Read mass-spectrum peaks and isotope abundances
Interpret m/e values and relative peak abundances as evidence about ions and isotopes, without describing instrument operation.
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 quantity is shown on the horizontal axis of a mass spectrum?
What you will learn
- Read an m/e value and relative peak intensity
- Recognise isotopic peaks
- Use isotope abundance patterns as chemical evidence
Read each peak as an ion signal
A mass spectrum plots relative abundance against m/e, the mass-to-charge ratio used by the syllabus. For ions carrying a single positive charge, the numerical m/e value corresponds to the ion's relative mass.
The vertical intensity is relative abundance, not automatically a percentage of the original sample. Compare peak heights or supplied percentages as stated in the question. The molecular ion and fragment ions have different roles in interpreting the spectrum.
Isotopes create related peaks
Isotopes of the same element have different masses but similar chemical behaviour. A molecule containing such an element can produce molecular-ion peaks separated by two m/e units, with relative intensities that reflect natural isotope abundances.
The AS outcome is spectrum analysis: the syllabus explicitly does not require knowledge of how a mass spectrometer works. Focus on the supplied m/e values and abundance pattern.
Pause and explain
What can two molecular-ion peaks separated by two m/e units indicate?
Worked example
A spectrum shows peaks at m/e 35 and 37 with relative abundances 75 and 25. What does the pattern suggest?
Show the worked solution
- The peaks are two units apart.
- Their abundance ratio is 75:25, or 3:1.
- Compare this with the supplied chlorine isotope pattern.
Answer The 3:1 pattern is consistent with the ³⁵Cl and ³⁷Cl isotopes of chlorine.
Try a topic test
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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 has molecular-ion peaks at m/e 86 and 88 in an intensity ratio of about 3:1. What element is suggested?
Give me a hint
Use the two-unit spacing and abundance ratio.
Compare my reasoning
- The masses differ by two.
- A 3:1 molecular-ion pair is characteristic of chlorine's common isotopes.
- State that the pattern suggests, rather than uniquely proves, the element.
The pattern suggests one chlorine atom, from the ³⁵Cl and ³⁷Cl isotopes.
Look for these in your work
- Spacing noted
- Ratio interpreted
- Inference appropriately qualified
A peak at m/e 43 is twice as high as a peak at m/e 29. What does the height comparison tell you, and what does it not tell you by itself?
Give me a hint
Read peak height as relative abundance, but avoid assigning a structure without context.
Compare my reasoning
- The m/e 43 ion is more abundant in this spectrum.
- The ratio is 2:1 if both heights share the same scale.
- Peak height alone does not uniquely identify either ion.
The m/e 43 ion has twice the relative abundance of the m/e 29 ion in the displayed spectrum; the heights alone do not uniquely establish their structures.
Look for these in your work
- Relative abundance compared
- Ratio stated
- Structural limitation explained
A student explains a spectrum by describing how ions are accelerated through the instrument. What should the answer focus on for this syllabus outcome?
Give me a hint
The syllabus asks for spectrum interpretation, not instrument operation.
Compare my reasoning
- Read the m/e values.
- Compare isotopic peak spacing and abundance.
- Use those data to infer mass or isotope evidence.
Focus on m/e values and relative isotopic abundances. Knowledge of how the mass spectrometer works is not required.
Look for these in your work
- Scope identified
- Relevant evidence named
- Out-of-scope explanation removed
Common mistakes
- Calling m/e the total mass of a neutral molecule for every peak
- Treating relative intensity as a percentage without checking the question
- Claiming the spectrum proves an entire structure from one peak
- Describing instrument operation that is outside the syllabus requirement
What can you explain now?
What quantity is shown on the horizontal axis of a mass spectrum?
Compare with the explanation
m/e
The syllabus uses mass-to-charge ratio values.
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Make it stick
What can peak position and relative abundance tell you, and what limit should you keep in mind?
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.