Oil fingerprinting by GC-MS

[MartinsOtokpa]

Hello,
I am trying to develop a method for crude oil biomarker fingerprinting by GC-MS. When I run hydrocarbon defining window (C8-C40) standard, the compounds are well eluted and resolved. However, when I run the saturate fractionated from the whole oil, I get humps with very poor resolution. Again, I ran the same saturate fraction on the GC-FID and I got the expected signature with all the compounds well resolved. I have reduced the injection volume from 1uL to 0.2uL, 0.5uL trimmed the column from the inlet end, diluted the crude (200, 500 and 1000 fold) to no avail (on the GC-MS). Let me also add that the GC-MS tune report is okay. I keep wondering what could be wrong; any suggestions please?

[James_Ball]

The MS will have a difference sensitivity to the different functional groups than what the FID will have. FID will have its highest response to the hydrocarbons in the crude, while MS has a lower response to hydrocarbons. MS can have a higher response to nitrated and sulfated organic molecules than FID, for instance CS2 will respond very well to MS but gives very little signal on the FID.

Does the spectra of the "humps" identify as simple hydrocarbons or as other functional group molecules? The Fingerprint of oils will appear differently between the MS and FID unless the oil is a simple mixture of pure alkanes, which crude oil is not.

[MartinsOtokpa]

Thank you James.
That makes perfect sense. The primary objective is to be able to extract the fragmentograms of target ions (hopanes, steranes and aromatics). Currently, the signature of the EICs especially the steranes (ions 217, 218) do not look like that of the reference oil. That is what I am trying to resolve. I can show the fragmentograms via email if you don't mind.

[mhr311]

Isotope ratio guys worked out the procedures for biomarkers awhile back (see Moldowan, Bott...). What are transfer line settings?

[TCCrab]

Isotope ratio guys worked out the procedures for biomarkers awhile back (see Moldowan, Bott...). What are transfer line settings?

Absolutely agree with mhr311, the Oil Company guys (Prof. Muldowan et. al.) figured all this out long long ago.
Probably your biggest difficulty would be getting them to share their techniques/methodology.
Most if not all are no doubt considered "Proprietary" and "Mission Critical" to the related Oil Companies and as such would likely be highly protected.

'Crabs