Principles and Practice of Chromatography - Chromatography Applications > Gas Chromatography Applications > Gasoline > Page 71

 

Nonpolar or dispersive stationary phases are employed for the separation of hydrocarbons (e.g. OV101, which is also a polyalkyl-siloxane, is widely used in packed columns). The flow velocity of 20 cm/sec., appears to have been taken from the ratio of the column length to the dead time. Thus, due to the pressure correction the actual effective linear velocity would be much less than that (see Dispersion in Chromatography Columns ). Helium was used as the carrier gas which was necessary to realize the high efficiencies with reasonable analysis times. The FID detector provided the necessary to wide quantitative dynamic range. The column temperature was held at 35˚C for 15 min. to effect the separation of the low boiling, low molecular weight hydrocarbons, the temperature was then increased to 200˚C at 2˚C/min. and finally held at 200˚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.