Mechanism of [M + H]+ Formation in Atmospheric Pressure Photoionization Mass Spectrometry: Identification of Propionitrile in Acetonitrile with High Mass Accuracy Measurement and Tandem Mass Spectrometry and Evidence for Its Involvement in the Protonation

The role of propionitrile in the production of [M + H]+ under atmospheric pressure photoionization (APPI) was investigated. In dopant-assisted APPI using acetone and anisole, protonated acetone and anisole radical cations were the most prominent ions observed. In dopant-free or direct APPI in acetonitrile, however, a major ion in acetonitrile was detected and identified as propionitrile, using high accuracy mass measurement and collision induced dissociation studies. Vaporizing ca. 10−5 M althiazide and bendroflumethazide under direct APPI in acetonitrile produced their corresponding protonated species [M + H]+. In addition to protonated acetonitrile, its dimers, and acetonitrile/water clusters, protonated propionitrile, propionitrile dimer, and propionitrile/water clusters were also observed. The role of propionitrile, an impurity in acetonitrile and/or a possible product of ion–molecule reaction, in the production of [M + H]+ of althiazide and bendroflumethazide was further investigated in the absence of dopant using propionitrile-d5. The formation of [M + D]+ species was observed, suggesting a possible role of propionitrile in the protonation process. Additionally, an increase in the [M + H]+ signal of althiazide and bendroflumethazide was observed as a function of propionitrile concentration in acetonitrile. Theoretical data from the literature supported the assumption that one possible mechanism, among others, for the formation of [M + H]+ could be attributed to photo-initiated isomerization of propionitrile. The most stable isomers of propionitrile, based on their calculated ionization energy (IE) and relative energy (ΔE), were assumed to undergo proton transfer to the analytes, and mechanisms were proposed.

Graphical AbstractWe have identified an ionic species (propionitrile) in acetonitrile with high mass accuracy measurement and tandem mass spectrometry, and studied its role as a possible proton donor for the formation of [M + H]+ of althiazide and bendroflumethazide under direct APPI assuming photo-initiated isomerization of propionitrile.Figure optionsDownload high-quality image (48 K)Download as PowerPoint slide