Impact of Ambient Vapors Present in an Electrospray Ionization Source on Gas-Phase Ion Structures

Abstract

According to current consensus, structures of protomeric (or deprotomeric) tautomers of gaseous ions generated by electrospray ionization depend primarily on the nature of the spray solvent. To probe the effect of the spray solvent on protonation, 4-aminobenzoic acid (PABA) has often been selected as the model compound. It is widely accepted that the protonation in the gas phase takes place primarily on the carbonyl oxygen atom when the sample is sprayed in methanol and on the nitrogen atom when acetonitrile is used as the spray solvent. Although this observation is valid, our current results indicate that the determination of the predominant protomer in the gas phase by the spray solvent is an indirect effect moderated by the solvent vapor molecules present in the ambient ion source. To investigate real-time changes in protomer distributions due to solvents, we used ion-mobility mass spectrometry (IM-MS). Initially, when a PABA solution in methanol was electrosprayed, the ion-mobility arrival-time profile recorded showed essentially one peak for the O-protomer. However, when acetonitrile or acetone vapors were introduced to the ambient-pressure ion source via the flowing desolvation gas, the intensity of the O-protomer peak diminished rapidly, and the N-protomer signal became dominant. The moment the acetonitrile (or acetone) vapors were removed from the ion source, the protomer-distribution signals began gradually reverting back to their original intensities. Furthermore, when PABA samples in methanol and acetonitrile were electrosprayed separately via a dual-sprayer setup, which allowed for the selective blocking of the gaseous ion-generation cascade of charged droplets from either sprayer, the predominant signal corresponded only to the N-protomer, irrespective of the position of the mechanical barrier. Because the mechanical barrier prevents only the gaseous ion formation, but not the physical access of solvent vapors to the ion source, it is evident that the solvent vapor that engulfs the ion source is the governing factor that decides the protomer distribution, not the nature of the spray solvent. Noticeably, acetonitrile wields a stronger effect on the manifested protomer distribution than many other solvents, including methanol, water, hexanes, and toluene. Apparently, the so-called "memory" of the solution-phase structures and the phenomenon described as "kinetic trapping" are both due to indirect effects caused by the solvent vapor engulfing the atmospheric-pressure ion source. Moreover, the so-called "memory" effect can either be "saved" or "erased" by exposing the initially formed gaseous ions to different solvent vapors from an alternative source.

Collision-assisted stripping for determination of microsolvation-dependent protonation sites in hydrated clusters by cryogenic ion trap infrared spectroscopy: the case of benzocaineH+(H2O)n

“…14,15 This solvent-dependent protonation-site switching is not only found in BC but prevails in other (bio-)organic molecules. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Much effort has been devoted to microsolvated clusters, in which an ion is solvated by a few to several hundreds of molecules, 14,[34][35][36] because the degree of solvation can be tuned by controlling the type and numbers of solvent molecules.…”

“…In particular, infrared photodissociation spectroscopy (IRPD) and ion mobility mass spectrometry (IMS) have frequently been applied to assign protomers of desolvated ions. [8][9][10][12][13][14][15][16][17][18][19][20][21][23][24][25][26][27]30,[32][33][34][35] For example, the protomers of isolated H + BC could clearly be separated by both IRPD and IMS. 15 In principle, IMS can separate isomers based on the collision cross section (CCS) with inert buffer gas, which strongly depends on charge distribution and shape of the molecule.…”

Influence Exerted by the Solvent Effect on the Mobility Peak of 1,8-Naphthalic Anhydride in Ion Mobility Spectrometry

“…Several reports have shown that solvents affect the formation of tautomers detected by IM-MS. − Chandran et al found solvent effects must be thoroughly evaluated before a validated “standard” protocol for the quantitative determination of an analyte can be developed and recommended. In addition, Morsa et al detected the mobility peaks of acetonitrile adducts of electrophilic benzylium cations using the electrospray ionization source by trapped ion mobility spectrometer (TIMS).…”

Untargeted urine metabolite profiling by mass spectrometry aided by multivariate statistical analysis to predict prostate cancer treatment outcome

“…[24][25][26][27] For this purpose, highresolution mass spectrometry is particularly useful because the high resolving power enables discrimination of so-called isobaric ions with different exact masses. 28 For this purpose, high-throughput biochemical techniques that do not require time-consuming elaborate sample preparation or chromatographic procedures for analysis are more attractive. The laser diode thermal desorption coupled with atmospheric pressure chemical ionization mass spectrometry (LDTD-APCI-MS) is a promising technique for rapidly determining the composition of biofluids.…”