Further fragmentation of 74 ion

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

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I was developing a neutral loss method for Methyl palmitate (270). In scan mode ,I got prominent EI+ peak at 74 (Mclaffarty rearrangement), there should be a loss of 196 {CH3(CH2)10CH=CH2}. When I put Neutral loss value as 196 and look for 270 peaks. I do not see it at all. why is that? Can someone with GCMS background help

270=74 (m/z) +196 (Neutral molecule)
bobs wrote:
I was developing a neutral loss method for Methyl palmitate (270). In scan mode ,I got prominent EI+ peak at 74 (Mclaffarty rearrangement), there should be a loss of 196 {CH3(CH2)10CH=CH2}. When I put Neutral loss value as 196 and look for 270 peaks. I do not see it at all. why is that? Can someone with GCMS background help

270=74 (m/z) +196 (Neutral molecule)



Hello Bob

The m/z 196 is neutral because the lost of one electron on the ionisation process is on the oxygen atom of the carbonyl group, when the Mclafferty arrangement occur, the fragment m/z 74 continues like radical catión and the 196 is neutral molecule.

Best regards

Henry
Also in EI+ GCMS the ionization is quite aggressive versus in ESI+ for LCMS. The molecular ion and the M+1 are tiny most of the time so it is difficult to find the 270 molecular mass versus other fragments.
The past is there to guide us into the future, not to dwell in.
Hi! James and Henry,

Thanks for your reply.

In my understanding, the way neutral loss scan works is it puts both Q1 and Q2 into scan mode, but with a mass offset (so if you tell it to scan for a neutral loss of 20, quad 1 would be scanning for mass M and quad 2 would be scanning for (Mass M - 20) and report the M from which 20 is coming from. So in my case since 270 is breaking into 196 (neutral) and cation 74. If I can scan for loss of 196, I should see 270 in my TIC cause it is coming from it. Am I wrong?
Hi there,
If the molecular ion is being converted to m/z 74 in the ion source (in source fragmentation) then there isn’t going to any molecular ion to select using the first quad so you won’t see the 270 to 74 transition using ms/ms
The only way you will see this is if the transition from 270 to 74 can be made to occur downstream of quad 1

Hope this makes sense.
Kevin.
Hi! Thanks,

I do see good enough molecular ion peak in regular scan so was expecting it to be seen in NL TIC even if it was very small.
If you are using gc/ms to separate methyl palmitate , at low levels, from other components you might do better to use positive chemical ionization.
This will concentrate the ion current typically in the [M+H]+ ion at m/z 271 to give better sensitivity.
Esters typically eliminate the alcohol (MeOH; -32 amu) followed by ketene (CH2=C=O; -42 amu), to give the ion at m/z 197.

If you go this route, please let us know if it works (or not)!
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