Gas-Chromatographic Analysis of Gasolines

In this experiment, you will analyze samples of gasoline by gas chromatography. From your analysis, you should learn something about the composition of these fuels. Although all gasolines are compounded from the same basic hydrocarbon components, each company blends these components in different proportions to obtain a gasoline with properties similar to those of competing brands. 

There are different octane rating requirements for regular and premium gasolines. You may be able to observe differences in the composition of these two types of fuels. You should pay particular attention to increases in the proportions of those hydrocarbons that raise octane ratings in the premium fuels. 

In some areas of the country, manufacturers are required from November to February to control the amounts of carbon monoxide produced when the gasoline burns. To do this, they add oxygenates, such as ethanol or methyl fcrf-butyl ether (MTBE), to the gasoline. You should try to observe the presence of these oxygenates, which may be observed in gasolines produced in carbon monoxide-containment areas. Because MTBE has been banned or partially baimed in most states (see previous essay), it is unlikely that you will observe MTBE. 

The class will analyze samples of regular unleaded and premium unleaded gasolines. If available, the class will analyze oxygenated fuels. If different brands are analyzed, equivalent grades from the different companies should be compared. 

Gasoline contains many highly volatile and flammable components. Do not breathe the vapors, and do not use open flames near gasoline. 

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Pauls RE (1995) Chromatographic characterization of gasolines. Advances in Chromatography 35 259-342. Wang ZD and Fingas M (1997) Developments in the analysis of petroleum products and oil-spill-related environmental samples by gas chromatography. Journal of Chromatography A 774 51-78. 

A gas chromatographic analysis showing 2 mol% pentanes and heavier in the gas plant absorber off-gas indicate 1 to 1 Vi vol% of gasoline production is being lost to fuel gas. 

Apart from innovations such as coupling spectrophotometric detection (Parker et al, Parker and Hudson ) or titrimetry (Parker et al ) at the outlet end of the gas chromatographic column, conventional thermal conductivity or flame ionization methods of gas chromatographic analysis are not effective in the chromatography of the trace tetraalkyl leads due to the extreme complexity of the gasoline base stock. 

Burford et al. [3] reported a coupled supercritical extraction-gas chromatographic method that can quantitatively extract and determine both gasoline and diesel range hydrocarbons from contaminated soils. The direct transfer of the extract to a gas chromatograph reduced analysis times to about 80min, compared to the 18h required for conventional sonication analysis. 

Kanai H, Inoye V, Goo R, et al. 1991. Gas chromatographic mass- spectrometric analysis of polar components in weathered gasoline/water matrix as an aid in identifying gasoline. Anal Lett 24 115-128. 

Fig. 6. Diagram of the low-frequency apparatus for decomposition of gasoline and in situ gas chromatographic assembly for product analysis. (Reproduced from Venugopalan, M., Scott, T. W. Z. physik. Chem. Neue Folge, 108,157 (1977), by permission of the authors and the publishers, Akademische Verlagsgesellschaft)...

The detection of low level concentrations of volatile petroleum hydrocarbons in either soil or water can be performed by static headspace analysis. In this technique, the gas phase in thermodynamic equilibrium with the matrix is analysed. The soil is placed in a headspace vial to which water and soluble salts such as sodium chloride are added to aid the transfer of hydrocarbons into the headspace. Internal standards and surrogate spikes can also be introduced. The vial is heated and an aliquot of the static headspace vapour is directly injected onto the column of the gas chromatograph. The advantages of this technique for volatiles such as gasoline range organics are less sample handling which minimises losses, no introduction of solvents which can interfere with the compounds of interest (MTBE), and the technique can be easily automated. 

DiSanzo, F. P. and Giarrocco, V. J., "Analysis of Pressurized Gasoline-Range Liquid Hydrocarbon Samples by Capillary Column and PIONA Analyzer Gas Chromatography, Journal of Chromatographic Science, Vol. 26, 1988, pp. 258-401. 

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