The key peaks
- Molecular ion, M⁺: the peak with the largest m/z (ignoring small M+1 and M+2 peaks). Its m/z is the relative molecular mass.
- Base peak: the tallest peak, set to 100%. It's the most stable fragment, not necessarily the molecular ion.
- M+1 peak: a small peak one unit above M⁺, from molecules containing a ¹³C atom. Its height is about 1.1% of M⁺ for every carbon, so the number of carbons ≈ (height of M+1 ÷ height of M⁺) × 100 ÷ 1.1.
- M+2 peak: shows chlorine or bromine (see below).
Chlorine and bromine
| Halogen | Isotopes | M⁺ : M+2 | Example |
|---|---|---|---|
| Chlorine | ³⁵Cl and ³⁷Cl, about 3:1 | 3 : 1 | chloroethane: m/z 64 and 66 |
| Bromine | ⁷⁹Br and ⁸¹Br, about 1:1 | 1 : 1 | bromoethane: m/z 108 and 110 |
Common fragments
| m/z | Fragment | Usually from |
|---|---|---|
| 15 | CH₃⁺ | any methyl group |
| 29 | C₂H₅⁺ or CHO⁺ | ethyl groups; aldehydes |
| 31 | CH₂OH⁺ | primary alcohols |
| 43 | C₃H₇⁺ or CH₃CO⁺ | propyl groups; methyl ketones |
| 45 | COOH⁺ | carboxylic acids |
| 57 | C₄H₉⁺ or C₂H₅CO⁺ | butyl groups; ethyl ketones |
| 77 | C₆H₅⁺ | benzene rings |
| 91 | C₇H₇⁺ | a CH₂ attached to a benzene ring |
| 105 | C₆H₅CO⁺ | aromatic ketones and esters |
Also look at the gaps from M⁺: a loss of 15 is CH₃, 18 is H₂O (common for alcohols), 29 is C₂H₅ or CHO, 31 is OCH₃, and 45 is COOH.
Worked examples
M⁺ = 72, base peak at 43, strong C=O in the IR. M = 72 fits C₄H₈O. A peak at 43 suggests CH₃CO⁺, and 72 − 43 = 29, a C₂H₅ group. So the compound is likely butanone, CH₃COCH₂CH₃.
The nitrogen rule
A molecule with an odd molecular mass contains an odd number of nitrogen atoms. If M⁺ is even, there are zero or two (or another even number of) nitrogens.
Quick answers
What is the molecular ion peak?
The molecular ion peak (M⁺) is the peak with the highest m/z value, apart from small isotope peaks. Its m/z equals the relative molecular mass of the compound.
What does an M+2 peak mean?
An M+2 peak two units above the molecular ion shows chlorine or bromine. Chlorine gives an M:M+2 ratio of about 3:1, and bromine about 1:1.
What is the base peak in mass spectrometry?
The base peak is the tallest peak in the spectrum, given a relative abundance of 100%. It comes from the most stable or most common fragment ion.
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