Use the molecular ion and fragments
Use the M⁺ peak to infer molecular mass and simple fragment peaks to suggest parts of an organic structure.
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. Use the functional-group data supplied by Cambridge; knowledge of how a mass spectrometer works is not required. Spectra and values in these questions are illustrative. Written practice is self-checked
Before you begin
- Recognise the functional groups from Topics 13–20 and calculate relative mass and weighted means.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does an identified M⁺ peak give for a singly charged molecular ion?
What you will learn
- Identify the molecular ion peak
- Deduce molecular mass from M⁺
- Suggest simple fragment-ion identities from m/e values
Use M⁺ for the molecular mass
The molecular ion peak, M⁺, represents the molecule after loss of one electron. For a singly charged molecular ion, its m/e value gives the molecular mass of the organic molecule.
Do not assume the tallest peak is M⁺. The most abundant peak may be a fragment; identify the molecular ion from the spectrum and context supplied.
Treat fragments as structural clues
A fragment peak is produced when a molecular ion breaks into smaller ions. Compare its m/e value with plausible fragment formulas and the molecular structure. For example, a CH₃CO⁺ fragment has m/e 43.
A fragment assignment should be simple and compatible with the molecule's atoms and mass. A peak alone may fit more than one ion, so use the molecular ion and other evidence rather than claiming a unique structure too early.
Pause and explain
Which statement about the tallest peak is safest?
Worked example
A spectrum has M⁺ at m/e 58 and a prominent fragment at m/e 43. Suggest the molecular mass and one possible fragment ion.
Show the worked solution
- Read the molecular ion separately from the tallest fragment.
- M⁺ at 58 gives molecular mass 58.
- A fragment at 43 may be CH₃CO⁺ if compatible with the molecule.
Answer Molecular mass = 58. A possible m/e 43 fragment is CH₃CO⁺; the assignment should be checked against the molecule's structure.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A spectrum identifies M⁺ at m/e 72. Deduce the molecular mass.
Give me a hint
Read the labelled molecular ion, not the most intense fragment.
Compare my reasoning
- M⁺ is singly charged in this interpretation.
- Use its m/e value.
- State the relative molecular mass.
The molecular mass is 72.
Look for these in your work
- Correct peak used
- m/e interpreted
- Answer stated as molecular mass
A molecule has M⁺ at m/e 58 and a fragment at m/e 43. Give one plausible formula for the m/e 43 ion and explain how you would check it.
Give me a hint
Use a simple fragment with total relative mass 43 and check its atoms occur in the molecule.
Compare my reasoning
- CH₃CO⁺ has mass 12+3+12+16 = 43.
- Compare its atoms with the supplied molecular formula or structure.
- Treat it as a possible assignment, not automatic proof.
CH₃CO⁺ is a plausible m/e 43 fragment. Check that the parent molecule contains the required C₂H₃O composition and that other spectral evidence is consistent.
Look for these in your work
- Ion mass adds to 43
- Parent composition checked
- Assignment qualified
The highest-intensity peak is at m/e 43, while a smaller peak is labelled M⁺ at m/e 58. A student says the molecular mass is 43. Correct the conclusion.
Give me a hint
Use the molecular-ion label rather than peak height.
Compare my reasoning
- The m/e 43 peak is more abundant.
- M⁺ is at m/e 58.
- The labelled molecular ion gives molecular mass 58.
The molecular mass is 58. The m/e 43 base peak is a fragment, not the molecular ion.
Look for these in your work
- Peak abundance distinguished from identity
- M⁺ located
- Correct mass stated
Common mistakes
- Using the base peak as M⁺ automatically
- Calling a fragment's m/e the molecular mass
- Suggesting a fragment with more atoms or mass than the molecule
- Claiming one fragment uniquely proves a whole structure
What can you explain now?
What does an identified M⁺ peak give for a singly charged molecular ion?
Compare with the explanation
The molecular mass
Its m/e value corresponds to the molecule's relative mass.
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Make it stick
How do the molecular-ion peak and fragment peaks answer different questions about a molecule?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.