Observed adducts on positive mode direct analysis in real time mass spectrometry - Proton/ammonium adduct selectivities of 600-sample in-house chemical library

In this study, direct analysis in real time adduct selectivities of a 558 in-house high-resolution mass spectrometry sample library was evaluated. The protonated molecular ion ([M+H] (+)) was detected in 462 samples. The ammonium adduct ion ([M+NH4](+)) was also detected in 262 samples. [M+ H] (+) and [M+H-4](+) molecular ions were observed simultaneously in 166 samples. These adduct selectivities were related to the elemental compositions of the sample compounds. [M+NH4] (+) selectivity correlated with the number of oxygen atom(s), whereas [M+H](+) selectivity correlated with the number of nitrogen atom(s) in the elemental compositions. For compounds including a nitrogen atom and an oxygen atom [M+H](+) was detected; [M+NH4](+) was detected for compounds including an oxygen atom only. Density functional theory calculations were performed for selected library samples and model compounds. Energy differences were observed between compounds detected as [M+H] (+) and [M+NH4](+), and between compounds including a nitrogen atom and an oxygen atom in their elemental compositions. The results suggested that the presence of oxygen atoms stabilizes [M+NH4](+), but not every oxygen atom has enough energy for detection of [M+NH4](+). It was concluded that the nitrogen atom( s) and oxygen atom(s) in the elemental compositions play important roles in the adduct formation in direct analysis in real time mass spectrometry.