Chromatography Techniques and Spectrometry

Gas Chromatography - Mass Spectrometry (GC-MS)

Gas Chromatography-Mass Spectrometry (GC/MS) is an Analytical instrumental technique, by which separation, identification and quantification of complex mixtures of chemicals can be done which comprises a gas chromatograph coupled to a mass spectrometer. This makes it eligible for the analysis of the hundreds of environmental materials relatively low molecular weight compounds. The compound must be sufficiently volatile and thermally stable in order to be analysed by GC/MS. Before GC/MS analysis, samples are usually analyzed as organic solutions consequently materials of interest (e.g. soils, sediments, tissues etc.) need to be extracted from solvent and the extract is subjected to various “wet chemical” techniques.

To the GC inlet the sample solution is injected where it is vaporized by the carrier gas (usually helium) and swept onto a chromatographic column. By virtue of their relative interaction with the coating of the column (stationary phase) and the carrier gas (mobile phase) the sample flows through the column and the compounds comprising the mixture of interest are separated. The compounds eluting from the column are converted to ions following part of the column passes through a heated transfer line and ends at the entrance to ion source.

For Ion production two potential methods exist. Electron ionisation (EI) is the most frequently used method is and chemical ionisation (CI) is the occasionally used alternative. A beam of electrons ionise the sample molecules resulting in the loss of one electron for Electron ionisation. A molecule with one electron missing is represented by M+ called as the molecular ion. In a mass spectrum when the resulting peak from this ion is seen, it gives the compound’s molecular weight. Usually fragments producing further smaller ions with characteristic relative abundances that provide a 'fingerprint' for that molecular structure due to the large amount of energy imparted to the molecular ion. This information may help to elucidate the structure of unknown components of mixtures and also used to identify compounds of interest.