CI is an indirect process involving an intermediate chemical reagent.
In CI, mass spectra are produced by reaction between neutral organic molecules and reagent gas ion plasma. Concentration of reagent exceeds that of the sample by several orders of magnitude.
CI sources are operated at high energies (200 to 500 eV) which favor production of thermal electrons. CI produces stable molecular ions with little fragmentation.
Several gases are used in CI ionization: methane, propane, isobutane, hydrogen, ammonia, water, tetramethyl silane, or dimethyl amine. The CI ion source is similar to the EI source but is designed to have an ionization chamber. In positive chemical ionization, the ion source is filled with a reagent gas which is ionized to create a species of the proton donator type that can form a protonated molecule with an analyte; for example, with methane
All the processes proceed simultaneously.
Methane forms characteristic molecular adducts
Such processes are called proton affinity. In order to generate a protonated molecule the proton affinity of an analyte must be greater than that of the reagent gas ion. Ammonia has a high proton affinity value and provides in many cases a better differentiation between the sample matrix and an analyte.
In electron capture negative ionization, thermal electrons are generated by collision of electrons emitted from the filament with buffer gas molecules located at high pressure in the ionization chamber of the source.
e *(70 eV) + CH4 (buffer gas)! e * (thermal electrons)(2 eV)
Suitable analytes exhibit high electron capture capacity or high electron affinity. CI is the technique of choice for the analysis of isomers in environmental samples. In APCI (Atmospheric Pressure Chemical Ionization) solvent evaporation and analyte ionization are two separate processes.
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