Depending on sample preparation, adduct ions can be present on a mass spectrum. The goal of the elucidator is to identify whether an adduct ion(s) is present and its contribution to the mass (or elemental composition) of the unknown compound.
The example ESI+ mass spectrum below exhibits 2 ion clusters at m/z 951.305 and 973.287, corresponding to ions [M+H]+ and [M+Na]+ respectively. For this spectrum, sodiation is identified by taking the difference between the two masses at 21.982 g/mol and comparing it to the mass of a sodium ion at 22.989 g/mol minus the mass of a proton at 1.007 g/mol. The mass of the unknown at 950.298 g/mol is calculated by subtracting the contribution of the sodium adduct or the mass of a proton at 1.007 g/mol from the respective peak.