Mass spectrometrists have ever been searching for ionization methods softer than
EI, because molecular weight determination is of key importance for structure elucidation.
Chemical ionization (CI) is the first of the soft ionization methods we are
going to discuss. Historically, field ionization (FI, Chap. 8) has been applied some
years earlier, and thus CI can be regarded as the second soft ionization method introduced
to analytical mass spectrometry. Several aspects of CI possess rather
close similarity to EI making its discussion next to EI more convenient. CI goes
back to experiments of Talrose in the early 1950s [1] and was developed to an
analytically useful technique by Munson and Field in the mid-1960s. [2-5] Since
then, the basic concept of CI has been extended and applied in numerous different
ways, meanwhile providing experimental conditions for a wide diversity of analytical
tasks. [5,6] The monograph by Harrison is especially recommended for
further reading