Identify chlorine and bromine from M+2 peaks
Use the two-unit isotope spacing and characteristic abundance ratios to recognise chlorine or bromine atoms.
Section 22 · Analytical techniques
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 M⁺:[M+2]⁺ ratio suggests one chlorine atom?
What you will learn
- Locate M⁺ and [M+2]⁺ peaks
- Use the chlorine 3:1 pattern
- Use the bromine 1:1 pattern and qualify the inference
Look for a peak two units above M⁺
A molecule containing a halogen isotope can show a molecular-ion partner two m/e units higher. Compare the intensity of [M+2]⁺ with M⁺ rather than using the spacing alone.
For one chlorine atom, the M⁺:[M+2]⁺ pattern is approximately 3:1 because ³⁵Cl is about three times as abundant as ³⁷Cl. For one bromine atom the two molecular-ion peaks are approximately 1:1 because ⁷⁹Br and ⁸¹Br have similar abundances.
State what the pattern suggests
A 3:1 M⁺:[M+2]⁺ pattern suggests one chlorine atom; a 1:1 pair suggests one bromine atom. Use the molecular-ion region and other supplied evidence to check the assignment.
These simple ratios describe one halogen atom. More than one halogen can produce a more complex pattern; do not force a single-atom ratio onto every spectrum.
Pause and explain
What pattern is typical for one bromine atom?
Worked example
A molecule has M⁺ and [M+2]⁺ peaks with nearly equal intensities. Which halogen is suggested?
Show the worked solution
- The peaks are separated by two m/e units.
- Compare the intensity ratio with the isotope patterns.
- A 1:1 pair is characteristic of bromine.
Answer One bromine atom is suggested by the approximately 1:1 M⁺:[M+2]⁺ pattern.
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 molecular-ion pair is separated by two m/e units and has intensities 150 and 50. Which halogen is suggested if one atom is present?
Give me a hint
Write the M⁺:[M+2]⁺ ratio, then match it.
Compare my reasoning
- 150:50 simplifies to 3:1.
- A single chlorine atom gives about 3:1.
- State the inference as a suggestion.
One chlorine atom is suggested by the 3:1 M⁺:[M+2]⁺ ratio.
Look for these in your work
- Ratio simplified
- Correct halogen matched
- Single-atom assumption stated
A molecular-ion pair has intensities 48 and 50 at m/e values separated by two. What halogen is suggested?
Give me a hint
The nearly equal peak heights indicate a 1:1 pattern.
Compare my reasoning
- Compare the intensities.
- A 1:1 pattern matches ⁷⁹Br and ⁸¹Br.
- Check that the peaks are the molecular-ion pair.
One bromine atom is suggested by the approximately 1:1 M⁺:[M+2]⁺ pattern.
Look for these in your work
- Spacing checked
- Ratio matched
- Inference qualified
A spectrum containing two halogen atoms does not show a simple single pair. Why should you not assign it using only the one-atom ratio?
Give me a hint
Think about the combinations of isotopes when more than one halogen is present.
Compare my reasoning
- Each halogen atom can be one of two isotopes.
- Several combinations can produce M, M+2 and M+4 peaks.
- The full pattern and other evidence are needed.
Two halogen atoms can produce a more complex isotope cluster, such as M⁺, [M+2]⁺ and [M+4]⁺. The single-atom 3:1 or 1:1 shortcut is not enough; interpret the complete pattern.
Look for these in your work
- Multiple isotope combinations recognised
- Possible M+4 peak explained
- Single-atom rule limited
Common mistakes
- Using peak spacing without the abundance ratio
- Swapping chlorine and bromine patterns
- Calling a 3:1 ratio bromine
- Applying the single-atom pattern unchanged to multiple halogens
What can you explain now?
What M⁺:[M+2]⁺ ratio suggests one chlorine atom?
Compare with the explanation
About 3:1
³⁵Cl is about three times as abundant as ³⁷Cl.
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Make it stick
How do peak spacing and relative intensity distinguish a one-chlorine pattern from a one-bromine pattern?
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.