Selective isotopic exchange of polyfimctional ions in tandem mass spectrometry: Methodology, applications and mechanism

Isotopic exchange of mass‐selected odd‐ and even‐electron molecular ions of aromatic compounds upon collision with deuterated gases was investigated as a function of reagent gas, interaction time and collision energy. Use of ND 3 as reagent allows exchange of all active hydrogens for the compound types studied, providing a count of the total number of active hydrogens present in the analyte. CH 3 OD exchanges specific types of active hydrogens, such as phenolic and carboxylic hydrogens, without exchanging amino hydrogens. This selectivity assists in the identification and enumeration of different types of active hydrogens present in polyfunctional compounds. The H–D exchange patterns serve to differentiate isomeric aromatic compounds containing methoxy, amino, hydroxy and carboxylic acid substituents. Trapping of mass‐selected ions in the collision region of a triple quadrupole mass spectrometer greatly enhances the degree of H–D exchange, thereby facilitating determination of the number of active hydrogens in the analyte. Triple stage mass spectrometric experiments, performed in a pentaquadrupole mass spectrometer, help elucidate the exchange process. Isotopic exchange in the collision region of a tandem mass spectrometer also provides insights into the site of protonation in molecules containing several functional groups. The proximity of the functional groups and the proton affinity difference between the analyte and the reagent gas are important factors in site‐specific H–D exchange in polyfunctional compounds. An investigation of the effects of collision energy reveals that cluster ion formation plays a major role in the exchange mechanism operating in the triple quadrupole and that H–D exchange, ion‐molecule adduct formation and endothermic fragmentation are competitive reaction channels.