Mass Spectrometry

Abstract

Mass spectrometer has long been used to measure the composition of a gas by ionizing the gas, typically by electron impact ionization, and separating the resulting ions by the mass-to-charge ratios, and the set up is typically as shown in Fig. 27. For a time-of-tlight mass analyzer with a flight tube of L, the ions are dispersed in time:

$$\begin{array}{*{20}{c}} {E = m{{v}^{2}}/2 = zV} \hfill \\ {v = \sqrt {{2E/m}} } \hfill \\ {t = L/v = L\sqrt {{m/2Vz}} } \hfill \\ \end{array}$$

(8)

The field created by the quadrupole structure causes the ions to transverse between the poles in a helical path that is stable for only a particular mass-to-charge ratio (Fig. 28). The ions that successfully pass through the mass filter arc collected and the resulting current is amplified to form the mass spectrometer signal. Varying the AC and DC potentials allows the selection or scanning of mass-to-charge ratios, as shown in Fig. 29.