DART正イオン化において観測されるアンモニウムイオン付加分子の生成機構

摘要

Ammonium ion adduct molecules [M+NH_(4)]~(+) that are produced in positive Direct Analysis in Real Time (DART) ionization mass spectrometry were examined using a selected set of compounds. The [M+NH_(4)]~(+)/[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+ND_(4)]~(+) ions. This observation suggests that hydrogen-bonded complexes between an ammonium ion ND_(4)~(+) and an acetophenone molecule M initially formed an excited complex [M_(****)D_(****)ND_(3)]~(+*), after which, some of the complexes then collapsed into [M+D]~(+). These results indicate that the mechanism responsible for the formation of [M+NH_(4)]~(+) ion under ammonia chemical ionization and DART ionization is similar irrespectively of the compound under investigation.