Processes of the charge-stripping type: $${\text{M}}^{+}+\text{X}\to {\text{M}}^{2+}+\text{X}+\text{e}$$ 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 $\frac{E}{2}$ mass spectrum. Interference from product ions from processes of the type: $${\text{M}}_1^{+}+\text{X}\to {\text{M}}_2^{+}+\text{X}+{\text{M}}_3$$ 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 $m_2=0.5m_1$, can arise in $\frac{E}{2}$ mass spectra.