Separate molecular-ion evidence from fragmentation, calculate candidate masses precisely and interpret chlorine and bromine isotope clusters.
Read the axes and the ionisation method
A mass spectrum plots relative abundance against mass-to-charge ratio, m/z. In electron ionisation, loss of one electron gives a molecular radical cation, written M⁺•. It can fragment into smaller charged and neutral species. The instrument detects ions, not the neutral fragments.
For a singly charged molecular ion, m/z is numerically the ion’s relative mass. The base peak is simply the most abundant detected peak, conventionally scaled to 100. It is not necessarily the molecular ion. A molecular-ion peak may be weak or absent.
Do not automatically call the rightmost peak M. Isotope peaks can appear at M+1, M+2 and beyond. A different ionisation method can produce ions such as [M+H]⁺, whose mass includes the added proton, or multiply charged ions. Use the species and charge specified in the question.
Nominal mass can hide different formulae
Use the precise isotopic masses supplied, not rounded periodic-table averages, when calculating an accurate molecular mass. Multiply each atomic mass by its atom count and add. Retain precision until the requested final rounding.
For this original example use ¹²C = 12.0000, ¹H = 1.0078 and ¹⁶O = 15.9949. Three candidate formulas all have nominal mass 58, but their calculated accurate masses differ.
| Candidate | Calculation | Calculated mass |
|---|---|---|
| C₃H₆O | 3(12.0000) + 6(1.0078) + 15.9949 | 58.0417 |
| C₂H₂O₂ | 2(12.0000) + 2(1.0078) + 2(15.9949) | 58.0054 |
| C₄H₁₀ | 4(12.0000) + 10(1.0078) | 58.0780 |
Check both mass and charge in a fragmentation proposal
A fragment peak can suggest a part of the structure. For example, propanone’s molecular radical cation can form CH₃CO⁺ and a methyl radical. The charged fragment has nominal m/z 43; the neutral radical has nominal mass 15 and is not a separate detected ion in this fragmentation step.
Nominal m/z 43 alone is not unique: C₃H₇⁺ also has nominal mass 43. A convincing assignment should fit the parent formula, possible bond cleavage and other peaks.
Halogen patterns use isotope combinations
Using approximate natural abundances, ³⁵Cl:³⁷Cl is about 3:1 and ⁷⁹Br:⁸¹Br about 1:1. Replacing a light isotope by the heavy one shifts a singly charged ion peak by two m/z units. These patterns concern ions retaining the relevant halogen atoms, not necessarily every fragment.
| Halogens retained | Peak positions | Relative pattern |
|---|---|---|
| One Cl | M, M+2 | 3:1 |
| One Br | M, M+2 | 1:1 |
| Two Cl | M, M+2, M+4 | 9:6:1 |
| Two Br | M, M+2, M+4 | 1:2:1 |
| Three Cl | M, M+2, M+4, M+6 | 27:27:9:1 |
Diagram placeholder
Illustrative two-chlorine isotope cluster
Labels to include:
- m/z axis
- relative abundance axis
- 98: ³⁵Cl/³⁵Cl
- 100: mixed isotopes
- 102: ³⁷Cl/³⁷Cl
- heights 9:6:1
The pending diagram should show three labelled sticks spaced two m/z units apart with heights in the ratio 9:6:1. This is an idealised molecular-ion cluster for C₂H₄Cl₂, not a complete measured spectrum; smaller carbon-isotope features and fragments are omitted.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. A spectrum has a base peak at 43 and a molecular ion at 72, both singly charged. Which gives the molecular mass?Show answer
The molecular ion at m/z 72 gives the nominal molecular mass. The base peak at 43 is simply more abundant.
Q2. Calculate the accurate mass of C₄H₈O using C = 12.0000, H = 1.0078 and O = 15.9949.Show answer
4(12.0000) + 8(1.0078) + 15.9949 = 72.0573. Do not replace the supplied oxygen mass with 16.0.
Q3. A suitable molecular-ion cluster has peaks two units apart in a 1:1 ratio. What halogen pattern does this suggest?Show answer
One bromine atom in the ion, from approximately equal abundances of ⁷⁹Br and ⁸¹Br. Check the full spectrum and candidate formulas rather than treating the ratio as proof in every possible sample.
Q4. Why is the M+2 peak larger than the M+4 peak for a molecule containing two chlorine atoms?Show answer
M+2 corresponds to one ³⁵Cl and one ³⁷Cl, with two possible arrangements. M+4 requires two less-abundant ³⁷Cl atoms. Their approximate probabilities are 6/16 and 1/16.
Q5. Can accurate molecular mass distinguish butanal from 2-methylpropanal?Show answer
No. Both are C₄H₈O and have identical calculated accurate molecular masses. Structural information from other evidence is needed.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise: Organic Analysis — Coverage checklist, pp1–3; original examples and questions.
- AQA 7405 specification — 3.3.6.1–3.3.6.3 and required practical 6.
- AQA June 2023 AS Paper 2 mark scheme — Q01.3 p12: precise masses and decimal places.
- AQA June 2023 AS Paper 2 examiner report — Q01.3 p3: high-resolution reasoning.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.