A mass spectrometer ionises a molecule and records the masses of the ions. The heaviest important peak gives the Mr, the fragments reveal the carbon skeleton, the tiny M+1 peak counts the carbon atoms and an M+2 peak gives away chlorine or bromine.
In the spectrometer a molecule loses an electron to form the molecular ion, M+. The M+ peak, at the highest m/z apart from its isotope peaks, gives the Mr. Some molecular ions break into a fragment ion and a neutral radical; only the ion is detected, so peaks such as 15 (CH3+), 29 (C2H5+ or CHO+) and 43 (C3H7+ or CH3CO+) reveal pieces of the structure. About 1.1% of carbon is 13C, so a small M+1 peak appears and the number of carbon atoms is n = 100 × (M+1 height) / (1.1 × M height). An M+2 peak one third the height of M shows one Cl atom (35Cl : 37Cl = 3 : 1); M and M+2 of equal height show one Br atom (79Br : 81Br = 1 : 1).
Build the mass spectrum of a molecule in the simulator. Find the molecular ion peak, then check which fragment ions appear and what was lost to make each one.
| m/z | Likely fragment ion | Often from |
|---|---|---|
| 15 | CH3+ | any compound with a methyl group |
| 29 | C2H5+ or CHO+ | ethyl groups; aldehydes |
| 31 | CH2OH+ | primary alcohols |
| 43 | C3H7+ or CH3CO+ | propyl groups; methyl ketones |
| 57 | C4H9+ or C2H5CO+ | butyl groups; ethyl ketones |
Every carbon atom has a 1.1% chance of being 13C, so the more carbon atoms in the molecule, the taller the M+1 peak relative to M.
| Height of M | Height of M+1 | n = 100 × (M+1) / (1.1 × M) | Carbon atoms |
|---|---|---|---|
| 30.0 | 0.66 | 66 / 33.0 = 2.00 | 2 |
| 25.0 | 1.10 | 110 / 27.5 = 4.00 | 4 (butanone, Fig. 1) |
| 50.0 | 2.75 | 275 / 55.0 = 5.00 | 5 |
| 12.5 | 0.82 | 82 / 13.75 = 5.96 | 6 (round to a whole number) |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What does the molecular ion peak of an organic compound show?
Which ion is most likely to cause a peak at m/z 15?
A compound shows peaks at m/z 64 and 66 with heights in the ratio 3 : 1. Which element does it contain?
Why does a small M+1 peak appear next to the molecular ion peak?
Chlorine and bromine each have two common isotopes two mass units apart, so a compound containing them shows a second molecular ion peak at M+2.
| Halogen in molecule | Isotopes | M : M+2 peak heights | Example |
|---|---|---|---|
| one Cl | 35Cl and 37Cl, 3 : 1 | 3 : 1 | C2H5Cl: m/z 64 and 66 |
| one Br | 79Br and 81Br, 1 : 1 | 1 : 1 | C2H5Br: m/z 108 and 110 |
| two Cl | three combinations | 9 : 6 : 1 (M : M+2 : M+4) | CH2Cl2: m/z 84, 86, 88 |
The M+ peak is the last significant peak before its own small isotope peaks, not the tallest (the tallest is the base peak). In the M+1 formula use peak heights, not m/z values, and put the M+1 height on top. Only ions are detected, never the radical lost. A 3 : 1 pair two units apart means chlorine; 1 : 1 means bromine, so do not swap them.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
The molecular ion peak of a compound has a relative height of 40.0 and the M+1 peak a height of 1.76. How many carbon atoms are in the molecule?
A ketone has Mr 86 and its mass spectrum has major peaks at m/z 57 and 29, but no peak at 43. What is the ketone?
Bromoethane, C2H5Br, is analysed. Which pair of molecular ion peaks, with their relative heights, is seen?
Which statement about the fragmentation M+ → CH3CO+ + C2H5• in butanone is correct?
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