We used positive mode electrospray ionization (ESI) mass spectrometry to examine 540 in-house high-resolution mass spectrometry (HRMS) samples that formed an adducted positive ion. Of the 540 samples, the sodium adduct ([M+Na]⁺) was detected in 480 samples, and the protonated molecule ([M+H]⁺) was detected in 92 samples; both [M+Na]⁺ and [M+H]⁺ were detected in 32 samples. No other adduct ions were predominant. The selectivities of these adducts were evaluated by a two-dimensional plot using topological polar surface area (tPSA) and molecular weight. Two predominant trends were observed: [M+H]⁺ converged around tPSA (Ų) = 20 and molecular weight = 250, and the selectivity for [M+Na]⁺ correlated with the tPSA value. These observations were found to be related to the elemental composition of the sample compounds. From the results obtained by positive mode ESI mass spectroscopy under our experimental conditions, predominant trends were observed with respect to adduct selectivity: compounds containing oxygen atom(s) form [M+Na]⁺, and compounds containing nitrogen but not oxygen atom(s) form [M+H]². Based on these trends, we developed the "Nitrogen-Oxygen rule" (NO rule) to predict the adduct formed by a given compound on positive mode ESI. This NO rule provides a guideline to estimate elemental composition using ESI-HRMS with methanol as mobile phase.

To provide a practical guideline for the selection of a mass spectrometer ion source, we compared the applicability of three types of ion source: direct analysis in real time (DART), electrospray ionization (ESI) and fast atom bombardment (FAB), using an in-house high-resolution mass spectrometry sample library consisting of approximately 600 compounds. The great majority of the compounds (92%), whose molecular weights (MWs) were broadly distributed between 150 and 1000, were detected using all the ion sources. Nevertheless, some compounds were not detected using specific ion sources. The use of FAB resulted in the highest sample detection rate (>98%), whereas the detection rates obtained using DART and ESI were slightly lower (>96%). A scattergram constructed using MW and topological polar surface area (tPSA) as a substitute for molecular polarity showed that the performance of ESI was weak in the low-MW (<400), low-polarity (tPSA<60) area, whereas the performance of DART was weak in the high-MW (>800) area. These results might provide guidelines for the selection of ion sources for inexperienced mass spectrometry users.

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 + NH]) was also detected in 262 samples. [M + H] and [M + NH] molecular ions were observed simultaneously in 166 samples. These adduct selectivities were related to the elemental compositions of the sample compounds. [M + NH] 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 + NH] 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 + NH], 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 + NH], but not every oxygen atom has enough energy for detection of [M + NH]. 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.

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. e M NH 4 / M H relative intensity ratio was dependent on the proton a nity 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. is 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 * , a er which, some of the complexes then collapsed into M D . ese 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.

Gas-phase hydrogen/deuterium (H/D) exchange between D 2 O, CH 3 OD, CH 3 CH 2 OD, ND 3 and ascorbic acid was examined, in an attempt to understand the mechanism responsible for the gas-phase ion/molecular reactions that occur in Direct Analysis in Real Time (DART) ．Ionized water/ammonium clusters played an important role in the H/D exchange process, known as a transient microenvironment (TME) ．Argon was used as an alternative DART gas to eliminate formation of ionized water clusters. ese experiments clearly characterized the gas-phase H/D exchange process of positive-ion DART.