The kinetic energy of the electrons is usually 70 eV in the electron ionization method. Typically 10-20 eV energy is transferred to the molecules. Around 10 eV energy is sufficient to cause ionization of most organic molecules; the radical cation is therefore left with an excess energy. Electron ionization, therefore often causes extensive fragmentation of the radical cation. Detection of these fragments can provide useful structural information about the molecule but can complicate the data for larger molecules. In some cases, molecular ion may not even be detected at all. The fragmentation is usually hemolytic, resulting in an even-electron cation and a neutral radical (Equation 11.10). Fragmentation into a neutral molecule and a smaller radical cation, however, is not uncommon (Equation 11.11).
...........................................................................(11.10)
..........................................................................(11.11)
Electron ionization method is limited to samples in the gas phase. Gaseous and highly volatile samples can be directly introduced into the ionization chamber. Liquid and solid samples can be heated to obtain molecules in gaseous phase but it depends on the thermostability of the samples.
Chemical Ionization (CI)
The ions are produced through the collision of the sample molecules with the primary ions produced by a gas (called a reagent gas) in the ionization chamber. The reagent gas is ionized through electron ionization.
The radical cations generated will undergo fragmentations and reactions. The most common reaction generating ions is a proton transfer from a gas cation (GH+) to the molecule. Methane, isobutane, and ammonia are the most common reagent gases. Let us take methane as an example to understand how chemical ionization occurs:
| Electron ionization: |  |
| Fragmentation: |
|
The radical cations and the carbocations can react with the reagent gas to give various protonated species:
| Reactions: |  |
The analyte molecules acquire the protons from any one of these cations:
 |
The ion, MH+ is called the quasimolecular or pseudomolecular ion as its mass is one unit more than the molecular mass. Saturated hydrocarbons usually ionize through hydride abstraction by the reagent gas cation:
 |
The chemical ionization method imparts little excess energy to the molecular ions thereby resulting in lesser fragmentation as compared to the electron ionization method. Furthermore, the degree of fragmentation depends on the reagent gas used for chemical ionization. The fragmentation caused by isobutane and ammonia is considerably less than that caused by methane. Like electron ionization, chemical ionization is also suitable only for gaseous samples limiting it to the gases and volatile liquids .
Fast Atom Bombardment (FAB)
Fast atom bombardment is a soft ionization technique i.e. it causes little fragmentation of the molecular ions generated. In fast atom bombardment ionization methods, the sample is dissolved in a non-volatile liquid and the ions are extracted by bombarding the sample with a beam of high energy atoms (~5 keV), usually argon (sometimes xenon). The commonly used liquid matrices include glycerol, thioglycerol, and m-nitrobenzyl alcohol. Fast moving Argon atoms are generated as shown in the Figure 11.6. The Argon radical cations (Ar•+), generated through electron ionization, are accelerated and focused as a sharp beam. The high energy Ar•+ ions are allowed to collide with the Ar atoms resulting in the neutralization of some of the Ar•+ ions in the beam. The residual Ar•+ in the beam are extracted out by applying an electric field, thereby resulting in a beam of fast moving atoms. The atoms collide with the sample dissolved in the liquid matrix extracting the ions into the gas phase.
Figure 11.6 Diagram showing the generation of fast moving atoms and sample ionization in FAB ionization source |
FAB causes little or no ionization but desorbs the ions already existing in the solution into the gas phase. FAB causes desorption of the ions present on the surface of the matrix; the compounds having higher surface activity are therefore detected better. FAB is particularly good for polar molecules with large molecular weights and molecules up to 10,000 Da can be detected. It is therefore possible to detect biomolecules like peptides, oligonucleotides, and oligosaccharides using FAB ionization.