I have isolated and identified limonin from my plant material. I have completed NMR analysis, now I have to do GC-MS analysis to confirm the molecular weight of the compound.
Structure of limonine has three carbonyl groups, therefore, is could be derivatized by oximation, e.g. by methylhydroxylamine, pentafluorobenzyl hydroxylamine, etc. You must consider the increase of molecular weight of your analyte by derivatization (and sometimes some artifacts). However, molecular ion could not be present in the mass spectra (it depends on molecular structure of your analyte and ionization technique used in your MS.
Tomáš Vrzal Would you please send me the method of oximation? Thank you very much for your syggestions.
You could find the method in a literature, this derivatization reaction is widely used in analysis of compounds with carbonyl group. For example, it was used in many metabolomic studies of plants performed by GC-MS.
You could try prepare methoxyamine solution in pyridin and add few microliters to your dried sample, and let to react at 45 °C for one or more hours. Of course, you could use another reagent like PFBHA (pentafluorobenzylhydroxylamine) which could be benefit in your MS detection. At the end, do not forget to include derivatization in molecular mass calculation and/or fragmentation.
Limonin is unlikely to GC either underivatized or with just methoximation.
i would suggest trying derivatization with hydroxylammine.HCl (2% w/v) in pyridine (40μl) 90min 30deg then MSTFA, 37deg, 30min.
You are not going to get MW confirmation from GC-MS for this compound; it is going to heavily fragment. Not sure who told you this would work but they are dead wrong. Move to LC-MS.
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