In chemical ionization new ionized species are formed when gaseous molecules
interact with ions. Chemical ionization may involve the transfer of an electron,
proton, or other charged species between the reactants. [8] These reactants are
i) the neutral analyte M and ii) ions from a reagent gas.
CI differs from what we have encountered in mass spectrometry so far because
bimolecular processes are used to generate analyte ions. The occurrence of bimolecular
reactions requires a sufficiently large number of ion-molecule collisions
during the dwelltime of the reactants in the ion source. This is achieved by significantly
increasing the partial pressure of the reagent gas. Assuming reasonable collision
cross sections and an ion source residence time of 1 µs, [9] a molecule will
undergo 30–70 collisions at an ion source pressure of about 2.5 × 102
Pa. [10] The
103
–104
-fold excess of reagent gas also shields the analyte molecules effectively