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Take this nicotine molecule:

enter image description here

And this mass spectrum:

enter image description here

I'm not sure how to explain all the peaks.

This might explain the peak at 42:

enter image description here

But can this peek als be explained by something else in the molecule?

Like the decomposition of the ring with the double bonds: enter image description here

This might explain the peak at 84:

enter image description here

And I'm clueless on the peak at 133

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The peak at 133 corresponds to a loss of 29 mass units (nicotine is 162). N-CH3 has a mass of 29

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  • $\begingroup$ How about the others? Do you think my guesses were correct? $\endgroup$ – Pickering Jan 26 '14 at 4:35
  • $\begingroup$ How can you know whether the peak at 42 is the decomposition of the ring with the double bonds or the ring without the double bonds? $\endgroup$ – Pickering Jan 26 '14 at 9:18
  • $\begingroup$ Yes, I think your other guesses are reasonable. As Klaus points out, you would need to look at the mass spectrum of some model compounds and/or some isotopically labeled compounds to know for sure. Also, especially in larger molecules, if you have a high resolution mass spectrometer that can help, because if you calculate the mass to 6 decimal places, different fragments with the same mass - but different formula - will give rise to different exact masses. As to which ring is fragmenting, the pyridine ring is aromatic and therefor has more stability then the saturated $\endgroup$ – ron Jan 26 '14 at 14:32
  • $\begingroup$ pyrolidine ring, making the pyridine less likely to crack. $\endgroup$ – ron Jan 26 '14 at 14:33
  • $\begingroup$ "pyrolidine ring, making the pyridine less likely to crack" How can you explain the fact that altghouh stable, there is no peak in the MS for 78... I would expect it to be the base peak. $\endgroup$ – Erez Berman Apr 17 '18 at 6:38
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I think that your assignment is reasonable.

As ron has already pointed out, cleavage of an endocyclic $\ce{C-N}$ bond (= opening of the pyrolidine ring) in nicotine is likely. The resulting open chain fragment then loses $\ce{NCH3}$.

The peak at 84 represents a N-methylpyrolidine fragment and indicates that the bond between the two heterocycles is cleaved. This also makes sense.

In general, it it a good idea to work from higher to lower masses and not to spend to much brain on the very small fragments.

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  • $\begingroup$ How can you know whether the peak at 42 is the decomposition of the ring with the double bonds or the ring without the double bonds? $\endgroup$ – Pickering Jan 26 '14 at 9:17
  • $\begingroup$ @Pickering You might be able to rule that out by comparison with the MS of pyridine. Further experimental proof could involve the specific isotopic labelling of the starting material. $\endgroup$ – Klaus-Dieter Warzecha Jan 26 '14 at 9:27

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