GC/MS is the coupling of a gas chromatography device (GC) with a mass spectrometer (MS). The gas chromatograph is used to separate the mixture of substances to be analysed and the mass spectrometer is used to identify and, if necessary, quantify the individual components. The column of a gas chromatography device consists of a thin tube or capillary as the stationary phase and is flowed through by an inert gas as the mobile phase. The substance mixture, which is also gaseous and consists of neutral molecules, is injected into this gas flow. Each component of the mixture of substances has a characteristic running speed, so that the mixture is separated into individual substances. Due to the limitations of gas chromatography, only vaporisable substances with a correspondingly relatively low molecular mass (M approx. < 1000 amu) can be investigated.  After passing through the chromatography column, the separated substances are ionised. To ionise the substances in the ion source, EI (electron impact ionisation) is usually used, but also CI (chemical ionisation) or FI (field ionisation) as well as several other ionisation techniques. Through ionisation, the molecules of the individual substance are either broken up (EI) or protonated (CI). From the mass numbers of the mole peak (CI) and characteristic fragments (EI) and any isotope patterns present, the structural and molecular formula of the substance can be deduced.

(https://www.chemie.de/lexikon/GC%2FMS.html