Mass spectrometry is an analytical method that determines the mass-to-charge ratio (m/z) of ions. Molecules are ionized in the source and accelerated by an electric field into the analyzer. In time-of-flight (TOF) mass spectrometry (see Figure 1), the analyzer is a chamber under vacuum that contains no electric fields. The ions drift through the analyzer with the kinetic energy obtained from the potential energy of the electric field. The kinetic energy the ions obtain is defined by Equation 1 where eV is the potential energy of the electric field, KE is the kinetic energy, m is mass-to-charge ratio, and v is velocity.
eV = KE = mv2/2 [Eq. 1]
Figure 1. Basic linear TOF mass spectrometer displaying two ions traversing the analyzer.
If all ions obtain the same kinetic energy, the ions of lesser m/z will have greater velocity than ions of greater m/z. Therefore, as ions traverse the analyzer, they separate in space. A detector is positioned at the end of the analyzer to measure the arrival time of ions. Ions of lesser m/z arrive first, followed by ions of greater m/z. A plot of intensity or abundance versus time is made to show the arrival time distribution of the ions detected (see Figure 2). The plot of abundance versus time is calibrated and replotted as intensity versus m/z (see TOF Calibration.)
Figure 2. Plot of abundance versus time for ions detected in TOF mass spectrometer.