Processes of the charge-@S06054@ 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 @K03402@/charge ratio half, or almost exactly half, that of the main ion beam. The product ions of the charge-@S06054@ 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 \(\frac{E}{2}\) 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 \(\text{M}_{1}^{+}\), \(\text{M}_{2}^{+}\) and \(\text{M}_{3}\), respectively, and where \(m_{2}=0.5\ m_{1}\), can arise in \(\frac{E}{2}\) mass spectra.