The mass spectrum of alkyl halides is distinctive because of the existence of two Cl isotopes in a 3:1 ratio and two Br isotopes in a 1:1 ratio.

This isotopic effect generates two different molecular ions, observed in the mass spectra of corresponding alkyl halides in the form of a molecular-ion peak and a molecular-ion-plus-two peak, differing by two units.

For example, 2‐chloropropane shows two molecular-ion peaks with relative heights in a 3:1 ratio.

Similarly, 1‐bromopropane exhibits two peaks with relative heights in a 1:1 ratio.  

Notably, the base peak is formed by the heterolytic cleavage of the molecular ion's C–X bond, forming a cation and a halogen atom.

Moreover, alkyl chlorides undergo α cleavage producing two peaks in the mass spectra. This happens because the C–Cl and the C–C bonds have similar strengths, and the cation formed is stable as the positive charge is shared between two atoms.

Conversely, alkyl bromides do not undergo α cleavage as the C–C bond is weaker than the C–Br bond.