1-bromodecane (C10H21Br) produces a mass spectrum with a parent molecular ion (M ) peak at m/z 220 (100%). A peak at m/z 222 (98%) also appears in the spectrum. What causes this signal in the mass spectrum

The presence of Br atom produces 2 ions (M and M+2) with the same abundance.

Explanation:

Mass spectrometry is an instrumental technique in which chemical substances are ionized and fragmented. The nature of the substances produce different fragmentations.

A way to know if a substance contains bromine is seeing the parent molecular ion (M, Subtance with ⁷⁹Br) and M+2 (Substance with ⁸¹Br) with the same relative abundance. Where parent molecular ion is the substance without fragmentations

That is because in nature, the isotope abundance of bromine is ⁷⁹Br with 51% and ⁸¹Br with 49%, producing the 49% of ions having  2 units of m/z higher than its consecutive ion.