in mainstream cigarette smoke.The limits of quantification for NNN,NNK,NAT and NAB reached the levels of 6.0,1.0,3.0 and 0.6 pg/cig,respectively,which were well below the lowest levels of TSNAs in MSS of current commercial cigarettes.The accuracy of the measurement of four TSNAs was from 92.8 to 107.3%.The relative standard deviations of intra-and inter-day analysis were less than 5.4%and 7.5%,respectively.The main advantages of the method developed are fairly high sensitivity,selectivity and accuracy of results,minimum sample pre-treatment,full automation,and high throughput.As a part of the validation procedure,the developed method was applied to evaluate TSNAs yields for 27 top-selling commercial cigarettes in China.