Chemical ionization (symbol: CI) is a mass-spectroscopic technique, used when the peak with the highest mass-to-charge ratio in a mass spectrum is suspected not to be the molecular ion peak and, therefore, can not be used to determine the molecular weight of the compound. In chemical ionization mass spectroscopy, the sample molecule is first protonated in gas phase using a very strong acid. The resultant cation, which is not a free radical and, therefore, always gives rise to a peak in the mass spectrum, is subjected to ionization.
Consider the hypothetical compound X.
The heaviest cation formed during the experiment is XH+. Thus, of all the cations formed, XH+ has the highest mass-to-charge ratio, excluding any XH+ ions containing heavier isotopes.
see also electron ionization