Gas Chromatography - Applications > Gasoline > Page 63
Gas chromatography has a very wide field of application but its first and main area of use is in the separation and analysis of multi component mixtures such as essential oils, hydrocarbons and solvents. Intrinsically, with the use of the flame ionization detector and the electron capture detector (which have very high sensitivities) gas chromatography can quantitatively determine materials present at very low concentrations. It follows, that the second most important application area is in pollution studies, forensic work and general trace analysis.
Gasoline is a multicomponent mixture containing a large number of hydrocarbons, many of which have very similar molecular weights and all are almost exclusively dispersive in interactive character. The structure of many of the hydrocarbons are also very similar and there are many isomers present. As a consequence, due to their interactive similarity the separation factors between individual components is very small.
It follows that columns of very high efficiency will be mandatory to achieve an effective separation. It is clear that open tubular columns are ideal for this type of separation problem. In fact, it would be impossible to separate the components of gasoline efficiently with a packed column, even one that is 50 ft long, and even if the inherent long analysis times could be tolerated. In addition this type of separation demands the maximum number of theoretical plates and therefore not only must open tubes be used but tubes of relatively small diameter to produce the maximum number of theoretical plates. In fact, several hundred thousand theoretical plates will be necessary and so the column must also be very long. As has already been discussed, it is necessary to use small radius open tubular columns with a split injection system. Furthermore, as a result of the wide range of molecular weight of the components present, gasoline has a relatively wide boiling range and so will require a temperature program that will heat the column to 200 ˚C or more. A thermally stable stationary phase must be employed. The individual gasoline components are present over a wide concentration range and thus, for accurate quantitative results, the linear dynamic range of the detector must also be large. These latter demands mandate that the detector must be the FID.