In GC-MS combination, compounds separated in GC are sent to mass spectrometer to receive chromatograph to determine mass spectrum for accurate identification.

In GC-MS volatile samples and volatilized by derivatization samples are quantitatively or qualitatively analyzed using EI, CI, PCI and NCI ionization techniques. Especially spectrums obtained in El mode has higher resolution which increases the screening potential using available Wiley, NIST, and Pesticide libraries.