After analysing my samples, I was able to detect alkanes bigger than C40 up to C60. Based on their estimated boiling point, I don't see how is it possible that I am detecting this type of peak, and thus find it difficult to discuss the reasoning behind it.
The instrument used was: GCMS-QP2010 Ultra, Shimadzu, Japan, with a column of silica Rtx-5ms, Restek, USA (length 30 m, 0.32 mm I.D., df = 0.25 μm) using Helium gas as a carrier.
Working conditions were as follows; the initial temperature at 90 C for 2 min., ramped to 320 C at 5 C per min., and held for 12 min. Samples were injected in split mode, with injector temperature at 300 C. MS transfer line temperature was 300 C, with electron energy of 70 V, the data sampling time was 4.5 - 60 min., and a scan time of 1 sec/scan.
Any help would be appreciated.
Dear Hector Jirau,
Unfortunately, I couldn't find a protocol that detects n-alkanes >C40 with GC-MS, and most of them detect n-alkanes between C7 to C40.
Please, try to download the last version of the Library Search Program of your GC-MS machine, perhaps it will solve the problem, since each second, there are alot of database data that are updated.
However, i wish you all the best to found a solution to this delicate situation.
Best wishes,
Sabri
We have been able to detect alkanes up to C70 by GC-MS and GCxGC-MS by using a high-temperature column and modifying our GC-TOF heated transfer line to permit temperatures up to 450 degrees C. We can detect the molecular ions by field ionization, although the molecular ion for C70 is _barely_ detectable in the GCxGC-MS data.
What makes you think you detected them? As Simon Hammann said, if it is an EI library spectral match I would be EXTREMELY cautious about reading too much into that...as the EI spectra of n-alkanes is virtually identical and their differentiation relies entirely on the nearly non-detectable molecular ions. Hence the matching algorithms never weight the distinguishing feature sufficiently and the match scores (even reverse) will be FAR higher for all alkanes (correct or incorrect) than even experimental spectra from standards of many heteroatom containing compounds.
Luke is right - database search for hydrocarbons is absolutely of limited value without more information. That's why we used field ionization to detect the molecular ions for our GCxGC-MS work. The molecular ion for C70 was weak, but clearly detected at m/z 983.1. For anything larger, we would have to do field desorption and sacrifice separation of isomers.
I do not agree to Lukes remark, indeed. It is absolutely possible to identify alkanes by their spectrum. And yes, with respect to the chain length of hydrocarbons single database search is of limited value. You get no or only very small molecular peaks and thus, you dont know much about the chain length. But this, the chain length, is not difficult to find this out via the retention time of respective standards. If there are no standards available you can work with some extrapolation.
Thus, it is no rocket science to find out which chain lenght will pass your GC system. Columns which allow oven temperatures up to 350°C and above (e.g. DB-1 ht) will allow to measure C70 or higher. Due to a high fragmentation rate and relatively high molecular weights the signal will decrease in GC-MS.
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