Gas Chromatography (GC) Analysis of Light Hydrocarbon Fractions
The light hydrocarbon fractions of crude oils 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 is mixed with deactivated alumina and transferred to the Sample Concentrator which is fitted with a tenax/silica gel/charcoal trap (OI trap #9) which is in turn connected to the gas chromatograph equiped with flame ionization detector (FID). The light hydrocarbon fraction is passed onto the GC column (J&W DB-1 60m x 0.32 mm x 0.25um), the temperature of which is initially 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. A GC trace is obtained showing the elution order of different compounds according to their physical properties and their relative abundances in the samples. Identification of individual peaks/compunds was based on co-injection of reference compounds and on comparison with published data in literature, and is shown as follows:
Identification of
Light Hydrocarbon GC
Gas Chromatography (GC) Analysis of Saturate Fractions
Preparation of oil samples
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.
Open column chromatography
A mixture of 28-200 mesh silica gel and 80-200 mesh alumina (1:3, w/w) was used as a support for the column. The support is activated at 120o-150oC for 12 hours. A glass wool plug is placed at the bottom of 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 vials and dried in a slow stream of nitrogen and weighed to a constant weight.
Gas chromatography
Saturate fractions were analysed using a Varian 3700 GC equipped with
a FID. A J&W DB-5 capillary column (30m x 0.32mm x 0.25um) was
used with helium as the carrier gas. The samples was injected using
a split injector at 320oC. The detector temperature was
maintained at 320oC. Different temperature programs have
been used , however, care has been taken to ensure data quality and consistency.
For selected samples, an internal standard (n-octadec-1-ene) has
been added to the saturate fraction for the purpose of quantification.
Idetification of individual peaks on the GC traces can be viewed here
(Identification
of Saturate Fraction GC)