	APCI was applied to GC/MS and LC/MS also by Horning's group in 1975. Analyte in LC eluate was vaporized and ionized in a heated block. High sensitivity and simple mass spectra were obtained through this application. In the later decades, the coupling of APCI with LC/MS became famous and caught a lot attention. The introduction of APCI and LC-MS had expanded dramatically the role of mass spectrometry in the pharmaceutical industry in the area of drug development. The sensitivity of APCI combined with the sensitivity and specificity of LC/MS and liquid chromatography-tandem mass spectrometry (LC-MS/MS) makes it the method of choice for the quantification of drugs and drug metabolites.