When is it better to use the electron bombardment (EI–MS)?
It is preferable to use electron impact when you want lots of fragments that will provide a fingerprint for identifying the molecule.
In EI, fast electrons collide with the compound and cause it to break down into fragment ions.
Advantages of EI
- It is a well-understood technique.
- It can be used with almost all volatile compounds.
- It gives reproducible results.
- The fragmentation gives structural information.
- There are huge searchable databases that can be searched for mass spectroscopic "fingerprints".
Disadvantages of EI
- Identification of fragments is not always easy.
- The molecular ion may not be detectable.
If you need a molecular ion peak, you can use the chemical ionization (CI) method.
In CI, fast electrons first ionize a reagent gas. The reagent gas ions then ionize the sample molecules.
Most of the ions are (M+H)⁺ ions, and there is littlefragmentation.
Advantages of CI
- Easy to determine molecular mass
- Simple spectra
Disadvantages of CI
- Little structural information due to lack of fragmentation
Below are the EI spectrum and the CI spectrum (in acetonitrile) of the toxic nerve agent VX.
Notice the wealth of fragmentation information in the EI spectrum and the ease of locating the molecular ion in the CI spectrum.