SOLVENT EXTRACTION AND GAS CHROMATOGRAPHY/MASS SPECTROMETRY

When cores are suspected to contain migrated petroleum (because of fluorescence, anomalous C₁/C₂ ratios, or other evidence), it may be useful to determine if the extractable organic matter in the cores has a molecular distribution that resembles petroleum. This can be done by extracting a small amount of dried sediment sample with hexane or methylene chloride (depending on the technique used), concentrating the extract solution to a small volume, and analyzing the extract using the GC/MSD.

To prepare the solvent extract, one can use, for example, the method described below.

Crushed, freeze-dried sediment (500 mg sample) is weighed and transferred into 2-dram screw-cap vials with Teflon-lined caps. Normal hexane (n-C₆H₁₄; 2 mL) and methanol (CH₃OH; 2 mL) are added to the vial, and the suspension is shaken occasionally for ~2 hr (if the sample is freeze-dried, methanol should not be used for extraction). The clear supernatant solution is pipetted into a second vial (2 mL) and the extraction is repeated with another 1-mL aliquot of n-C₆H₁₄. The combined extract is evaporated to near dryness under a nitrogen blow-down at ~40°C and is then taken up in n-hexane to a volume of 50 µL. A 1- to 8-µL sample is then injected in the GC/MSD.

The GC/MSD is an HP 5973 system that consists of a HP 6890 GC, a mass selective detector, and an HP 7683 automatic liquid sampler (ALS). The 6890 GC is equipped with an EPC split-splitless inlet and has been fitted with an HP-5MS 5% phenyl methyl siloxane capillary column with dimensions of 30 m x 250 µm and a film thickness of 0.25 µm. The column enters the MSD through a heated conduit called the GC/MSD interface. Injections are done automatically using the ALS. The MSD itself (referred to as the analyzer) is composed of an electron impact ion source, mass filter, detector, analyzer heater, and radiators. Helium is used as the carrier gas. An alkenone method developed during Leg 185 can be used for both qualitative and quantitative analyses. The run time is 78 min.

In this method, the GC parameters are as follows:

Injection volume = 1 µL,

Front inlet temperature = 300°C,

Splitless injection mode = selected,

Column flow = 1 mL/min (constant mode selected),

Column outlet = MSD (vacuum),

Initial oven temperature = 40°C (hold 2 min),

Oven ramp = 20°C/min to 130°C (no hold) then 4°C/min to 320°C (hold 20 min) then 70°C/min to 40°C,

Thermal AUX2 temperature = 280°C (used for heating MSD transfer line), and

RTE integrator = data analysis selected.

In this method, the MSD acquisition parameters are as follows:

Tune file = ATUNE.A,

Acquisition mode = scan,

Solvent delay = 5 min,

Electron multiplier offset = 0,

Low mass = 27,

High mass = 500,

Threshold = 500,

Sampling = 2,

MSD quadropole temperature = 150°C, and

MSD source temperature = 230°C.

An HP MS ChemStation computer data acquisition and analysis system is used to run the GC/MSD. Chromatographic response is calibrated against a preanalyzed high molecular weight (HWM) Supelco standard (containing C₁₀ to C₃₆ hydrocarbons), and the concentrations are reported in nanograms per liter.