Gasoline Components

area is in pollution studies, forensic work and general trace analysis. Gasoline 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

the necessary selectivity to resolve it from the other, slightly more excluded, enantiomer. The Analysis of Gasoline Gasoline is a multi-component hydrocarbon mixture of which many of the components have very similar molecular weights and all have interactive properties that are almost exclusively dispersive (with the exception of the aromatic hydrocarbons that can interact by induced dipoles if the stationary phase is strongly polar). The structure of many of the hydrocarbons components are also very similar and the mixture contains many isomers. It follows that the separation factors between individual components are likely to be very small and, consequently, to achieve a separation, columns of very high efficiency will be essential. Open tubular columns are ideal for this type of separation and, in fact, it is impossible to separate gasoline efficiently with a packed column, (and that will be true even if the column is 50 ft long and the inherent long

nbsp; As with all small radius open tubular columns a split injection system must be used. In addition, the relatively wide boiling range of the gasoline will require a temperature program that will heat the column to 200oC or more and thus the stationary phase must be thermally stable. The components of the gasoline 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 requisites mandates the use of an FID. A separation of gasoline components is shown in figure 30. The stationary phase used was Petrocol which is the trade

will be predominantly polar and in LC the eluting mobile phase would-be made dispersive (cf. normal phase chromatography). An example of dispersive and polar interactions is afforded by the separation of the gasoline sample on both a highly dispersive stationary phase, and a strongly polar stationary phase. The separations are shown in figure 2. GC gives a clear indication of the retentive character of the stationary phase as there are no significant interactions in the mobile phase. Gasoline has a relatively high proportion of aliphatic hydrocarbons which can only interact dispersively with any stationary phase. However, it has also a significant number of different aromatic hydrocarbons present which, as already has been discussed (book 1), can be polarized and consequently interact with a polar stationary phase. Thus if a sample of gasoline is chromatographed on a strongly dispersive stationary phase the components would be separated roughly on a basis of molar volume.

730;C for 5 min. to ensure the complete elution of the higher boiling components. An excellent separation is obtained giving clearly separated peaks for the marker compounds which are of importance in fuel evaluation. Nevertheless, due to the complexity of the sample, exceedingly high efficiencies were necessary and so, the analysis time was about 100 min. Long analysis times are directly related to the use of long columns The complete analysis was carried out using only 0.1 ml of gasoline with a split of 100:1 at 250˚C (ca 1 mg) confirming the remarkable sensitivity of the FID for general analysis

The separation of the gasoline components is shown in figure 41. The stationary phase used was Petrocol (the trade name for a special poly(dimethysiloxane)) that is actually intra-column polymerized and thus bonded to the surface and, as a result, is very thermally stable. The alkane chains in the polymer contribute strong dispersive properties to the stationary phase. The necessary high efficiency was obtained by using a column, 100 m long, 250 mm I.D. carrying a film of stationary phase 0.5 mm thick. The