EXPERIMENTAL

Preparation of crude oils

About 30-45 ml of oil were poured into a tared flask, boiling chips were added and the oil was heated up to 210ºC. The fraction boiling below 210ºC was distilled into a separate flask and weighed. The remaining fraction was cooled and weighed. About 4-5 grams of the fraction boiling above 210ºC was deasphalted by adding an excess of pentane (40 volumes). About 100 milligrams of each deasphalted oil were then fractionated using open column liquid chromatography.

Analysis of gasoline fraction hydrocarbons

The gasoline range hydrocarbons (iC5-nC8) were analysed on a HP5890 Gas Chromatograph connected to an OI Analytical 4560 purge-and-Trap Sample Concentrator. A small amount of the whole crude oil was mixed with deactivated alumina and transferred to the Sample Concentrator which was fitted with a tenax/silica gel/charcoal trap (OI trap #9). This was connected to a split/splitless injector on the Gas Chromatograph which was equipped with a 60m x 0.32 mm DB-1 column. The initial temperature was held at 30ºC for 10 minutes and then programmed to 40ºC at a rate of 1ºC/min. The final temperature was held for 25 minutes. The eluting hydrocarbons were detected using a flame ionization detector.

Liquid chromatography

A mixture of 28-200 mesh Silica Gel (MCB) and 80-200 mesh alumina (ALCOA) (1/3:2/3 by weight respectively) was used as a support for the column. The support is activated by heating at 120º-150ºC for 12 hours. A glass wool plug is placed at a bottom of the column and covered with a 1 cm thick layer of sand. The support, weighed as 1 g of support/10 mg of deasphalted sample, is slowly settled in pentane and any air trapped is released by gentle tapping on the column. A deasphalted sample, dissolved in a minimal amount of previously measured pentane, is then added to the column. Saturates are recovered by eluting with pentane (3.5 ml/g support), aromatics with a 50:50 mixture of pentane and dichloromethane (4 ml/g support), resins with methanol (4 ml/g support) and any remaining asphaltenes with chloroform. The solvents are rotary-evaporated, separate fractions transferred to tared 1 dram vials, dried in a slow stream of nitrogen and weighed to constant weight.

Gas chromatography

Saturate fractions were analysed using gas chromatography (GC). A Varian 3700 FID gas chromatograph was used with 30m DB-1 column with helium as the carrier gas. The temperature programmed was 60ºC to 300ºC at a rate of 6ºC/min and then isothermal for 30 min. The eluting compounds were detected and quantitatively determined using a hydrogen flame ionization detector.


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