Charge Retention/Charge Depletion in ESI-MS: Experimental Evidence

Dynamics of the Aspiration of Charged Droplets into a LC-ESI-MS System

Electrospray ionization (ESI) enables coupling between liquid chromatography (LC) and mass spectrometry (MS). Since it is a gentle ionization method, it is frequently used for the analysis of large biomolecules. In recent years, several experimental setups have demonstrated that the use of ESI results in the formation of charged droplets that are aspirated into the vacuum systems of mass spectrometers. This results in a variety of consequences, such as instrument contamination, which can impede the analytical performance. We investigate the signatures of aspirated charged droplets with a commercial LC-ESI-MS system at analytical conditions. Previous observations without LC coupling are reproduced and show that significant droplet aspiration is probably taking place at analytical LC-ESI-MS conditions. This common phenomenon likely decreases the instrument sensitivity. Analyte can be released by isolation and fragmentation of droplet fragments; thus, aspirated droplets can mask analyte even in the mass analyzer region. The complex morphology of droplet MS/MS mass spectra is highly reproducible at the same experimental conditions. This implies the existence of distinct molecular reaction pathways of the droplet fragments. To assess the effect of droplet aspiration on analytical applications, relevant method and ion source parameters, which are commonly varied during method optimization, were investigated. Further variations of the solvent composition revealed that the aspirated droplets and their fragmentation are particularly sensitive to the solvent composition and thus also to the LC solvent gradient in an analytical experiment.

Influence of Solvent Relative Permittivity in Swab Spray Mass Spectrometry

The influence of solvent properties on ion generation by swab spray ionization was investigated. The ability of a variety of solvents of different relative permittivity, surface tension, and viscosity to form a stable and reproducible electrospray was examined. It is demonstrated that in swab spray ionization, a crucial balance between solvent composition, applied potential, and the solvent flow fed to the swab head must be maintained. The solvent composition was found to significantly affect the shape of the Taylor cone and the emerging cone jet, which eventually have an impact on the resulting ion yield. The results indicate that the relative permittivity of solvents measured under standard conditions is the main factor governing jet shaping, and consequently, the ionization efficacy. Short jets, which are required for maximum ion yield, were observed for solvents with relative permittivity εr higher than 25. Solvents exhibiting lower relative permittivity required the addition of 20% to 60% methanol to limit the jet length and to avoid the ineffective dripping pulsation. The observed effects were compared to conventional electrospray ionization and paper spray ionization.

Elucidating the Role of Ion Suppression in Secondary Electrospray Ionization

Ion suppression is a known matrix effect in electrospray ionization (ESI), ambient pressure chemical ionization (APCI), and desorption electrospray ionization (DESI), but its characterization in secondary electrospray ionization (SESI) is lacking. A thorough understanding of this effect is crucial for quantitative applications of SESI, such as breath analysis. In this study, gas standards were generated by using an evaporation-based system to assess the susceptibility and suppression potential of acetone, deuterated acetone, deuterated acetic acid, and pyridine. Gas-phase effects were found to dominate ion suppression, with pyridine exhibiting the most significant suppressive effect, which is potentially linked to its gas-phase basicity. The impact of increased acetone levels on the volatiles from exhaled breath condensate was also examined. In humid conditions, a noticeable decrease in intensity of approximately 30% was observed for several features at an acetone concentration of 1 ppm. Considering that this concentration is expected for breath analysis, it becomes crucial to account for this effect when SESI is utilized to quantitatively determine specific compounds.

Chemical Transformations Can Occur during DMS Separations: Lessons Learned from Beer's Bittering Compounds

While developing a DMS-based separation method for beer's bittering compounds, we observed that the argentinated forms of humulone tautomers (i.e., [Hum + Ag]⁺) were partially resolvable in a N₂ environment seeded with 1.5 mol % of isopropyl alcohol (IPA). Attempting to improve the separation by introducing resolving gas unexpectedly caused the peaks for the cis-keto and trans-keto tautomers of [Hum + Ag]⁺ to coalesce. To understand why resolution loss occurred, we first confirmed that each of the tautomeric forms (i.e., dienol, cis-keto, and trans-keto) responsible for the three peaks in the [Hum + Ag]⁺ ionogram were assigned to the correct species by employing collision-induced dissociation, UV photodissociation spectroscopy, and hydrogen-deuterium exchange (HDX). The observation of HDX indicated that proton transfer was stimulated by dynamic clustering processes between IPA and [Hum + Ag]⁺ during DMS transit. Because IPA accretion preferentially occurs at Ag⁺, which can form pseudocovalent bonds with a suitable electron donor, solvent clustering also facilitated the formation of exceptionally stable microsolvated ions. The exceptional stability of these microsolvated configurations disproportionately impacted the compensation voltage (CV) required to elute each tautomer when the temperature within the DMS cell was varied. The disparity in CV response caused the peaks for the cis- and trans-keto species to merge when a temperature gradient was induced by the resolving gas. Moreover, simulations showed that microsolvation with IPA mediates dienol to trans-keto tautomerization during DMS transit, which, to the best of our knowledge, is the first observation of keto/enol tautomerization occurring within an ion-mobility device.

Validation of Field-Dependent Ion-Solvent Cluster Modeling via Direct Measurement of Cluster Size Distributions

Ion mobility spectrometry is widely used in analytical chemistry, either as a stand-alone technique or coupled to mass spectrometry. Ions in the gas phase tend to form loosely bound clusters with surrounding solvent vapors, artificially increasing the collisional cross section and the mass of the ion. This, in turn, affects ion mobility and influences separation. Further, ion-solvent clusters play an important role in most ionization mechanisms occurring in the gas phase. Consequently, a deeper understanding of ion-solvent cluster association and dissociation processes is desirable to guide experimental design and interpretation. A few computational models exist, which aim to describe the amount of clustering as a function of the reduced electric field strength, bath gas pressure and temperature, and the chemical species probed. It is especially challenging to model ion mobility under high reduced electrical field strengths due to the nonthermal conditions created by the field. In this work, we aim to validate a recently proposed first-principles model by comparing its predictions with direct measurements of cluster size distributions over a range of 20-120 Td as observed using a High Kinetic Energy Ion Mobility Spectrometer coupled to a mass spectrometer (HiKE-IMS-MS). By studying H⁺(H₂O)_n, [MeOH + H + n(H₂O)]⁺, [ACE + H + n(H₂O)]⁺, and [PhNH₂ + H + n(H₂O)]⁺ as test systems, we find very good agreement between model and experiment, supporting the validity of the computational workflow. Further, the detailed information gained from the modeling yields important insights into the cluster dynamics within the HiKE-IMS, allowing for better interpretation of the measured ion mobility spectra.

The hitchhiker's guide to dynamic ion-solvent clustering: applications in differential ion mobility spectrometry

This article highlights the fundamentals of ion-solvent clustering processes that are pertinent to understanding an ion's behaviour during differential mobility spectrometry (DMS) experiments. We contrast DMS with static-field ion mobility, where separation is affected by mobility differences under the high-field and low-field conditions of an asymmetric oscillating electric field. Although commonly used in mass spectrometric (MS) workflows to enhance signal-to-noise ratios and remove isobaric contaminants, the chemistry and physics that underpins the phenomenon of differential mobility has yet to be fully fleshed out. Moreover, we are just now making progress towards understanding how the DMS separation waveform creates a dynamic clustering environment when the carrier gas is seeded with the vapour of a volatile solvent molecule (e.g., methanol). Interestingly, one can correlate the dynamic clustering behaviour observed in DMS experiments with gas-phase and solution-phase molecular properties such as hydrophobicity, acidity, and solubility. However, to create a generalized, global model for property determination using DMS data one must employ machine learning. In this article, we provide a first-principles description of differential ion mobility in a dynamic clustering environment. We then discuss the correlation between dynamic clustering propensity and analyte physicochemical properties and demonstrate that analytes exhibiting similar ion-solvent interactions (e.g., charge-dipole) follow well-defined trends with respect to DMS clustering behaviour. Finally, we describe how supervised machine learning can be used to create predictive models of molecular properties using DMS data. We additionally highlight open questions in the field and provide our perspective on future directions that can be explored.

Enhancement of sensitivity and quantification quality in the LC-MS/MS measurement of large biomolecules with sum of MRM (SMRM)

Liquid chromatography (LC) coupled with tandem mass spectrometry (MS/MS) provides a simple and efficient means for the measurement of analytes in biological matrices with high selectivity and specificity. LC-MS/MS plays an important role in the pharmaceutical industry and biomedical research, but it requires analytes to be in an ionized form in order to be detected. This can pose a challenge for large molecules such as proteins and peptides, because they can exist in multiple charged forms, and this will reduce the total analyte signal by distributing it into multiple ion peaks with a different number of charges in a mass spectrum. In conventional LC-MS/MS analysis of such macromolecules, one charged form is selected as the precursor ion which is then fragmented by collision-induced dissociation (CID) in MS/MS to generate product ions, a process referred to as multiple-reaction monitoring (MRM). The MRM method minimizes interference from endogenous molecules within biological matrices that share the same molecular weight of the precursor ion, but at the expense of signal intensity as compared to precursor ion intensity. We describe here an approach to boost detection sensitivity and expand dynamic range in the quantitation of large molecules while maintaining analytical specificity using summation of MRM (SMRM) transitions and LC separation technique. Protein image from PDB-101 (PDB101.rscb.org).

The Charge-State and Structural Stability of Peptides Conferred by Microsolvating Environments in Differential Mobility Spectrometry

The presence of solvent vapor in a differential mobility spectrometry (DMS) cell creates a microsolvating environment that can mitigate complications associated with field-induced heating. In the case of peptides, the microsolvation of protonation sites results in a stabilization of charge density through localized solvent clustering, sheltering the ion from collisional activation. Seeding the DMS carrier gas (N₂) with a solvent vapor prevented nearly all field-induced fragmentation of the protonated peptides GGG, AAA, and the Lys-rich Polybia-MP1 (IDWKKLLDAAKQIL-NH₂). Modeling the microsolvation propensity of protonated n-propylamine [PrNH₃]⁺, a mimic of the Lys side chain and N-terminus, with common gas-phase modifiers (H₂O, MeOH, EtOH, iPrOH, acetone, and MeCN) confirms that all solvent molecules form stable clusters at the site of protonation. Moreover, modeling populations of microsolvated clusters indicates that species containing protonated amine moieties exist as microsolvated species with one to six solvent ligands at all effective ion temperatures (T_eff) accessible during a DMS experiment (ca. 375-600 K). Calculated T_eff of protonated GGG, AAA, and Polybia-MPI using a modified two-temperature theory approach were up to 86 K cooler in DMS environments seeded with solvent vapor compared to pure N₂ environments. Stabilizing effects were largely driven by an increase in the ion's apparent collision cross section and by evaporative cooling processes induced by the dynamic evaporation/condensation cycles incurred in the presence of an oscillating electric separation field. When the microsolvating partner was a protic solvent, abstraction of a proton from [MP1 + 3H]³⁺ to yield [MP1 + 2H]²⁺ was observed. This result was attributed to the proclivity of protic solvents to form hydrogen-bond networks with enhanced gas-phase basicity. Collectively, microsolvation provides analytes with a solvent "air bag," whereby charge reduction and microsolvation-induced stabilization were shown to shelter peptides from the fragmentation induced by field heating and may play a role in preserving native-like ion configurations.

Sulfolane-Induced Supercharging of Electrosprayed Salt Clusters: An Experimental/Computational Perspective

It is well-known that supercharging agents (SCAs) such as sulfolane enhance the electrospray ionization (ESI) charge states of proteins, although the mechanistic origins of this effect remain contentious. Only very few studies have explored SCA effects on analytes other than proteins or peptides. This work examines how sulfolane affects electrosprayed NaI salt clusters. Such alkali metal halide clusters have played a key role for earlier ESI mechanistic studies, making them interesting targets for supercharging investigations. ESI of aqueous NaI solutions predominantly generated singly charged [Na_nI_(n-1)]⁺ clusters. The addition of sulfolane resulted in abundant doubly charged [Na_nI_(n-2)Sulfolane_s]²⁺ species. These experimental data for the first time demonstrate that electrosprayed salt clusters can undergo supercharging. Molecular dynamics (MD) simulations of aqueous ESI nanodroplets containing Na⁺/I^- with and without sulfolane were conducted to obtain atomistic insights into the supercharging mechanism. The simulations produced [Na_nI_i]^z+ and [Na_nI_iSulfolane_s]^z+ clusters similar to those observed experimentally. The MD trajectories demonstrated that these clusters were released into the gas phase upon droplet evaporation to dryness, in line with the charged residue model. Sulfolane was found to evaporate much more slowly than water. This slow evaporation, in conjunction with the large dipole moment of sulfolane, resulted in electrostatic stabilization of the shrinking ESI droplets and the final clusters. Hence, charge-dipole stabilization causes the sulfolane-containing droplets and clusters to retain more charge, thereby providing the mechanistic foundation of salt cluster supercharging.

Nested-channel for on-demand alternation between electrospray ionization regimes

On-demand electrospray ionization from different liquid channels in the same emitter was realized using filamented capillary and gas phase charge supply. The solution sub-channel was formed when back-filling solution to the emitter tip by capillary action along the filament. Gas phase charge carriers were used to trigger electrospray ionization from the solution meniscus at the tip. The meniscus at the tip opening may be fully filled or partially empty to generate electrospray ionization in main-channel regime and sub-channel regime, respectively. For emitters with 4 μm tip opening, the two nested electrospray (nested-ESI) channels accommodated ESI flow rates ranging from 50 pL min-1 to 150 nL min-1. The platform enabled on-demand regime alternations within one sample run, in which the sub-channel regime generated smaller charged droplets. Ionization efficiencies for saccharides, glycopeptide, and proteins were enhanced in the sub-channel regime. Non-specific salt adducts were reduced and identified by regime alternation. Surprisingly, the sub-channel regime produced more uniform responses for a peptide mixture whose relative ionization efficiencies were insensitive to ESI conditions in previous picoelectrospray study. The nested channels also allowed effective washing of emitter tip for multiple sampling and analysis operations.

Simple, Rapid Lipidomic Analysis of Triacylglycerols in Bovine Milk by Infusion-Electrospray Mass Spectrometry

Bovine milk is a complex mixture of lipids, proteins, carbohydrates, and other factors of which lipids comprise 3-5% of the total mass. Rapid analysis and characterization of the triacylglycerols (TAG) that comprise about 95% of the total lipid is daunting given the numerous TAG species. In the attached methods paper, we demonstrate an improved method for identifying and quantifying TAG species by infusion-based "shotgun" lipidomics. Because of the broad range of TAG species in milk, a single internal standard was insufficient for the analysis and required sectioning the spectrum into three portions based upon mass range to provide accurate quantitation of TAG species. Isobaric phospholipid interferences were removed using a simple dispersive solid-phase extraction step. Using this method, > 100 TAG species were quantitated by acyl carbon number and desaturation level in a sample of commercially purchased bovine milk.

Computational analysis of the proton-bound acetonitrile dimer, (ACN)2 H. RAPID COMMUNICATIONS IN MASS SPECTROMETRY : RCM 2020;34:e8767. [PMID: 32115782 DOI: 10.1002/rcm.8767] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

RATIONALE

In atmospheric pressure ionization mass spectrometry the theoretical thermodynamic treatment of proton-bound cluster stabilities helps us to understand the prevailing chemical processes. However, such calculations are rather challenging because low-barrier internal rotations and strong anharmonicity of the hydrogen bonds cause the breakdown of the usually applied harmonic approximation. Even the implemented anharmonic treatment in standard ab initio software failed in the case of (ACN)₂ H⁺ .

METHODS

For a case study of the proton-bound acetonitrile dimer, (ACN)₂ H⁺ , we scan the potential energy surface (PES) for the internal rotation and the proton movement in all three spatial directions. We correct the partition functions by treating the internal rotation as a free rotor and by solving the nuclear Schrödinger equation explicitly for the proton movement. An additional PES scan for the dissociation surface further improves the understanding of the cluster behavior.

RESULTS

The internal rotation is essentially barrier free (V₀  = 2.6 × 10^-6 eV) and the proton's movement between the two nitrogen atoms follows a quartic rather than quadratic potential. As a figure of merit we calculate the free dissociation enthalpy of the dimer. Our description significantly improves the standard results from about 118.3 kJ/mol to 99.6 kJ/mol, compared with the experimentally determined value of 92.2 kJ/mol. The dissociation surface reveals strong crosstalk between modes and is essentially responsible for the observed errors.

CONCLUSIONS

The presented corrections to the partition functions significantly improve their accuracy and are rather easy to implement. In addition, this work stresses the importance of alternative theoretical methods for proton-bound cluster systems besides the standard harmonic approximations.

Charge Retention/Charge Depletion in ESI-MS: Theoretical Rationale

Gas phase modification in ESI-MS can significantly alter the charge state distribution of small peptides and proteins. The preceding paper presented a systematic experimental study on this topic using Substance P and proposed a charge retention/charge depletion mechanism, explaining different gas- and liquid-phase modifications [Thinius et al. J. Am. Soc. Mass Spec. 2020, 10.1021/jasms.9b00044]. In this work, we aim to support this rational by theoretical investigations on the proton transfer processes from (multiply) charged analytes toward solvent clusters. As model systems we use small (di)amines as analytes and methanol (MeOH) and acetonitrile (ACN) as gas phase modifiers. The calculations are supported by a set of experiments using (di)amines, to bridge the gap between the present model system and Substance P used in the preceding study. Upon calculation of the thermochemical stability as well as the proton transfer pathways, we find that both ACN and MeOH form stable adduct clusters at the protonation site. MeOH can form large clusters through a chain of H-bridges, eventually lowering the barriers for proton transfer to an extent that charge transfer from the analyte to the MeOH cluster becomes feasible. ACN, however, cannot form H-bridged structures due to its aprotic nature. Hence, the charge is retained at the original protonation site, i.e., the analyte. The investigation confirms the proposed charge retention/charge depletion model. Thus, adding aprotic solvent vapors to the gas phase of an ESI source more likely yields higher charge states than using protic compounds.