Gas‐phase protomers of p‐(dimethylamino)chalcone investigated by travelling‐wave ion mobility mass spectrometry (TWIMS)

Abstract

Results from ion-mobility (IM) separation experiments demonstrate that O- and N-protomers of p-(dimethylamino)chalcone (p-DMAC) can coexist in the gas phase. The relative populations of the two protomers strongly depend on the ion-generating settings and the conditions the precursor ions experience from the point of their gas-phase inception to the time of their detection. Under relatively dry source conditions, the ratio of the gas-phase protomers generated under helium-plasma ionization (HePI) conditions is biased towards the thermodynamically favored O-protomer. However, when the humidity of the enclosed ion source was increased, the IM arrival-time distribution profile of the mass-selected protonated precursor of p-DMAC changed rapidly to one dominated by the N-protomer. Under spray-ionization conditions, the formation of the thermodynamically less favored protomer has been generally attributed to a phenomenon called kinetic trapping. Herein, we demonstrate that the population of thermodynamically less favored N-protomer can be dramatically increased simply by introducing water vapor to the HePI ion source.

Solvent-Mediated Proton-Transfer Catalysis of the Gas-Phase Isomerization of Ciprofloxacin Protomers

“…The mechanism of protonation during ESI is particularly intriguing when the analyte contains multiple protonation sites and the thermodynamically preferred site of protonation switches from solution to gas phase. Of course, it is well-known that biological molecules are protonated at many molecular sites following ESI, and since ion populations can contain multiple protomers, this results in a broader distribution of product ions that enable structural analysis. − Recent studies on protomer systems have reported that ionization source conditions can drastically influence the protomer population ratios ,− and that this, in turn, affects product ion distributions upon collision-induced dissociation (CID). − Since the protons are key agents in these dissociation mechanisms, gaining control over their placementand rationalizing their location following ESIis key to optimal sensitivity and selectivity in mass spectrometry workflows.…”

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

“…For example, mass spectral profiles recorded under ASAP conditions in conjunction with principal component analysis (PCA) have been recommended as a protocol for routine analysis to monitor the varieties of the oranges . This study and other previous investigations ,, have demonstrated that mass spectral profiles acquired under plasma-ionization conditions are highly affected by the nature of vapor molecules present in the ion source enclosure. Therefore, it is vital that ambient conditions are carefully monitored and controlled when ASAP and other ambient ionization methods are used for sample profiling.…”

“…17 analytes to be unambiguously identified because, for many multifunctional compounds, the fragmentation spectra of different tautomers are significantly different from each other. 18,19 The overall ion-generation scenario under atmospheric spray conditions is a complex process. The manifested tautomer ratios are known to depend on many factors including the nature of the solvent, the conjugate acid/base chemical equilibria that exist in solution, the ionic strength and pH of the solution, and various ion-source parameters.…”

“…Thus, several different tautomeric species are expected to form from the same analyte. A clear understanding of how different tautomers are formed is important to ensure analytes to be unambiguously identified because, for many multifunctional compounds, the fragmentation spectra of different tautomers are significantly different from each other. , …”

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.…”