Optimization and Application of APCI Hydrogen–Deuterium Exchange Mass Spectrometry (HDX MS) for the Speciation of Nitrogen Compounds

Abstract

Abstract. A systematic study was performed to investigate the utility of atmospheric pressure chemical ionization hydrogen-deuterium exchange mass spectrometry (APCI HDX MS) to identify the structures of nitrogen-containing aromatic compounds. First, experiments were performed to determine the optimized experimental conditions, with dichloromethane and CH 3 OD found to be good cosolvents for APCI HDX. In addition, a positive correlation between the heated capillary temperature and the observed HDX signal was observed, and it was suggested that the HDX reaction occurred when molecules were contained in the solvent cluster. Second, 20 standard nitrogen-containing compounds were analyzed to investigate whether speciation could be determined based on the different types of ions produced from nitrogencontaining compounds with various functional groups. The number of exchanges occurring within the compounds correlated well with the number of active hydrogen atoms attached to nitrogen, and it was confirmed that APCI HDX MS could be used to determine speciation. The results obtained by APCI HDX MS were combined with the subsequent investigation of the double bond equivalence distribution and indicated that resins of shale oil extract contained mostly pyridine type nitrogen compounds. This study confirmed that APCI HDX MS can be added to previously reported chemical ionization, electrospray ionization, and atmospheric pressure photo ionization-based HDX methods, which can be used for structural elucidation by mass spectrometry.

Hydrogen/deuterium exchange in mass spectrometry

“…High desolvation temperatures may lead to exchange of non‐labile protons during in‐ESI source HDX of benzoic acid derivatives and aromatic amino acids . The influence of capillary and source temperatures were also investigated on APCI/APPI HDX for nitrogen, oxygen, and sulfur compounds . The ionization source temperature should not be too low or too high due to the direct correlation of source temperature with internal energy of the cluster ion .…”

“…The APPI source of krypton‐discharge lamp emitting 10.6 eV photons and dopant can be used for APPI HDX. To get the best sensitivity in HDX experiments, sample solution should be diluted with 30:70 (v:v) deuterated solvent:co‐solvent for in‐APCI source and 10:90 (v:v) deuterated solvent:dopant for in‐APPI source . The optimum flow rate for in‐APCI and in‐APPI source is 200 and 100 μL/min, respectively.…”

“…Because high‐resolution mass spectrometry is very useful to analyze the crude oil samples, the combination of high‐resolution mass spectrometry with H/D‐exchange emerged as an analytical technique that can be helpful to provide structural information on individual compound. In the previous work, the speciation of nitrogenous compounds in crude oil sample by APCI HDX in combination with high resolution MS was reported . In the study, a total of 20 standard nitrogen‐containing compounds were analyzed and structural assignments were performed based on HDX MS spectra.…”

“…138 The influence of capillary and source temperatures were also investigated on APCI/APPI HDX for nitrogen, oxygen, and sulfur compounds. [139][140][141] The ionization source temperature should not be too low or too high due to the direct correlation of source temperature with internal energy of the cluster ion. 142 Very low source temperature may cause insufficient HDX due to the insufficient internal energy of the ion and very high temperature may induce undesirable effects such as thermal dissociation.…”

CID fragmentation, H/D exchange and supermetallization of Barnase-Barstar complex

“…Once the complex is ionized and the 3D structure of the ion is likely to be as close as possible to obtain to the structure in the solution, then the H/D exchange reaction in the gas phase labels the outer surface of the complex and the following MS n experiments reveal the exchange-protected sites. Recently it was suggested to perform the H/D exchange reaction directly in the ESI source 28, 29 . This approach was applied to study proteins 30–32 , oligosaccharides 33 , natural complex mixtures 34–37 and small organic molecules 38 .…”