I am measuring CHCs of insects and I am having problems trying to identify methyl branched alkanes. My GC-MS (ion trap) does not give me EI mass spectra with high molecular ions. When runing samples on CI mode I only have molecular weight. Retention indexes are not good for compounds that elute so close each other and when are not available as standards. So, how could I get good results?. It seems quadrupoles are better mass spectrometers for this type of research.
Hi R. N.,
EI is a hard ionization technique, it is used for getting maximum probable fragment ion of an analyte. If your analytes does not give higher ion, then anybody can do nothing for getting so, this is the main limitation of EI technique. For overcome this limitation CI technique arises. In CI by doing MS you get only molecular ion peak. But doing MS/MS you can get corresponding characteristics fragment ions. This MS/MS data helps for structure elucidation of your targeted analyte. Again by doing MS/MS in Precursor ion scan mode, Product ion scan mode, Neutral loss scan mode you can provide complete structural information of your molecule.
In general MS alone is not very good for branched alkanes. You need to maximize your GC separation and even then you may not get a definitive identification with just MS, as it is not a great tool for isomeric/isobaric compounds. MS/MS will help but with alkanes you tend to get the same fragments. Use a really long (100 m) non-polar column and perhaps switch to hydrogen as a carrier as that will help you get more molecular ion, then use a combination of your Kovats RI data and the MS to help identify the compounds.
Inject a known mixture of n-alkanes. Between the linear alkanes the branched one are eluted.i.e. between nC7 and nC8 you will get all i-C8.
Further identyf. Can be made as suggested by Mark. Check a method called DHA (detailed hydrocarbon analysis). Remember that it is dedicated for petroleum samples. Other matrix may contain different compounds with same retention index values.
Regards
G
I think Chandras idea using CI with MS-MS might work. On the MS side there is also a chance to modify things without using MS-MS. I would recommend to use a mixed EI/CI volume in CI mode which might be available for your instrument. Then a mixed EI/CI spectrum is obtained.
A mixed EI/CI volume is a CI volume with slightly enlarged orifices.
Dear all, thanks a lot for your answers. Using a long 100m column to help separation and then use RI could work when you have a list of RI values of methyl branched alkanes, where could I find such a list?. Regarding the changes on my instrument, a ten years old Varian 450GC 240MS, such as mixed EI/CI or H2 as a carrier gas, I would prefer not to touch to much because this is the only instrument I have to do many different type of analysis. But my doubt is how people do this analysis because you can find a lot of papers of CHCs with detailed identifications of methyl branched alkanes... Is it just a question of instrument? Does single quad MS work better than ion traps? or perhaps High resolution machines?
Regards
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