A previous blog described how a sodiated ion peak can be used to locate or calculate the molecular ion for an unknown compound. In a similar fashion, the dimer ion peak can be used to identify the mass of the unknown even if the molecular ion is no visible.
The ESI+ MS data below shows 4 ion peaks at m/z 401.0, 422.9, 801.0 and 823.0. Assuming the ion peak at m/z 801.0 is 2M+H+, the weight of the unknown is (801.0-1.007) / 2 = 400.0 Da. The same holds true if the sodiated dimer ion (2M+Na+) is used: (823.0-22.989) / 2 = 400.0 Da.