Dear Karthick, I think that no benzene fragmentation at all can really happen in the gas chromatograph injector or during elution in the column. It is a very stable molecule. The only fragmentation you have is of course in the ion source at -70 eV. If you reduce the energy of the filament electrons (e.g. to -10 eV) you should reduce benzene fragmentation almost totally and verify that only ion source is able to fragment benzene in your system. Some GC-MS instruments have the possibility to modify the energy of the filament electrons.