This lab is adapted from Theory of Gas Chromatography with Operating Instructions and Experiments using the CERA Gas Chromatograph, published by Sargent-Welch, 1992.

Introduction:

Crude petroleum consists of a variety of hydrocarbons. The organic compound classes vary from highly volatile light paraffins to high molar mass substances called asphaltenes.

The following table shows the approximate composition of petroleum fractions and their intended uses.

FRACTION	COMPOSITION	COMMERCIAL USE
Natural Gas	C1 - C4	Heating Fuel
Gasoline	C5 - C10	Motor Fuel
Kerosene	C11 - C12	Jet Fuel
Light Gas Oil	C13 - C17	Heat, Diesel
Heavy Gas Oil	C18 - C25	Motor Oil, Wax
Residuum	C26 - C60	Asphalt, Wax

Gasoline is a distillation product of petroleum. It is also obtained by "reforming" other fractions to increase the yield of gasoline from crude.

Gasoline products are classified by a system called Octane Rating. The anti-knock properties of gasoline are compared to that of a standard mixture of heptane and 2,2,4-trimethly pentane (Iso-octane). The selection of iso-octane for this standard gives the system its name, Octane Rating.

The sample chromatogram below is of a gasoline product which is rated at an octane number of 89.

A gasoline sample which has the same anti-knock properties as a given mixture of heptane/iso-octane is assigned the numerical value of the iso-octane composition. A gasoline with an octane rating of 89 will have anti-knock characteristics comparable to mixture of 11% heptane and 89% octane.

Procedure:

Setup of the GC:

• Helium gas flow should be set to about 20 mL/min. Check the flow by using the soap film meter. Power should be "On" and the oven control should be set to "Low" (80 ^oC).

• Check the temperature to make sure it is at 80^oC and then turn the Detector switch to "On". The Attenuation should be at X10. Adjust the Balance Control to set the base line on the data recording device.

Experimental:

• Inject 1.0 mL of hexane (including 2.0 mL of air) and press "Enter". Allow the chromatogram to to develop fully.

• Repeat with 1.0 mL or heptane/air, then with 1.0 mL nonane/air.

• Now inject 1.0 mL of the gasoline sample(s). Allow the chromatogram to run for several minutes until all the peaks seem to have passed. Note that there will be many peaks, some not very well defined in the gasoline sample. There will also be peaks later in the chromatogram showing the presence of hydrocarbons with higher molar masses than nonane.

Save the data onto a 3 1/2" diskette and then go to one of the other PC's in the room in order to print off your chromatograms. Make copies for each member of the group.

Results and Discussion

Compare the comatogram(s) for your unknown gasoline(s) with those of the standards. What hydrocarbons can you identify as being in the gasoline(s)? How do the quantities of any hydrocarbons present compare with each other and with the standards?

Refinery operations and gasoline blending facilities are monitored by gas chromatography for quality control. Comment on the ability to "profile" a given gasoline mixture for consistency and quality before shipment to distributors.

Referring to the definition of "Octane Number" comment on the differences in the amounts of various hydrocarbon fractions between different octane rated gasolines.

Does the definition of "Octane Number" correlate to the GC analysis of your sample(s)?