Factors affecting reactivity in ammonia chemical ionization mass spectrometry

Several factors affecting reactivity in ammonia chemical ionization mass spectrometry (NH₃ CI) have been examined. These include the sample proton affinity, the preferred site of protonation and NH₄]⁺ attachment, and substituent effects. In general, compounds having proton affinities ?787 kJ mol^?1 do not yield analytically useful intensities of the M·NH₄]⁺ adduct ion. Substituted aromatic compounds in which the ring is the most basic site yield little (if any) M·NH₄]⁺ ion even if the proton affinity of the compound is greater than 787 kJ mol^?1. On the other hand, some aromatic compounds in which the substituent is the most basic site yield relatively abundant adduct ions. The spectra of compounds possessing a good leaving group (X) exhibit only weak M·NH₄]⁺ ions, but intense M·NH₄ ? HX]⁺ and M ? X]⁺ ions formed by substitution and elimination reactions. Electronic effects strongly influence these processes. Several examples are presented in which isomers are readily differentiated because of different reactivities under ammonia chemical ionization conditions.