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Hi, All:
I am doing metabolite ID using LTQ with ESI, and I found a -15Da fragment with high abundance in MS3 spectral chromatograms. And the most possible loss is -CH3 from trimethoxybene part, but it will form a radical structure based on this. As what I know, ESI is a soft-ionization pattern and hard to produce radical, so I am not sure if it is possible to cleavage a -CH3 to form a radical in LTQ apparatus. Can anyone help me? Thanks!
Jian
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The following message was posted to: PharmPK
ESI can produce radicals. For example, see
Van Berkel GJ, McLuckey SA, Glish GL. 1992. Electrochemical
origin of radical cations observed in electrospray ionization
mass-pectra.
Analytical Chemistry 64, 1586-1593.
However, ESI has got nothing (very little) to do with what you see at
MS3 stage as this is not "ionizaton" but "CID" or simply
"fragmentation". Gas-phase reactions, incl. methyl radicals, have been
known to occur in MS.
HTH
Frederik
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Hi, Frederik:
Thank you very much for your response. This has puzzled me for a long time and I can not find any useful papers about radical cations formation in MS fragmentation. What I saw was mostly neutral loss fragmentation. So does it mean neutral loss fragmentation is much easier than radical cation formation? If so, in what case the fragment with radical cation can dominate in MSn fragmentation profiles?
Jian
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The following message was posted to: PharmPK
Dear Jian,
I can't really answer your question(s) because I am no expert and
because it depends.
There is actually quite a few papers on the subject so I am a little
surprised that you couldn't find anything useful. I have just checked in
Web of Science and the paper I mentioned in my previous post has been
cited 186 times. In addition, you could do a search on each of the
authors. This will probably give you more than enough to make & keep you
happy for a while.
The generation of a radical in the gas-phase/vacuum (very low partial
pressures in MS) can be modelled (in fact, anything can be modelled...)
and if the radical is stabilised (e.g. through delocalisation) this will
increase the probability of it being formed and staying in existence
long enough to be detected. It is all about thermodynamics etc.
Different molecules (ions) will fragment in different ways. For example,
glutathione conjugates show a characteristic neutral loss of 129 amu.
Other ions lose water (18 amu), which is very common but not very
informative. Aromatic/highly conjugated molecules may favour radical
formation. However, neutral loss is more common because of the lower
activation barriers involved.
Last but not least, the collisional energies in MS can get fairly high.
HTH
Frederik
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The following message was posted to: PharmPK
Dear Jian,
I can't really answer your question(s) because I am no expert and
because it depends.
There is actually quite a few papers on the subject so I am a little
surprised that you couldn't find anything useful. I have just checked in
Web of Science and the paper I mentioned in my previous post has been
cited 186 times. In addition, you could do a search on each of the
authors. This will probably give you more than enough to make & keep you
happy for a while.
The generation of a radical in the gas-phase/vacuum (very low partial
pressures in MS) can be modelled (in fact, anything can be modelled...)
and if the radical is stabilised (e.g. through delocalisation) this will
increase the probability of it being formed and staying in existence
long enough to be detected. It is all about thermodynamics etc.
Different molecules (ions) will fragment in different ways. For example,
glutathione conjugates show a characteristic neutral loss of 129 amu.
Other ions lose water (18 amu), which is very common but not very
informative. Aromatic/highly conjugated molecules may favour radical
formation. However, neutral loss is more common because of the lower
activation barriers involved.
Last but not least, the collisional energies in MS can get fairly high.
HTH
Frederik
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The following message was posted to: PharmPK
Forming an odd-electron (radical) fragment from even-electron parent ions is quite possible. We have assigned radical structures to fragment ions of metabolites in several occasions.
The formation of odd-electron parent (precursor) ions in the ESI is also possible. I have seen it and others have reported it in LC-MS analyses of endogenous retinoids.
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