Which one of the following statements about the mass spectrum of CH₃Br is correct?
- Last two peaks are of equal size at m/z values of 94 and 96
- Last two peaks are in the ratio of 3:1 at m/z value of 94 and 96
- Just one peak for molecular ion at m/z values of 95
- Just one peak for molecular ion at m/z value of 44
Explanation
CH₃Br molecular ion peaks come from bromine isotopes.
Br has 2 stable isotopes: ⁷⁹Br ≈ 50.7% and ⁸¹Br ≈ 49.3% → basically 1:1 ratio
So CH₃Br gives 2 M⁺ peaks:
1) ¹²C + 3(¹H) + ⁷⁹Br = 12 + 3 + 79 = m/z 94
2) ¹²C + 3(¹H) + ⁸¹Br = 12 + 3 + 81 = m/z 96
Because the isotopes are ~equal in nature, the two peaks are equal size/height.
Last verified on 05-06-2026
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