Upon entering the mass spectrometer in the gas phase, the molecules are commonly ionized by either removing an electron or by adding a proton. Removal of an electron is achieved by bombarding the molecules with a high energy electron beam, commonly of 70 eV electrons. This electron beam is of much higher energy than the ionization energy of the molecules being bombarded. The technique is called electron ionization (EI). The resulting ion is a radical ion because it has one unpaired electron. In mass spectral parlance, it is an odd electron ion. Predicting which electron is actually removed from the molecule is often difficult, but it is usually an electron in one of the higher energy occupied molecular or atomic orbitals. With a simple saturated hydrocarbon such as propane, this assignment is usually not attempted and the ion is written with the molecular structure or formula in brackets and a plus charge to the right of the brackets. Often a plus-dot is written to the right side of the brackets to emphasize that the ion is an odd electron radical ion. With molecules bearing a heteroatom such as nitrogen or oxygen the electron usually comes from a non-bonding atomic orbital on the heteroatom. With molecules bearing bonds, the electron often comes from the orbital; the resulting ion is written with the -bond replaced with a dot for the remaining electron on one carbon of the original bond for the odd electron, and a positive charge on the other carbon. In actual fact, the structure is one resonance structure with the other resonance structure having the dot and positive charge reversed. Examples of odd electron ions created by electron impact ionization are illustrated in Figure 5.
Figure 5. Examples of M+ ions created by electron impact ionization (EI) of molecules bearing double bonds or heteroatoms.
A second method of ionization is by protonation with a very strong, gas phase, Bronsted acid such as CH5+. The strong acid is created by electron impact ionization of methane followed by reaction of [CH4]+ with a molecule of methane.
This technique is called chemical ionization (CI) and yields ions with m/z = M+1 where M is the molecular mass. This technique works well with molecules bearing bonds and/or heteroatoms that are good sites for protonation. Chemical ionization often yields ions that are less prone to fragmentation and becomes particularly important when a molecular ion is not observed with electron impact ionization. Examples of [M+1]+ ions created by chemical ionization are illustrated in Figure 6.
Figure 6. Examples of [M+1]+ ions created by chemica ionization (CI).
Next section: Instrumentation: Measurement of the mass to charge ratio
Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.