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Chemical Ionization Advantages / Disadvantages


Advantages:
  • CI transfers less energy to the molecule than EI.  Therefore CI is less likely to cause fragmentation.
  • CI is especially well suited to the negative ionization mode.
Disadvantages:
  • Interactions between the reagent carrier gas and the sample may complicate the spectrum analysis.
  • The sample must be in the gas phase to ionize, therefore heating of the sample may be required.
  • The higher source pressures required to run CI may make high resolution tuning of the ion source difficult or impossible.
Note:
Chemical ionization is not performed on a routine basis.  Please see LiLi if your research calls for CI.  The reagent gases available are ammonia and isobutane.  Ammonia has a proton affinity of 207 kcal/mole and a hydride affinity of 232 kcal/mole.  A reagent gas with a higher proton affinity is less likely to cause fragmentation.  Proton affinity of the reagent gas must be lower than that of the sample to create the [M+H]+ ions (which are the species seen in the spectrum).  Likewise, hydride affinity of the reagent gas must be lower than that of the sample in order to create the [M-H]- ions.