Mass spectrum of a molecule showed molecular ion peak and its isotopic peak in 1:1 ratio. What is the possibility of the atom present in the molecule?

Since it is mentioned that it has a molecular ion peak it gives the fact that it must be aromatics, olefins and not alcohols or branched hydrocarbons since the latter have less probability to show molecular ion peaks . Since they contain isotopic peaks it must mean they have chloro or bromo compounds. But I do not understand how to use ratios to identify the atom present in the molecule

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    $\begingroup$ The abundance of the isotopes of Chlorine 35:37 is approx 3:1 and is characteristic of chlorinated compounds. $\endgroup$
    – Waylander
    Apr 30, 2019 at 7:17

1 Answer 1


Each element has different natural abundances for its isotopes. Two peaks with approximately 1:1 ratio indicates the presence of bromine. $\ce{^{79}Br}$ is the most common isotope, corresponding to $0.5069$ of naturally occurring bromine. The isotopic composition for $\ce{^{81}Br}$ is $0.4931$.

What this means is that, for a molecule contaning a single bromine atom, approximately, half of the molecular ions will have $\ce{^{79}Br}$ and the other half will have $\ce{^{81}Br}$, resulting in two peaks with roughly the same intensity with $m/Z$ differing by two units.


[1] https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses


  • $\begingroup$ In addition to the peak pattern indicating the presence of bromine, it also indicates the presence of a single bromine atom in the molecule. $\endgroup$
    – Curt F.
    Jul 8, 2019 at 15:25
  • $\begingroup$ @CurtF, thanks! Very good point. I'll edit that in answer. $\endgroup$ Jul 9, 2019 at 19:07

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