The main disadvantage of this technique is that it can analyse only the compounds having vapour pressure exceeding around 10-10 torr. Analysis can be done only for the compounds with low pressures. By this technique determination of the position on the aromatic ring is difficult. Some isomeric compounds cannot be differentiated by mass spectrometry; they can be separated only by chromatography. This technique shows incompatibility between two techniques. The compound used in the gas chromatograph is a trace component present in GC-carrier gas. It requires pressure about 760 torr. But the mass spectrometer uses vacuum about 10-6 to 10-5 torr for the operation. It is a major problem in this technique10. In this technique lower E-collisions and incomplete fragmentations were observed. It is not suitable for pulsed ionization methods. The mass range and resolution were limited. MS/MS spectra is mostly instrument dependent8.