Chemical ionization

Chemical ionization (CI) is an ionization technique used in mass spectrometry. Chemical ionization is a lower energy process than electron ionization (EI). The lower energy yields less or sometimes no fragmentation, and usually a simpler spectrum. The lack of fragmentation limits the amount of structural information that can be determined about the ionated species. However, a typical CI spectra has an easily identifiable protonated molecular ion peak [M+1]⁺ which allows for determination of molecular mass. CI is thus useful in cases where the energy from the bombarding electrons in EI is so great that impact almost exclusively causes fragmentation to occur and molecular ions are not formed in large enough quantities to produce an identifiable molecular ion peak.

Mechanism

In a CI experiment, ions are produced through the collision of the analyte with ions of a reagent gas that are present in the ion source. Some common reagent gases include: methane, ammonia, and isobutane. Inside the ion source, the reagent gas is present in large excess compared to the analyte. Electrons entering the source will preferentially ionize the reagent gas. The resultant collisions with other reagent gas molecules will create an ionization plasma. Positive and negative ions of the analyte are formed by reactions with this plasma.