Formation of Ammonium Ion Adduct Molecules in Positive Direct Analysis in Real Time (DART) Ionization Mass Spectrometry

Abstract

Ammonium ion adduct molecules [M+NH₄]⁺ that are produced in positive Direct Analysis in Real Time (DART) ionization mass spectrometry were examined using a selected set of compounds. The [M+NH₄]⁺/[M+H]⁺ relative intensity ratio was dependent on the proton affinity (PA) of the compounds and ammonia (PA 846 kJ/mol). Ar-DART mass spectra of acetophenone (PA 850 kJ/mol) showed [M+D]⁺ and [M+ND₄]⁺ ions. This observation suggests that hydrogen-bonded complexes between an ammonium ion ND₄⁺ and an acetophenone molecule M initially formed an excited complex [M_****D_****ND₃]^+*, after which, some of the complexes then collapsed into [M+D]⁺. These results indicate that the mechanism responsible for the formation of [M+NH₄]⁺ ion under ammonia chemical ionization and DART ionization is similar irrespectively of the compound under investigation.