How does mass spectrometry reveal relative molecular mass and structural fragments of an organic compound?
Interpret a mass spectrum to identify the molecular ion and relative molecular mass, deduce fragments from peaks, and explain isotope patterns including the M+2 peak of chlorine and bromine compounds
A focused answer to the H2 Chemistry learning outcome on mass spectrometry. Identifying the molecular ion and relative molecular mass, deducing common fragments from mass differences, and interpreting isotope patterns such as the characteristic M and M+2 peaks of chlorine and bromine compounds.
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What this dot point is asking
SEAB wants you to interpret a mass spectrum: identify the molecular ion peak and read off the relative molecular mass, deduce fragments from the mass differences between peaks, and explain isotope patterns, especially the M and M+2 peaks that reveal chlorine and bromine. Reading and the chlorine/bromine isotope pattern are dependable exam tasks.
The answer
How a mass spectrometer works (in brief)
The sample is vaporised and ionised (usually by electron impact) to form positive ions, which are accelerated and separated by their mass-to-charge ratio (). The detector records the abundance of each ion, producing the spectrum of peaks against .
The molecular ion and relative molecular mass
When a molecule loses one electron it forms the molecular ion (). The peak at the highest (ignoring isotope peaks) is the molecular ion peak, and its value gives the relative molecular mass directly. Identifying is usually the first step in any structure determination.
Fragmentation
The molecular ion can break apart, giving fragment ions at lower . The mass difference between a fragment and the molecular ion reveals the group lost. Common losses:
| Mass lost | Group lost |
|---|---|
| 15 | (methyl) |
| 17 | OH |
| 29 | CHO or |
| 31 | |
| 45 | COOH |
Common fragment ions include (15), (29) and the acylium ion (43).
Isotope patterns: chlorine and bromine
Some elements have more than one common isotope, producing extra peaks:
- Chlorine has and in a roughly 3:1 ratio. A molecule with one chlorine shows two molecular ion peaks two units apart (M and M+2) in a 3:1 ratio.
- Bromine has and in a roughly 1:1 ratio. A molecule with one bromine shows M and M+2 peaks of roughly equal height.
So an M and M+2 pattern is a strong clue to the presence of chlorine (3:1) or bromine (1:1).
Examples in context
Example 1. Confirming a molecular formula. Once the relative molecular mass is read from the molecular ion, it can be checked against a candidate molecular formula from combustion or other data. SEAB often gives the molecular ion in a structure-determination question so that candidates can confirm before interpreting the IR and NMR spectra.
Example 2. Spotting a halogen at a glance. Forensic and environmental chemists use the M and M+2 isotope pattern to flag chlorine- or bromine-containing pollutants quickly, because the distinctive peak ratios stand out in the spectrum. This applied use illustrates why SEAB emphasises the isotope-pattern interpretation.
Try this
Q1. A mass spectrum has its highest non-isotope peak at . State the relative molecular mass and suggest the compound. [2 marks]
- Cue. ; consistent with ethanol ().
Q2. Explain why a compound containing one chlorine atom shows two molecular ion peaks in a 3:1 ratio. [2 marks]
- Cue. Chlorine has and in a 3:1 abundance, giving M and M+2 peaks in that ratio.
Q3. A molecular ion at gives a fragment at . Identify the group lost. [1 mark]
- Cue. Mass lost , a methyl group ().
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Specimen (9729)4 marksThe mass spectrum of an organic compound shows a molecular ion peak at m/z = 58 and a strong fragment peak at m/z = 43. Suggest the relative molecular mass, identify the fragment lost, and suggest a structure consistent with the data.Show worked answer →
The molecular ion peak gives the relative molecular mass directly: Mr = 58.
The mass lost in fragmenting from 58 to 43 is 58 - 43 = 15, which corresponds to the loss of a methyl group (CH3, mass 15).
A fragment at m/z = 43 could be CH3CO+ (the acylium ion) or C3H7+.
A structure consistent with Mr = 58 and loss of CH3 to give CH3CO+ is propanone, CH3COCH3, which fragments to CH3CO+ (43) by losing CH3 (15).
Markers reward Mr = 58 from the molecular ion, the loss of 15 identified as CH3, the fragment at 43 (CH3CO+), and a consistent structure such as propanone.
2023 (style)3 marksThe mass spectrum of a chloro compound shows two molecular ion peaks at m/z = 78 and m/z = 80 in an approximate 3:1 ratio. Explain this pattern and state what it indicates about the molecule.Show worked answer →
Chlorine has two isotopes, chlorine-35 and chlorine-37, in an approximate 3:1 abundance ratio.
A molecule containing one chlorine atom therefore shows two molecular ion peaks two mass units apart (M and M+2): the M peak (78) contains chlorine-35, and the M+2 peak (80) contains chlorine-37.
The 3:1 ratio of the two peaks matches the 3:1 abundance of the two isotopes, confirming the molecule contains exactly one chlorine atom.
Markers reward the two chlorine isotopes with the 3:1 ratio, the M and M+2 peaks two units apart, and the conclusion of one chlorine atom.
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