How to read a mass spectrum (with examples)
This guide shows you how to interpret an electron-ionisation (EI) mass spectrum step by step: find the molecular ion, use isotope patterns and the nitrogen rule, identify the base peak and decode fragmentation. Every spectrum is annotated and redrawn from NIST reference data.
On this page
What a mass spectrum shows
In electron-ionisation (EI) mass spectrometry, a molecule is hit by 70 eV electrons, loses one electron and becomes a radical cation, the molecular ion, M⁺•. Many molecular ions break apart; the mass spectrometer sorts the positive ions by mass-to-charge ratio (m/z). Because almost all ions carry a +1 charge, m/z is simply the ion's mass.
The spectrum is a bar chart: x-axis = m/z, y-axis = relative abundance, scaled so the tallest peak is 100%. Neutral fragments (radicals and small molecules) are not detected, so you work them out from the gaps between peaks.
How to read a mass spectrum in 7 steps
- Find the molecular ion (M⁺•). It is usually the highest-m/z peak that is not an isotope peak. It may be small (alcohols, branched alkanes) or absent; aromatic compounds give strong M⁺•.
- Apply the nitrogen rule. An odd M means an odd number of nitrogen atoms; an even M means zero or an even number of N.
- Read the isotope peaks. M+2 about ⅓ of M means one Cl; M and M+2 almost equal means one Br; M+2 about 4.4% means S. The M+1 height ÷ 1.1% estimates the number of carbons.
- Identify the base peak. The tallest peak (100%) is the most stable ion, often a clue to the main functional group.
- Calculate the losses from M. M−15 (CH₃), M−18 (H₂O), M−29 (C₂H₅ or CHO), M−43 (C₃H₇ or CH₃CO)… see the table below.
- Look for characteristic ions: 43 (CH₃CO⁺ or C₃H₇⁺), 77 (C₆H₅⁺), 91 (tropylium C₇H₇⁺), 105 (C₆H₅CO⁺), 31 (CH₂=OH⁺), 30 (CH₂=NH₂⁺).
- Propose a structure and check it with the molecular formula, degree of unsaturation, IR and NMR data.
Molecular ion peak and the nitrogen rule
The molecular ion gives the nominal molecular mass (using ¹²C = 12, ¹H = 1, ¹⁶O = 16, ¹⁴N = 14). With a high-resolution instrument you get the exact mass (e.g. 72.0575 for C₄H₈O) and can fix the formula with the molecular formula finder or the exact mass calculator.
Nitrogen rule: nitrogen has an even mass (14) but an odd valence, so a molecule with an odd number of N atoms has an odd nominal mass. Pyridine C₅H₅N = 79; aniline C₆H₇N = 93; ethanol C₂H₆O = 46 (even, no N).
What is the base peak in a mass spectrum?
The base peak is the tallest peak in the spectrum; all other intensities are expressed as a percentage of it. It is the most abundant ion, not necessarily the molecular ion. In 2-butanone the base peak is m/z 43 (CH₃CO⁺); in ethanol it is m/z 31 (CH₂=OH⁺); in chlorobenzene the molecular ion (m/z 112) is also the base peak, because the aromatic ring stabilises it.
Isotope patterns: M+1 and M+2 peaks (Cl, Br, S)
| Element | Isotopes (natural abundance) | Pattern you see |
|---|---|---|
| C | ¹²C 98.9%, ¹³C 1.1% | M+1 ≈ 1.1% × number of C |
| Cl | ³⁵Cl 75.8%, ³⁷Cl 24.2% | M : M+2 ≈ 3 : 1 (one Cl); 9 : 6 : 1 (two Cl) |
| Br | ⁷⁹Br 50.7%, ⁸¹Br 49.3% | M : M+2 ≈ 1 : 1 (one Br); 1 : 2 : 1 (two Br) |
| S | ³²S 95.0%, ³⁴S 4.2% | M+2 ≈ 4.4% of M per S |
| N, O, F, I, P | essentially one isotope (¹⁹F, ¹²⁷I, ³¹P) or small minor isotopes | no strong M+2; iodine gives a large mass gap (M−127) |
Use the M+1 peak to estimate carbon count: n(C) ≈ (height of M+1 ÷ height of M) × 100 ÷ 1.1. For chlorobenzene, 6.6 ÷ 100 × 100 ÷ 1.1 ≈ 6 carbons.
Mass spectrometry fragmentation: how molecules break
Fragmentation favours pathways that give the most stable cation (tertiary > secondary > primary; resonance-stabilised acylium, allyl, benzyl, oxonium and iminium ions).
Alpha-cleavage
The bond next to the carbon bearing a heteroatom (O, N) or next to a C=O breaks. Ketones give acylium ions (R–C≡O⁺); alcohols give oxonium ions (CH₂=OH⁺ at m/z 31 for primary alcohols); amines give iminium ions (CH₂=NH₂⁺ at m/z 30). The larger alkyl group is lost preferentially.
McLafferty rearrangement
A carbonyl compound with a hydrogen on the γ-carbon transfers that H to the C=O oxygen through a six-membered transition state and loses an alkene. The product is an even-mass radical cation (for compounds without N): methyl ketones give m/z 58, aldehydes m/z 44, methyl esters m/z 74, carboxylic acids m/z 60.
Other common patterns
- Alkanes: clusters 14 apart (29, 43, 57, 71…); branching weakens M⁺•.
- Alcohols: weak M⁺•, M−18 (water loss), α-cleavage ions.
- Alkylbenzenes: benzylic cleavage to the tropylium ion, m/z 91.
- Halides: loss of X• (M−35/37, M−79/81, M−127) and the isotope pattern.
Common fragment losses and ions (fragmentation table)
| Loss from M | Neutral lost | Suggests |
|---|---|---|
| M−1 | H• | aldehydes, alcohols, amines |
| M−15 | CH₃• | methyl group (branch, methyl ketone) |
| M−17 | OH• | carboxylic acids |
| M−18 | H₂O | alcohols |
| M−26 | C₂H₂ | aromatic rings |
| M−28 | CO or C₂H₄ | ketones, phenols / ethyl groups, McLafferty |
| M−29 | C₂H₅• or CHO• | ethyl group, aldehydes |
| M−31 | CH₃O• | methyl esters, methyl ethers |
| M−35/37 | Cl• | chlorides |
| M−43 | C₃H₇• or CH₃CO• | propyl group, methyl ketones |
| M−45 | C₂H₅O• or COOH• | ethyl esters, carboxylic acids |
| M−79/81 | Br• | bromides |
| M−127 | I• | iodides |
| Common ion (m/z) | Formula | Typical source |
|---|---|---|
| 15, 29, 43, 57, 71 | CₙH₂ₙ₊₁⁺ | alkyl chains |
| 30 | CH₂=NH₂⁺ | primary amines |
| 31 | CH₂=OH⁺ | primary alcohols, ethers |
| 43 | CH₃CO⁺ | methyl ketones, acetates |
| 44 / 58 / 60 / 74 | McLafferty ions | aldehydes / methyl ketones / acids / methyl esters |
| 77 | C₆H₅⁺ | monosubstituted benzenes |
| 91 | C₇H₇⁺ (tropylium) | benzyl compounds, alkylbenzenes |
| 105 | C₆H₅CO⁺ | benzoyl compounds |
Worked examples
Example 1: 2-butanone (Figure 1)
M⁺• = 72 (even → 0 N). M+1 ≈ 1.1 of 25 → ≈ 4 C. Formula C₄H₈O fits 72 with one degree of unsaturation (C=O, confirmed by IR at 1715 cm⁻¹). Base peak 43 = 72 − 29: α-cleavage losing C₂H₅•. Small 57 = 72 − 15: loss of CH₃•. Both acylium ions point to CH₃–CO–CH₂CH₃.
Example 2: bromoethane (Figure 3)
Two peaks of equal height at 108 and 110 → one Br. 108 − 79 = 29 → C₂H₅. Strong m/z 29 confirms loss of Br•.
Example 3: chlorobenzene (Figure 2)
112 : 114 = 3 : 1 → one Cl. 112 − 35 = 77 → C₆H₅. A large M⁺• and m/z 77, 51 (C₄H₃⁺, loss of C₂H₂ from 77) are typical of a benzene ring.
Example 4: ethanol (Figure 4)
Even M = 46; base peak 31 = 46 − 15 shows CH₃ next to a CH₂–OH carbon (α-cleavage). Compare dimethyl ether (also 46), which gives a base peak at m/z 45 and only a weak 31, a reminder to confirm with IR (O–H) and NMR.
Check your answer with MolDraw tools
Draw a candidate in the mass spectrum predictor to see its fragments, find formulas from an exact mass with the molecular formula finder, and confirm functional groups with the IR guide and the IR chart. For the C–H framework, continue with how to read an NMR spectrum.
Practice quiz
Tap an answer to check it.
1. A spectrum shows peaks at m/z 156 and 158 of nearly equal height. Which element is present?
⁷⁹Br and ⁸¹Br are almost 1:1, giving M and M+2 of equal height (bromobenzene, C₆H₅Br, M = 156).
2. The molecular ion is at m/z 73. What can you conclude?
Nitrogen rule: odd nominal mass → odd number of N atoms (e.g. C₄H₁₁N, butylamine).
3. A methyl ketone gives a strong even-mass peak at m/z 58. Which process forms it?
McLafferty rearrangement of a methyl ketone with a γ-hydrogen gives the enol radical cation CH₂=C(OH)CH₃⁺•, m/z 58.
4. A peak at M−18 most likely indicates…
Loss of 18 mass units = H₂O, typical of alcohols.
5. An alkylbenzene shows a base peak at m/z 91. This ion is…
Benzylic cleavage gives C₇H₇⁺, which rearranges to the aromatic tropylium ion, m/z 91.
6. M = 100 and M+1 = 6.6 (relative). Roughly how many carbons?
6.6 ÷ 100 × 100 ÷ 1.1 ≈ 6 carbons.
Frequently asked questions
How do I interpret a mass spectrum?
Find the molecular ion (highest m/z, not an isotope peak), apply the nitrogen rule, read M+1/M+2 isotope peaks for C, Cl, Br and S, identify the base peak, then work out neutral losses from M (e.g. M−15 = CH₃, M−18 = H₂O) and match characteristic ions such as 43, 77 and 91.
What is the base peak in a mass spectrum?
The tallest peak, set to 100% relative abundance. It is the most abundant (usually most stable) ion and is often a fragment rather than the molecular ion.
What is the difference between the molecular ion and the base peak?
The molecular ion M⁺• is the intact molecule minus one electron and gives the molecular mass. The base peak is simply the most intense peak; sometimes they are the same peak (e.g. chlorobenzene, m/z 112).
How do you read a mass spectrometry chart?
The x-axis is the mass-to-charge ratio (m/z) and the y-axis is the relative abundance (%) compared with the base peak. Each bar is an ion; differences between bars correspond to neutral fragments that were lost.
What does an M+2 peak mean?
A peak two units above M from a heavier isotope. M+2 about one-third of M indicates chlorine; M and M+2 nearly equal indicates bromine; about 4% suggests sulfur.
Can you explain mass spectrometry to a beginner?
A mass spectrometer turns molecules into charged ions, breaks some of them into pieces and weighs each piece. The resulting bar chart tells you the mass of the whole molecule and of its fragments, which you piece together like a puzzle to deduce the structure.
What is the nitrogen rule?
A compound with an odd nominal molecular mass contains an odd number of nitrogen atoms; an even mass means zero or an even number of nitrogens.
References and data sources
- NIST Chemistry WebBook, SRD 69: EI mass spectra of 2-butanone, bromoethane, chlorobenzene, ethanol and 2-hexanone (webbook.nist.gov).
- SDBS Spectral Database for Organic Compounds, AIST Japan.
- McLafferty, F. W.; Tureček, F. Interpretation of Mass Spectra, 4th ed., University Science Books, 1993.
- Pavia, Lampman, Kriz & Vyvyan, Introduction to Spectroscopy, 5th ed., Cengage, ch. 8.
- Silverstein, Webster & Kiemle, Spectrometric Identification of Organic Compounds, 8th ed., Wiley, ch. 1.
- Isotope abundances: IUPAC CIAAW, isotopic compositions of the elements.
Spectra on this page are redrawn schematically from the reference data above (peak positions and approximate relative intensities) for teaching; check the original database entry before citing exact intensities.