hydrocarbon biomarker ID problem

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

5 posts Page 1 of 1
Hi Forum,

Any geochemists here, particularly organic geochemists?
I'm currently having issues making an unequivocal identification of a troublesome hydrocarbon biomarker, namely gammacerane, using single quad GCMS. Using MSMS also will not work as many similar biomarkers have the same parent and daughter ions.

Gammacerane: due to it's molecular symmetry it is well known that it gives a relatively simple mass spec with a very dominant base ion of 191 mz.

However it also elutes in an area of the chromatogram filled with other biomarkers, mainly all sorts of hopanes.
The reference oil we use shows a tiny abundance of gammacerane, along with a mass spec that does NOT show a dominant 191 mz peak. I suspect, but can't be sure, this gammacerane peak has been mis-identified in our reference sample.

In my samples, (which admittedly give very messy spectra),the tentatively identified gammacerane also shows a mass spec with a non-dominant 191 mz peak, often with a sizeable 205 mz peak (usually a methylhopane peak).
Can anyone with much greater knowledge of this biomarker than I suggest whether co-elution would have the effect of making the 191 mz peak appear less dominant? Or could it be more likely that the gammacerane HAS been mis-ID'd? Difficult to answer when I haven't provided a chromatogram, I know, but do your best if you can. I'd be very grateful for any guidance on this, it's driving me round the twist. It is out of the question to buy a synthetic gammacerane standard, sadly (Phd student, no money).
The reference oil we use shows a tiny abundance of gammacerane, along with a mass spec that does NOT show a dominant 191 mz peak. I suspect, but can't be sure, this gammacerane peak has been mis-identified in our reference sample.
In my samples, (which admittedly give very messy spectra),the tentatively identified gammacerane also shows a mass spec with a non-dominant 191 mz peak, often with a sizeable 205 mz peak (usually a methylhopane peak).
Can anyone with much greater knowledge of this biomarker than I suggest whether co-elution would have the effect of making the 191 mz peak appear less dominant? Or could it be more likely that the gammacerane HAS been mis-ID'd? Difficult to answer when I haven't provided a chromatogram, I know, but do your best if you can. I'd be very grateful for any guidance on this, it's driving me round the twist. It is out of the question to buy a synthetic gammacerane standard, sadly (Phd student, no money).


Both of your scenarios, co-elution & mis-identification, are very possible.
Unlike LC-ESI-MS, where co-elution can and does lead to suppression of ionization, under GC-EI-MS conditions there is usually no suppression.
It sounds like you desparately need a reference crude oil that has a relatively high level of gammacerane (G) in order to accurately locate it in the EIC.
Using your reference oil, is it possible to ID the G peak by using the relative retention times (RRT's) of unequivocally known peaks that bracket the G peak ?

Your use of the phrase "driving me round the twist" makes me think that you are located in the UK. I seem to remember that Prof. Eglinton at Bristol is/was the king of organic geochemistry---have you tried contacting him for a reference oil that has a relatively high level of G ?
BP at Sunbury should have a library of relevant samples, if not G itself.
A Google search for G reveals many publications that show chromatograms of m/z 191; perhaps you can pick an author that has reported a relatively high G content in an oil within the last 5-10 yrs and request a sample of that?

Regards,
JMB
Search "preparation of gammacerane" gives,

Sediments, Diagenesis, and Sedimentary Rocks
R.P. Philp, in Treatise on Geochemistry, 2003

7.09.4.2 Depositional Environments
The utility of biomarkers as indicators of depositional environments arises from the fact that certain types of compounds are associated with organisms, or plants that grow in specific types of depositional environments. It has been possible, in some situations, to characterize depositional environments through a composite of biomarker parameters, e.g., the case of offshore Brazil (Mello et al., 1988). However, the majority of work in this area has utilized single compounds. For example, gammacerane has long been used as an indicator of hypersalinity (de Leeuw and Sinninghe Damste, 1990), although, as noted in Peters and Moldowan (1992), gammacerane is present in relatively low concentrations in virtually all oils from marine depositional environments. However, there is a general consensus that with increasing salinity the ratio of the concentration of gammacerane to that of hopane increases quite significantly. Oils or extracts from evaporitic environments, particularly lacustrine environments, typically have terpane chromatograms where gammacerane is the most abundant component in the chromatogram (Figure 9). It is also noteworthy that for many years the exact nature of the precursor of gammacerane was unclear until the work of Venkatesen (1989) established that gammacerane was derived from tetrahymanol precursor. It is of interest to note that while gammacerane is an indicator of salinity, its precursor, tetrahymanol, is widespread in freshwater and marine ciliates. Sinninghe Damste et al. (1995) proposed that in certain Miocene and Upper Jurassic formations, gammacerane is derived from bacterivorous ciliates that fed on green sulfur bacteria. If their diet is deprived of sterols, these anaerobic ciliates synthesize tetrahymanol, and hence gammacerane can be thought of as an indicator of water column stratification and not simply hypersalinity, as suggested by Schoell et al. (1994).

Fig 9 is an m/z 191 chromatogram that shows G(ammacerane) at about 85-90% relative abundance compared to C30-Hopane

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Figure 9. Depositional environments can be characterized by individual biomarkers or groups of biomarkers. The classic example is gammacerane, whose relative concentration has been shown to vary with salinity in many well-defined cases.

Regards,
JMB
JMB - thank you very much for your replies. Yes, I absolutely DO need a reference oil with a high abundance of G. Someone has recently promised to send me one of their extracts containing lots of G so I'm looking forward to receiving that, it will be very useful.
Yes I'm in the UK and yes, Prof Eglinton is THE biomarker guru. Sadly he passed away a couple of years ago, and I came to geochemistry way too late to ever meet him. An opportunity missed for me.
I will check out BP at Sunbury as you suggest, and I have accessed the reference you kindly provided.
From our current low G reference oil I am indeed using relative retention times to ID the G. The G peak appears somewhere between the doublets for C31 hopane and the doublets for C32 hopane.
Interestingly though, looking at the individual peak assignments provided by all the different labs which report results for this reference oil, exactly where in between these two pairs of doublets is hard to pin down, and is not helped by lack of a dominant m/z 191, because these labs are not in agreement on this. Some of them assign G to a peak roughly midway between the pairs, and others assign the peak to a very small one which elutes not far behind the 'R' peak of the C31 doublet. Hence my lingering doubts about the correct ID.
I need to get my mitts on that high G extract I've been promised!
When you receive the high G sample, you might want to spike your current low G oil with it.
I have done this with HPLC samples, and the trick is to not overload the original low G peak so that it is swallowed whole by the large G peak.
Ideally you want to add just enough of the spike to double/triple the low G peak height.
That should clearly ID the G peak in the low oil sample.

Good luck with your project!

Regards,
JMB
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