Processes of the charge-stripping type: \[\text{M}^{+}+\text{X}\rightarrow \text{M}^{2+}+\text{X}+\text{e}\] occuring in a collision cell (containing a gas, X) located in a field-free region preceding a magnetic and electric sector combination placed in either order, may be detected as follows. If the instrument slits are wide and if the electric sector field E is set to half the value required to transmit the main ion-beam, the only ions to be transmitted will be those with a kinetic energy/charge ratio half, or almost exactly half, that of the main ion beam. The product ions of the charge-stripping process fulfil this condition. If the magnetic field B is scanned, a mass spectrum of such doubly-charged product ions, and thus of their singly-charged precursors, is obtained. Such a spectrum is called an E/2 mass spectrum. Interference from product ions from processes of the type: \[\text{M}_{1}^{+}+\text{X}\rightarrow \text{M}_{2}^{+}+\text{X}+\text{M}_{3}\] where m 1, m 2, and (m 1- m 2) are the masses of M 1 +, M 2 + and M 3, respectively, and where m 2 = 0.5 m 1, can arise in E/2 mass spectra.