Nitrosamines are compounds with the general
structure R₂N–N=O. In mass spectrometry, nitrosamines have some
characteristic fragmentation patterns due to the relatively weak N–N and
N–O bonds.
Here’s the typical mass spectral behaviour of nitrosamines:
1. Molecular Ion (M⁺)
- You usually observe a molecular
ion peak (M⁺) for nitrosamines, but it might be weak
because these molecules fragment easily.
- The M⁺
corresponds to the intact nitrosamine molecule.
2. Loss of NO (Nitric Oxide)
- A very common and strong
fragmentation is the loss of NO (30 Da) from the molecular ion.
- So you will see a
significant peak at [M–30]⁺.
3. Further Fragmentations
- After losing NO, the
resulting amine fragment (R₂N⁺)
can fragment further:
- Cleavage of alkyl groups (R
groups)
leading to characteristic smaller ions.
- Formation of iminium ions
(R–N⁺=CHR'), particularly if the R groups are simple
alkyl chains.
For
example, in simple nitrosamines like dimethylnitrosamine (CH₃)₂N–NO:
- M⁺ at 74
m/z (molecular weight 74)
- Loss of NO gives fragment at
44 m/z (74–30 = 44), corresponding to (CH₃)₂N⁺.
4. Important Peaks Summary
|
Fragment |
Description |
Observed m/z (example for Dimethylnitrosamine) |
|
M⁺ |
Molecular
ion |
74 |
|
M–30 |
Loss of
NO (forming amine cation) |
44 |
|
Others |
Alkyl
cleavage / rearrangement |
Variable
depending on R groups |
5. Special Notes
- NO loss is extremely diagnostic. If
you see a consistent loss of 30 Da in nitrosamine-related compounds, it's
a strong clue.
- In higher nitrosamines
(like N-nitrosodiethylamine, N-nitrosodibutylamine, etc.), you will still
observe a major M–30 peak.