Ammonium Salts as a Source of Small Molecules Observed with High-Resolution Electron-Impact Ionization Mass Spectrometry

Hänni, Nora Phillys; Gasc, Sébastien; Etter, Adrian; Schuhmann, Markus; Isaac R.H.G., Schroeder I; Wampfler, Susanne; Schürch, Stefan; Rubin, Martin; Altwegg, Kathrin (2019). Ammonium Salts as a Source of Small Molecules Observed with High-Resolution Electron-Impact Ionization Mass Spectrometry. Journal of physical chemistry. A, 123(27), pp. 5805-5814. American Chemical Society 10.1021/acs.jpca.9b03534

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Recent high-resolution in situ mass spectrometry at
comet 67P/Churyumov−Gerasimenko visited by European Space
Agency’s Rosetta spacecraft raised the question, if sublimating
ammonium salts can unequivocally be detected in the cometary
coma. In laboratory experiments with the twin model of the space
instrument, two prototypic ammonium salts NH4B, namely, ammonium
chloride (B = Cl−) and ammonium formate (B = HCOO−) (as well as
methodologically relevant isotopologues), were allowed to sublimate in
vacuum while mass spectra were collected. High-resolution electronimpact
ionization mass spectrometry provides an outstanding
experimental tool to investigate the complex physicochemical processes
occurring during the sublimation of ammonium salts. Sublimation of
ammonium chloride led to the observation of the ammonium cation
NH4
+ and the chloramide molecule NH2Cl in the neutral gas mode of the instrument. These observations could be jointly
interpreted as indirect evidence for the existence of a neutral gaseous parent species (either as the molecular complex NH3···HB
or the double-ionic species NH4
+···B−). However, the qualitative fragmentation pattern we present for 13C15N-ammonium
formate suggests an alternative route of NH4
+ production within the ionization region of the instrument, namely, by
protonation/hydrogenation. Besides NH4
+, other species were observed that were formed in protonation/hydrogenation
reactions. Moreover, together with the two major species from the decomposition of the salt, ammonia and formic acid, three
minor species also contributed to the fragmentation pattern: HCN/HNC, HOCN/HNCO, and CH3NO. Like chloramide,
formamide (CH3NO) also is a secondary species probably formed in a pseudo-intramolecular chemical reaction while ammonia
and the respective acid are in a state of association. HCN/HNC and HOCN/HNCO are ternary products coming out of
formamide decomposition reactions. We discuss our experimental findings, summarized in a tentative chemical reaction
network, in light of the available theoretical literature and highlight their relevance for the interpretation of in situ measurements
in space research.
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