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Chemistry LibreTexts

Chemical Ionization

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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.

chemicalionization1.png

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.

chemicalionization2.png

see also electron ionization


This page titled Chemical Ionization is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Gamini Gunawardena via source content that was edited to the style and standards of the LibreTexts platform.

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