Gas Chromatography coupled with a Mass Spectrometer

Louvain-La-Neuve

Gas Chromatography coupled with Mass Spectrometry is a widely used technique for the analysis and quantification of organic compounds.

The sample is injected into the GC inlet where it is vaporized and eluted on a chromatographic column by the carrier gas, generally helium. The sample flows through the column and the compounds of the mixture of interest are separated thanks to their relative interaction with the stationary phase (coating of the column) and the mobile phase (carrier gas).

The column outlet passes through a heated transfer line and ends at the source inlet where the compounds eluting from the column are ionized.

There are two modes for the production of ions. Electronic Ionization (EI) which is the most frequently used method where an electron beam ionizes the molecules of the sample, resulting in the loss of an electron. A molecule with a missing electron is called the molecular ion. The second method is Chemical Ionization (CI).

When the peak of this molecular ion is observed in a mass spectrum, it gives information on the molecular weight of the compound. Fragmentation of the molecular ion occurs due to the large amount of energy that is transmitted to it. The fragments obtained are smaller ions with characteristic relative abundances which provide a fingerprint for this molecular structure. This information can help identify compounds of interest and help clarify the structure of unknown compounds.

The next component is a mass analyzer, quadrupole in our case, which acts as a filter by separating the positively charged ions according to the parameters requested from this analyzer. The separated ions enter a detector, the output of which is amplified to increase the signal. There are two ways to perform ion acquisition, FullScan mode which identifies new compounds and SIM mode which increases the selectivity and sensitivity of the analysis. These two methods are complementary.

The detector converts electrical impulses into peaks and sends the information to a computer which records all the results. The comparison of the mass spectra obtained with those of the database constitutes an aid to the identification of the compounds.