Impact of Ambient Vapors on Spectra of 4-Nitroaniline Recorded under Atmospheric Solids Analysis Probe (ASAP) Mass Spectrometric Conditions

Abstract

Thermally desorbed 4-nitroaniline (4-NA), upon atmospheric pressure chemical ionization (APCI), generates gaseous ions for its protonated species. The APCI mass spectrum recorded under mild in-source ion-activating conditions from 4-NA showed a peak at m/z 139, whereas that acquired under high ion-activating conditions showed two additional peaks at m/z 122 ( • OH loss) and 92 ( • NO loss). The spectrum changed instantaneously when acetonitrile vapor was introduced to the source. In the new spectrum, both m/z 122 and 92 peaks were absent, while a new peak appeared at m/z 93. Ion-mobility separation carried out with the m/z 139 ion revealed that the initial ion represented the thermodynamically favored nitroprotonated tautomer. The ion population changed to an ensemble dominated by the less-favored amino-protomer when acetonitrile vapor was introduced to the ion source. The amino-protomer, upon collisional activation, loses • NO 2 to generate an m/z 93 ion, which was confirmed to be the 4dehydroanilinium ion. Ion mobility provided a practical way to monitor the changes secured by acetonitrile vapor because the two protomers showed different arrival times. Under spray-ionization conditions, the formation of the thermodynamically less favored protomer has been attributed to kinetic trapping. Our study demonstrated that the less favored amino-protomer could be generated by introducing acetonitrile vapor under nonspray conditions. Apparently, under APCI conditions, protonated water vapor attaches to the nitro group to generate a proton-bound heterodimer, which upon activation dissociates to yield the nitro-protomer. In contrast, protonated acetonitrile makes a tighter complex preferentially with the amino group, which upon activation breaks to the amino-protomer.

Gas-Phase Internal Proton-Transfer of Protonated para-Aminobenzoic Acid Catalyzed by One Methanol Molecule

“…Several studies ,− have focused on the final stages of desolvation under ESI where, following the expulsion of the microsolvated ion from a charged droplet, the stability of the protonation site for the last few solvent molecules ultimately dictates the protomer population. Other studies provide evidence that at atmospheric pressure, the controlled delivery of selected solvent vapor can affect the final protomer ratios, which may point toward additional processes. ,, Even in these small cluster ions, at these final stages of desolvation, there are complex kinetics leaving some ambiguity to the mechanism(s) at play.…”

“…This finding is consistent with cryogenic ion mobility studies that derived a collisional cross section consistent with the bridged [ p ABA + H] + (H 2 O) 6 (Grotthuss) intermediate. Solvent-bound heterodimers have also been implicated as an important intermediate in protomer formation. ,, A recent study has shown that for para -nitroaniline, the nitro and amino protomer ratio was governed by the formation of either water or acetonitrile bound dimers in the atmospheric pressure ion source. Fragmenting the water dimer produced the nitro protomer, and the acetonitrile dimer produced the amino protomer.…”

“…For the two p ABA protomers, previous work has shown that they can be separated in space or time by both differential ion mobility , and drift ion mobility , techniques, while spectral discrimination can be achieved by characteristic CID product ions, ,, ion–molecule reactions, or spectroscopic methods. ,, Protomer populations have been altered with the use of different ESI solvents. Notably, the N -protomer of p ABA interconverts to the O -protomer with protic solvents, such as methanol and water. , Multiple mechanisms are proposed to explain solvent-mediated protomer isomerism: (i) proton transfer via a solvent bridge referred to as a Grotthuss mechanism; (ii) formation of solvent-bound heterodimers that selectively stabilize charge sites; (iii) through a single solvent molecule acting as a “vehicle” to transport the proton within the ion-dipole complex . Evidence for the Grotthuss mechanism is reported from differential ion mobility, where each protomer population is mobility-selected and then allowed to interact with methanol and water vapor .…”

Dependence of Collision-Induced Mass Spectra of Protonated Michler’s Ketone on the Nature of LC-MS Mobile Phase

“…17,22 Many mechanisms have been proposed to rationalize the observed transformations: (i) proton transfer from one site to the other via a solvent vapor bridge (Grotthuss mechanism); 23 (ii) a single solvent vapor molecule acting as a carrier vehicle to transport the proton within the ion−dipole complex; 24 and (iii) formation and dissociation of functional-group-specific proton-bound heterodimers created with solvent vapors present in the ion source. 25 Although a large body of research data support the Grotthuss-type mechanism, 8,12,26,27 the involvement of solvent vapor molecules in the mediation process, whether it is a single molecule or multiple molecules, appears to rely on the nature of the vapor molecules and the pressure exerted on the interacting entities. For example, under relatively low pressure and short ion storage ion-trap conditions, the number of solvent vapor molecules available is insufficient to facilitate the Grotthuss mechanism.…”

“…Many mechanisms have been proposed to rationalize the observed transformations: (i) proton transfer from one site to the other via a solvent vapor bridge (Grotthuss mechanism); (ii) a single solvent vapor molecule acting as a carrier vehicle to transport the proton within the ion–dipole complex; and (iii) formation and dissociation of functional-group-specific proton-bound heterodimers created with solvent vapors present in the ion source …”