How Hot are Your Ions Really? A Threshold Collision-Induced Dissociation Study of Substituted Benzylpyridinium "Thermometer" Ions

Perspective: intrinsic interactions of metal ions with biological molecules as studied by threshold collision-induced dissociation and infrared multiple photon dissociation

In this perspective, gas-phase studies of group 1 monocations and group 12 dications with amino acids and small peptides are highlighted. Although the focus is on two experimental techniques, threshold collision-induced dissociation and infrared multiple photon dissociation action spectroscopy, these methods as well as complementary approaches are summarized. The synergistic interplay with theory, made particularly powerful by the small sizes of the systems explored and the absence of solvent and support, is also elucidated. Importantly, these gas-phase methods permit quantitative insight into the structures and thermodynamics of metal cations interacting with biological molecules. Periodic trends in how these interactions vary as the metal cations get heavier are discussed as are quantitative trends with changes in the amino acid side chain and effects of hydration. Such trends allow these results to transcend the limitations associated with the biomimetic model systems.

Tuning the Internal Energy of Ions Produced by Atmospheric and High-Pressure Electrospray by Modulating the Gas Throughput into the First Vacuum Stage

In a high-pressure environment, electrospray ionization (ESI) can be achieved without discharge between the emitter and the counter electrode, thus enabling the generation of gas-phase ions from liquid with high surface tension, such as pure water, which requires a high onset voltage for stable ESI. In this study, the ion dissociation during the transferring of ions/charged droplets from a superatmospheric pressure environment to vacuum has been systematically investigated using benzyl ammonium thermometer ions. The ion source pressure did not affect the internal energy distribution of ions, whereas the gas throughput into the first vacuum stage clearly influences the internal energy distribution of the ions. The increase in the gas throughput increased the density of molecules/atoms presented in ion transfer/focusing electrodes located in the first vacuum stage. As a result, the mean free path of ions in the first vacuum stage decreases, and the energy of ions decreases by decreasing the kinetic energy involved in each collision between ions and residue gas. The gas throughput into the first vacuum stage is found to describe the internal energy distribution of ions associated with the local conditions more quantitatively instead of using the measured pressure of the vacuum stage, which is different from the effective local pressure. This study also demonstrated the controlled dissociation of ions using the ion transfer settings of the instrument in combination with ion inlet tubes of different sizes.

Pentafluorobenzylpyridinium: new thermometer ion for characterizing the ions produced by collisional activation during tandem mass spectrometry

In this study, pentafluorobenzylpyridinium (F5-BnPy+), which has the highest dissociation energy among the reported benzylpyridinium thermometer ion, is proposed to characterize the internal energy distributions of ions activated by higher energy collisional dissociation (HCD) and ion-trap collision-induced dissociation (CID) during tandem mass spectrometry. The dissociation threshold energies of F5-BnPy+ was determined using quantum chemistry calculations at the CCSD(T)/6-311++G(d,p)//M06-2X-D3/6-311++G(d,p) level of theory, and the appearance energies for ion dissociation in HCD and ion-trap CID were estimated using Rice-Ramsperger-Kassel-Marcus theory. The main differences between HCD and ion-trap CID are the collision energies used and the timescales of collisional activation. For both HCD and ion-trap CID, the average internal energy of the ions increased with increasing collision energy. In contrast, the average value for the internal energy of the ions activated by ion-trap CID was lower than that of ions activated by HCD, probably because of the smaller collisional energy and longer activation time of the ion-trap CID experiments. The reported method will aid in the determination of the optimum tandem mass spectrometry parameters for the analysis of small molecules such as metabolites.

Differences in the internal energies of ions in electrospray ionization mass spectrometers equipped with capillary-skimmer and capillary-RF lens interfaces

Small metabolites are commonly analyzed using electrospray ionization mass spectrometry (ESI-MS). Although the protonated form of a compound of interest is typically the target ion in ESI-MS, the protonated forms of small metabolites occasionally undergo fragmentation during ion transmission from ambient conditions to vacuum conditions, hindering the unambiguous identification of analyte molecules. To estimate the fragmentation efficiency during ESI processes, the internal energy distribution of the ions (P(E)) must be evaluated. The common approach for the P(E) evaluation is the survival yield method, which uses thermometer ions. In this study, the P(E) of ions produced by an ESI source in a commercial triple quadrupole mass spectrometer equipped with a capillary-skimmer and capillary-RF lens interfaces was evaluated using benzyl ammonium thermometer ions. Furthermore, this study proposes the use of 3-(aminomethyl)indole and related compounds, which have the lowest Eapp values among the reported thermometer ions, to obtain P(E) values of the ions more accurately. Results showed that P(E) strongly depends on whether a capillary-skimmer interface or capillary-RF lens interface was used for ion transport to the vacuum. ESI-MS with a capillary-skimmer interface provided a considerably lower and narrower P(E) of ions than that with a capillary-RF lens interface, thereby producing intact protonated molecules without significant fragmentation of most small metabolites. However, ESI-MS equipped with capillary-RF lens interfaces provided a higher efficiency of ion transmission than ESI-MS equipped with a capillary-skimmer interface, allowing for highly sensitive analysis of metabolites.

Evaluation for Ion Heating of H2A-H2B Dimer in Ion Mobility Spectrometry-Mass Spectrometry

Ion mobility spectrometry-mass spectrometry (IMS-MS) provides m/z values and collision cross sections (CCSs) of gas-phase ions. In our previous study, an intrinsically disordered protein, the H2A-H2B dimer, was analyzed using IMS-MS, resulting in two conformational populations of CCS. Based on experimental and theoretical approaches, this resulted from a structural diversity of intrinsically disordered regions. We predicted that this phenomenon is related to ion heating in the IMS-MS instrument. In this study, to reveal the effect of ion heating from parameters in the IMS-MS instrument on the conformational population of the H2A-H2B dimer, we investigated the arrival time distributions of the H2A-H2B dimer by changing values of three instrumental parameters, namely, cone voltage located in the first vacuum chamber, trap collision energy (trap CE) for tandem mass spectrometry, and trap bias voltage for the entrance of IMS. These results revealed that the two populations observed for the H2A-H2B dimer were due to the trap bias voltage. Furthermore, to evaluate the internal energies of the analyte ions with respect to each parameter, benzylpyridinium derivatives were used as temperature-sensitive probes. The results showed that the trap CE voltage imparts greater internal energy to the ions than the trap bias voltage. In addition, this slight change in the internal energy caused by the trap bias voltage resulted in the structural diversity of the H2A-H2B dimer. Therefore, the trap bias voltage should be set with attention to the properties of the analytes, even if the effect of the trap bias voltage on the internal energy is negligible.

Dual Tunability for Uncatalyzed N-Alkylation of Primary Amines Enabled by Plasma-Microdroplet Fusion

The fusion of non-thermal plasma with charged microdroplets facilitates catalyst-free N-alkylation for a variety of primary amines, without halide salt biproduct generation. Significant reaction enhancement (up to >200×) is observed over microdroplet reactions generated from electrospray. This enhancement for the plasma-microdroplet system is attributed to the combined effects of energetic collisions and the presence of reactive oxygen species (ROS). The ROS (e.g., O2 ⋅- ) act as a proton sink to increase abundance of free neutral amines in the charged microdroplet environment. The effect of ROS on N-alkylation is confirmed through three unique experiments: (i) utilization of radical scavenging reagent, (ii) characterization of internal energy distribution, and (iii) controls performed without plasma, which lacked reaction acceleration. Establishing plasma discharge in the wake of charged microdroplets as a green synthetic methodology overcomes two major challenges within conventional gas-phase plasma chemistry, including the lack of selectivity and product scale-up. Both limitations are overcome here, where dual tunability is achieved by controlling reagent concentration and residence time in the microdroplet environment, affording single or double N-alkylated products. Products are readily collected yielding milligram quantities in eight hours. These results showcase a novel synthetic strategy that represents a straightforward and sustainable C-N bond-forming process.

Bond-Dissociation Energies to Probe Pyridine Electronic Effects on Organogold(III) Complexes: From Methodological Developments to Application in π-Backdonation Investigation and Catalysis

In this work, we report on the synthesis of several organogold(III) complexes based on 4,4'-diterbutylbiphenyl (C^C) and 2,6-bis(4-terbutylphenyl)pyridine (C^N^C) ligands and bond with variously substituted pyridine ligands (pyrR). Altogether, 33 complexes have been prepared and studied with mass spectrometry using higher-energy collision dissociation (HCD) in an Orbitrap mass spectrometer. A complete methodology including the kinetic modeling of the dissociation process based on the Rice-Ramsperger-Kassel-Marcus (RRKM) statistical method is proposed to obtain critical energies E0 of the pyrR loss for all complexes. The capacity of these E0 values to describe the pyridine ligand effect is further explored, at the same time as more classical descriptors such as 1H pyridinic NMR shift variation upon coordination and Au-NpyrR bond length measured by X-ray diffraction. An extensive theoretical work, including density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster theory (DLPNO-CCSD(T)) methods, is also carried out to provide bond-dissociation energies, which are compared to experimental results. Results show that dissociation energy outperforms other descriptors, in particular to describe ligand effects over a large electronic effect range as seen by confronting the results to the pyrR pKa values. Further insights into the Au-NpyrR bond are obtained through an energy decomposition analysis (EDA) study, which confirms the isolobal character of Au+ with H+. Finally, the correlation between the lability of the pyridine ligands toward the catalytic efficiency of the complexes could be demonstrated in an intramolecular hydroarylation reaction of alkyne. The results were rationalized considering both pre-catalyst activation and catalyst reactivity. This study establishes the possibility of correlating dissociation energy, which is a gas-phase descriptor, with condensed-phase parameters such as catalysis efficiency. It therefore holds great potential for inorganic and organometallic chemistry by opening a convenient and easy way to evaluate the electronic influence of a ligand toward a metallic center.

Structural, Thermodynamic, and Spectroscopic Evolution in the Hydration of Copper(II) Ions, Cu2+(H2O)2-8

Gas-phase clusters of the hydrated Cu(II) cation with 2-8 water molecules were investigated using ab initio quantum chemistry. Isomer structures, energies, and vibrational spectra were computed across this size range, yielding a qualitative picture of this ion as an intact Cu2+ hydrate that also partially oxidizes the surrounding water network at equilibrium. At sufficient cluster sizes, these ion hydrates also become thermodynamically preferred over competitive Cu(II) hydroxide hydrates. Competitive coordination environments were found to exist at some cluster sizes, due to both hydrogen-bonding and d-orbital chemical effects, and the dominant coordination number was found in some cases to be temperature-dependent. Clear spectral signatures of the ion's coordination environment were computed to exist at each cluster size, which should make experimental verification of these computational predictions straightforward. Through comparison to recent studies of hydrated CuOH+, the effective charge on the metal center was shown to converge to approximately +1.5 in both cases, despite qualitatively different behavior of their radical spin densities. Therefore, nominally Cu(II) ions exhibit considerable electronic, chemical, and structural flexibility. The electronic origins of this flexibility─including key roles played by the water network itself─are investigated in this work and should provide a conceptual foundation for future studies of copper-based, water-oxidation catalysts.

Energetics and mechanisms for decomposition of cationized amino acids and peptides explored using guided ion beam tandem mass spectrometry

Fragmentation studies of cationized amino acids and small peptides as studied using guided ion beam tandem mass spectrometry (GIBMS) are reviewed. After a brief examination of the key attributes of the GIBMS approach, results for a variety of systems are examined, compared, and contrasted. Cationization of amino acids, diglycine, and triglycine with alkali cations generally leads to dissociations in which the intact biomolecule is lost. Exceptions include most lithiated species as well as a few examples for sodiated and one example for potassiated species. Like the lithiated species, cationization by protons leads to numerous dissociation channels. Results for protonated glycine, cysteine, asparagine, diglycine, and a series of tripeptides are reviewed, along with the thermodynamic consequences that can be gleaned. Finally, the important physiological process of the deamidation of asparagine (Asn) residues is explored by the comparison of five dipeptides in which the C-terminal partner (AsnXxx) is altered. The GIBMS thermochemistry is shown to correlate well with kinetic results from solution phase studies.

Using Density Functional Theory for Testing the Robustness of Mobile-Proton Molecular Dynamics Simulations on Electrosprayed Ions: Structural Implications for Gaseous Proteins

Current experiments only provide low-resolution information on gaseous protein ions generated by electrospray ionization (ESI). Molecular dynamics (MD) simulations can yield complementary insights. Unfortunately, conventional MD does not capture the mobile nature of protons in gaseous proteins. Mobile-proton MD (MPMD) overcomes this limitation. Earlier MPMD data at 300 K indicated that protein ions generated by "native" ESI retain solution-like structures with a hydrophobic core and zwitterionic exterior [Bakhtiari, M.; Konermann, L. J. Phys. Chem. B 2019, 123, 1784-1796]. MPMD redistributes protons using electrostatic and proton affinity calculations. The robustness of this approach has never been scrutinized. Here, we close this gap by benchmarking MPMD against density functional theory (DFT) at the B3LYP/6-31G* level, which is well suited for predicting proton affinities. The computational cost of DFT necessitated the use of small peptides. The MPMD energetic ranking of proton configurations was found to be consistent with DFT single-point energies, implying that MPMD can reliably identify favorable protonation sites. Peptide MPMD runs converged to DFT-optimized structures only when applying 300-500 K temperature cycling, which was necessary to prevent trapping in local minima. Temperature cycling MPMD was then applied to gaseous protein ions. Native ubiquitin converted to slightly expanded structures with a zwitterionic core and a nonpolar exterior. Our data suggest that such inside-out protein structures are intrinsically preferred in the gas phase, and that they form in ESI experiments after moderate collisional excitation. This is in contrast to native ESI (with minimal collisional excitation, simulated by MPMD at 300 K), where kinetic trapping promotes the survival of solution-like structures. In summary, this work validates the MPMD approach for simulations on gaseous peptides and proteins.

Phenyl Sulfate Derivatives: New Thermometer Ions for Characterization of Internal Energy of Negative Ions Produced by Electrospray Ionization

Although positive thermometer ions are widely used for evaluating the internal energy distribution of gas-phase ions, negative thermometer ions have not yet been proposed. In this study, phenyl sulfate derivatives were tested as thermometer ions to characterize the internal energy distribution of ions produced by electrospray ionization (ESI) in the negative mode because the activation of phenyl sulfate preferentially undergoes SO₃ loss, providing a phenolate anion. The dissociation threshold energies for the phenyl sulfate derivatives were determined using quantum chemistry calculations at the CCSD(T)/6-311++G(2df,p)//M06-2X-D3/6-311++G(d,p) level of theory. The values for the appearance energies of the fragment ions of the phenyl sulfate derivatives depend on the dissociation time scale in the experiment; therefore, the dissociation rate constants of the corresponding ions were estimated using the Rice-Ramsperger-Kassel-Marcus theory. The phenyl sulfate derivatives were used as thermometer ions to determine the internal energy distribution of negative ions activated by the in-source collision-induced dissociation (CID) and higher-energy collisional dissociation. Both mean and full width at half-maximum values increased with increasing ion collision energy. In the in-source CID experiments, the internal energy distributions obtained by phenyl sulfate derivatives are similar to that when all voltages are mirrored, and the traditional benzylpyridinium thermometer ions are used. The reported method will aid in determining the optimum voltage for ESI mass spectrometry and the subsequent tandem mass spectrometry of acidic analyte molecules.

Hydrogen Bonding Effect on the Oxygen Binding and Activation in Cobalt(III)-Peroxo Complexes

Cobalt(III)peroxo complexes serve as model metal complexes mediating oxygen activation. We report a systematic study of the effect of hydrogen bonding on the O₂ binding energy and the O-O bond activation within the cobalt(III)peroxo complexes. To this end, we prepared a series of tris(pyridin-2-ylmethyl)amine-based cobalt(III)peroxo complexes having either none, one, two, or three amino groups in the secondary coordination sphere. The hydrogen bonding between the amino group(s) and the peroxo ligand was investigated within the isolated complexes in the gas phase using helium tagging infrared photodissociation spectroscopy, energy-resolved collision-induced dissociation experiments, and density functional theory. The results show that the hydrogen bonding stabilizes the cobalt(III)peroxo core, but the effect is only 10-20 kJ mol^-1. Introducing the first amino group to the secondary coordination sphere has the largest stabilization effect; more amino groups do not change the results significantly. The amino group can transfer a hydrogen atom to the peroxo ligands, which results in the O-O bond cleavage. This process is thermodynamically favored over the O₂ elimination but entropically disfavored.

Periodic trends in the hydration energies and critical sizes of alkaline earth and transition metal dication water complexes

This review encompasses guided ion beam tandem mass spectrometry studies of hydrated metal dication complexes. Metals include the Group 2 alkaline earths (Mg, Ca, Sr, and Ba), late first-row transition metals (Mn, Fe, Co, Ni, Cu, and Zn), along with Cd. In all cases, threshold collision-induced dissociation experiments are used to quantitatively determine the sequential hydration energies for M2+ (H2 O)x complexes ranging in size from one to 11 water molecules. Periodic trends in these bond dissociation energies are examined and discussed. Values are compared to other experimental results when available. In addition to dissociation by simple water ligand loss, complexes at a select size (which differs from metal to metal) are also observed to undergo charge separation to yield a hydrated metal hydroxide cation and a hydrated proton. This leads to the concept of a critical size, xcrit , and the periodic trends in this value are also discussed.

Cationic Gold(II) Complexes: Experimental and Theoretical Study

Gold(II) complexes are rare, and their application to the catalysis of chemical transformations is underexplored. The reason is their easy oxidation or reduction to more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of [Au^II (L)(X)]⁺ complexes (L=ligand, X=halogen) from the corresponding gold(III) precursors and investigated their stability and spectral properties in the IR and visible range in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [Au^II (L)(X)]⁺ complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to those of analogous stable copper(II) complexes.

Characterization of the Internal Energy of Ions Produced by Electrospray Ionization Using Substituted Benzyl Ammonium Thermometer Ions

We propose the use of substituted benzyl ammonium species as thermometer ions to characterize the internal energy distribution of the ions produced by electrospray ionization (ESI). Crucially, we found that the activation of the benzyl ammonium species preferentially provided a benzyl cation via N-Cα bond cleavage. In addition, calculations at the CCSD(T)/cc-PVTZ//M06-2X-D3/6-311++G(d,p) level of theory revealed that the threshold energies of fragmentation of the tested model ions ranged from 86 to 192 kJ mol^-1, significantly lower than those of conventional 4-substituted benzylpyridinium thermometer ions. Thus, the substituted benzyl ammoniums are suitable for the characterization of the ESI process under typical experimental conditions. Further, the internal energies of the ions were found to depend on the radiofrequency voltage of the ion funnel, which is used to increase the transport efficiency of the ions from atmospheric to vacuum conditions. Our reported method will aid the determination of the optimum ion-funnel radiofrequency voltage for the analysis of small molecules such as metabolites. Furthermore, benzyl ammoniums are commercially available, which will facilitate the routine and widespread measurement of the internal energy distributions of ions.

Thermodynamics and Reaction Mechanisms for Decomposition of a Simple Protonated Tripeptide, H+GGA: From H+GGG to H+GAG to H+GGA

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated glycylglycylalanine (H⁺GGA) with Xe using a guided ion beam tandem mass spectrometer. Kinetic energy dependent cross sections for nine ionic products were obtained and analyzed to provide 0 K barriers for the five primary products, [b₂]⁺, [y₁ + 2H]⁺, [b₃]⁺, [y₂ + 2H]⁺, and [a₁]⁺; and four secondary products, [a₂]⁺, [a₃]⁺, high-energy [y₁ + 2H]⁺, and CH₃CHNH₂⁺, after accounting for multiple ion-molecule collisions, the internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-GD3BJ/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the five primary and three secondary products (with the mechanism of the other secondary product previously established). Geometry optimizations and single point energy calculations of reactants, products, intermediates, and TSs were performed at several levels of theory. These theoretical energies are compared with experimental threshold energies and found to give reasonable agreement, with B3LYP-GD3BJ and M06-2X levels of theory performing slightly better than MP2 and better than B3LYP. The results obtained here are compared with previous results for decomposition of H⁺GGG and H⁺GAG to probe the effect of changing the amino acid sequence. Methylation in H⁺GGA has a significant effect on the competition between the primary sequence products, [b₂]⁺ and [y₁ + 2H]⁺, suppressing the [b₂]⁺ cross section by raising its threshold energy, while enhancing that of [y₁ + 2H]⁺ by lowering its threshold energy.

Sodium Binding Interactions with Aliphatic Amino Acids: A Guided Ion Beam and Computational Study

Metal binding affinities play a vital role in medicinal, biological, and industrial applications. In particular, metal cation-amino acid (AA) interactions contribute to protein stability such that analyzing analogous prototypical interactions is important. Here, we present a full description of the interactions of sodium cations (Na⁺) and six aliphatic amino acids (AA), where AA = glycine (Gly), alanine (Ala), homoalanine (hAla), valine (Val), leucine (Leu), and isoleucine (Ile). Experimentally, these interactions are evaluated using threshold collision-induced dissociation carried out in a guided ion beam tandem mass spectrometer, allowing for the determination of the kinetic-energy-dependent behavior of Na⁺-AA dissociation. Analysis of these dissociation cross sections, after accounting for multiple ion-molecule collisions, internal energy of reactant ions, and unimolecular decay rates, allows the determination of absolute Na⁺-AA bond dissociation energies (BDEs) in kJ/mol of Gly (164.0), Ala (166.9), hAla (167.9), Val (172.7), Leu (173.7), and Ile (174.6). These are favorably compared to quantum chemical calculations conducted at the B3LYP, B3P86, MP2(full), B3LYP-GD3BJ, and M06-2X levels of theory. Our combination of structural and energetic analyses provides information regarding the specific factors responsible for Na⁺ interactions with amino acids. Specifically, we find that the BDEs increase linearly with increasing polarizability of the amino acid.

Observation of charged droplets from electrospray ionization (ESI) plumes in API mass spectrometers

Electrospray ionization (ESI) generates bare analyte ions from charged droplets, which result from spraying a liquid in a strong electric field. Experimental observations available in the literature suggest that at least a significant fraction of the initially generated droplets remain large, have long lifetimes, and can thus aspirate into the inlet system of an atmospheric pressure ionization mass spectrometer (API-MS). We report on the observation of fragment signatures from charged droplets penetrating deeply the vacuum stages of three commercial mass spectrometer systems with largely different ion source and spray configurations. Charged droplets can pass through the ion source and pressure reduction stages and even into the mass analyzer region. Since droplet signatures were found in all investigated instruments, the incorporation of charged droplets is considered a general phenomenon occurring with common spray conditions in ESI sources.

"Thermometer" Ions Can Fragment Through an Unexpected Intramolecular Elimination: These Are Not the Fragments You Are Looking For

Benzylpyridinium analogs are effective thermometer ions since monitoring the formation of the benzylium fragment produced from heterolytic cleavage of the C-N bond can be linked to the ion's internal energy. In this study, three para-substituted benzylpyridinium ions containing ethoxy (OEt), isopropoxy (OiPr) and tert-butoxy (OtBu) substitutents were synthesized and evaluated as chemical thermometers. Intriguingly, the product ion spectra of the three benzylpyridinium ions were dominated by m/z 107 instead of the anticipated benzylium species. Deuterium labeling suggested that the m/z 107 fragment resulted from an intramolecular elimination (E_i), which formed via a four-membered transition state (TS). The fragmentation pathway appears to be an anomaly within the mass spectrometry literature, as four-membered pericyclic TSs are usually accompanied by the formation of an exceptionally stable neutral molecule (e.g., CO₂). Quantum-chemical calculations confirmed our hypothesis that stabilization of the strained TS is afforded by hyperconjugation (ΔG^‡ tert-butoxy < isopropyoxy < ethoxy).

Direct Detection of Free and Counterion-Bound Carbanions by Electrospray-Ionization Mass Spectrometry

We propose electrospray-ionization (ESI) mass spectrometry as a robust and powerful method for the in situ analysis of carbanions. ESI mass spectrometry selectively probes the charged components of the sampled solution and, thus, is ideally suited for the detection of free carbanions. We demonstrate the potential of this method by analyzing acetonitrile solutions of 15 different carbon acids AH, whose acidities cover a range of 11.1 ≤ pK_a(DMSO) ≤ 29.5. After treatment with KO^tBu as a strong base, all but the two least acidic compounds were successfully detected as free carbanions A^- and/or as potassium-bound aggregates [K_n-1A_n]^-. The association equilibria can be shifted toward smaller aggregates and free carbanions by the addition of the crown ether 18-crown-6, which facilitates the evaluation of the mass spectra. When KO^tBu was replaced by other bases (LiOH, LiNⁱPr₂, NaH, NaOH, KOH, NBu₄OH) or when tetrahydrofuran or methanol was used as a solvent, carbanions were also successfully observed. For further demonstrating the utility of the proposed method, we applied it to the analysis of the Michael addition of deprotonated dimedone to butenone. ESI mass spectrometry allowed us to follow the decrease of the reactant carbanion and the buildup of the product carbanion in time.

Influence of a Hydroxyl Group on the Deamidation and Dehydration Reactions of Protonated Asparagine-Serine Investigated by Combined Spectroscopic, Guided Ion Beam, and Theoretical Approaches

Deamidation of asparaginyl (Asn) peptides is a spontaneous post-translational modification that plays a significant role in degenerative diseases and other biological processes under physiological conditions. In the gas phase, deamidation of protonated peptides is a major fragmentation channel upon activation by collision-induced dissociation. Here, we present a full description of the deamidation process from protonated asparagine-serine, [AsnSer+H]⁺, via infrared (IR) action spectroscopy and threshold collision-induced dissociation (TCID) experiments in combination with theoretical calculations. The IR results demonstrate that deamidation proceeds via bifurcating reaction pathways leading to furanone- and succinimide-type product ion structures, with a population analysis indicating the latter product dominates. Theory demonstrates that nucleophilic attack of the peptidyl amide oxygen onto the Asn side chain leads to furanone formation, whereas nucleophilic attack by the peptidyl amide nitrogen onto the Asn side-chain carbonyl carbon leads to the formation of the succinimide product structure. TCID experiments find that furanone formation has a threshold energy of 145 ± 12 kJ/mol and succinimide formation occurs with a threshold energy of 131 ± 12 kJ/mol, consistent with theoretical energies and with the spectroscopic results indicating that succinimide dominates. The results provide information regarding the inductive and steric effects of the Ser side chain on the deamidation process. The other major channel observed in the TCID experiments of [AsnSer+H]⁺ is dehydration, where a threshold energy of 104 ± 10 kJ/mol is determined. A complete IR and theoretical analysis of this pathway is also provided. As for deamidation, a bifurcating pathway is found with both dominant oxazoline and minor diketopiperazine products identified. Here, the Ser side chain is directly involved in both pathways.

Unimolecular Fragmentation Properties of Thermometer Ions from Chemical Dynamics Simulations

Thermometer ions are widely used to calibrate the internal energy of the ions produced by electrospray ionization in mass spectrometry. Typically, benzylpyridinium ions with different substituents are used. More recently, benzhydrylpyridinium ions were proposed for their lower bond dissociation energies. Direct dynamics simulations using M06-2X/6-31G(d), DFTB, and PM6-D3 are performed to characterize the activation energies of two representative systems: para-methylbenzylpyridinium ion (p-Me-BnPy⁺) and methyl,methylbenzhydrylpyridinium ion (Me,Me-BhPy⁺). Simulation results are used to calculate rate constants for the two systems. These rate constants and their uncertainties are used to find the Arrhenius activation energies and RRK fitted threshold energies which give reasonable agreement with calculated bond dissociation energies at the same level of theory. There is only one fragmentation mechanism observed for both systems, which involves C-N bond dissociation via a loose transition state, to generate either benzylium or benzhydrylium ion and a neutral pyridine molecule. For p-Me-BnPy⁺ using DFTB and PM6-D3 the formation of tropylium ion, from rearrangement of benzylium ion, was observed but only at higher excitation energies and for longer simulation times. These observations suggest that there is no competition between reaction pathways that could affect the reliability of internal energy calibrations. Finally, we suggest using DFTB with a modified-Arrhenius model in future studies.

Fragmentation study of tryptophan-derived metabolites induced by electrospray ionization mass spectrometry for highly sensitive analysis

ESI of tryptophan-derived metabolites produced an intense signal of fragment ion with a spiro[cyclopropane-indolium] backbone. The use of corresponding fragment ions for the precursor of MRM transitions could improve the detection limit.

Inexpensive Ultrasonic Nebulization Coupled with Direct Current Corona Discharge Ionization Mass Spectrometry for Liquid Samples and Its Fundamental Investigations

The concept of direct mass-spectrometric analysis, especially exploited by ambient desorption/ionization (ADI) methods, provides numerous means for convenient sample analysis. While many simple and versatile ionization sources have been developed, challenges lay in achieving efficient sample introduction. In previous work, a sample introduction method employing direct current corona discharge (CD) coupled to a surface acoustic wave nebulization (SAWN) device enhanced sampling performance for both polar and nonpolar analytes by up to 4 orders of magnitude. In fact, the SAWN-CD method generated a multiply charged peptide ion signal comparable to that of conventional ESI. Unfortunately, the high cost of the SAWN devices themselves limits their accessibility. Herein, we report on an analogous implementation of CD with an inexpensive ultrasonic nebulizer (USN) on the basis of a commercial room humidifier demonstrating equivalent exemplary performance. We subsequently compare the two methods of SAWN-CD and USN-CD in a screening application of milk for the detection of two antibiotic drugs, ciprofloxacin and ampicillin. Finally, we further investigate the relative softness of these CD-coupled acoustic nebulization methods in comparison to that of ESI using a survival yield study of the thermometer ion nitrobenzylpyridinium.

Evaluation of Trapped Ion Mobility Spectrometry Source Conditions Using Benzylammonium Thermometer Ions

A key aspect of reduced pressure ion mobility spectrometry (IMS) experiments is to identify experimental conditions that minimize the role of collisional energy transfer that allows for assessing effective ion-neutral collision cross sections of metabolites, peptides, and proteins in "native-like" or compact states. Across two separate experimental campaigns using a prototype trapped ion mobility spectrometer (TIMS) coupled to a time-of-flight mass spectrometer, we present independent findings that support the results recently published by Morsa et al. using a different set of thermometer ions (Morsa et al. Anal. Chem. 2020, 92 (6), 4573-4582). First, using five para-substituted benzylammonium ions, we conducted survival yield experiments to assess ion internal energy across different experimental settings. Results from the present set of experiments illustrate that greater ion heating occurs at lower pressures and higher voltage settings applied to the TIMS. At the "softest" settings where the benzylammonium thermometer ions have an effective average energy of 1.73 eV, we observe the majority of bradykinin in the compact state. Under more extreme operating conditions where the energy of the benzylammonium ions varies from 1.83 to 1.86 eV, the bradykinin transitions from the compact to the elongated state. In addition to independently confirming the findings of Morsa et al., we also report the mobilities for the benzylammonium parent and fragment ions using the tandem drift-tube-TIMS calibration procedure described by Naylor et al. ( J. Am. Soc. Mass Spectrom. 2019, 30 (10), 2152-2162).

How Hot Are Your Ions in Differential Mobility Spectrometry?

Ions can experience significant field-induced heating in a differential mobility cell. To investigate this phenomenon, the fragmentation of several para-substituted benzylpyridinium "thermometer" ions (R = OMe, Me, F, Cl, H, CN) was monitored in a commercial differential mobility spectrometer (DMS). The internal energy of each benzylpyridinium derivative was characterized by monitoring the degree of fragmentation to obtain an effective temperature, T_eff, which corresponds to a temperature consistent with treating the observed fragmentation ratio using a unimolecular dissociation rate weighted by a Boltzmann distribution at a temperature T. It was found that ions are sufficiently thermalized after initial activation from the ESI process to the temperature of the bath gas, T_bath. Once a critical field strength was surpassed, significant fragmentation of the benzylpyridinium ions was detected. At the maximum bath gas temperature (450 K) and separation voltage (SV; 4400 V) for our instrument, T_eff for the benzylpyridinium derivatives ranged from 664 ± 9 K (p-OMe) to 759 ± 17 K (p-H). The extent of activation at a given SV depends on the ion's mass, degrees of freedom, (N_DoF), and collision frequency as represented by the ion's collision cross section. Plots of T_eff vs the product of ion mass and N_DoF and the inverse of collision cross section produce strong linear relationships. This provides an attractive avenue to estimate ion temperatures at a given SV using only intrinsic properties. Moreover, experimentally determined T_eff correlate with theoretically predicted T_eff using with a self-consistent method based on two-temperature theory. The various instrumental and external parameters that influence T_eff are additionally discussed.

Intact Transition Epitope Mapping - Thermodynamic Weak-force Order (ITEM - TWO)

We have developed an electrospray mass spectrometry method which is capable to determine antibody affinity in a gas phase experiment. A solution with the immune complex is electrosprayed and multiply charged ions are translated into the gas phase. Then, the intact immune-complex ions are separated from unbound peptide ions. Increasing the voltage difference in a collision cell results in collision induced dissociation of the immune-complex by which bound peptide ions are released. When analyzing a peptide mixture, measuring the mass of the complex-released peptide ions identifies which of the peptides contains the epitope. A step-wise increase in the collision cell voltage difference changes the intensity ratios of the surviving immune complex ions, the released peptide ions, and the antibody ions. From all the ions´ normalized intensity ratios are deduced the thermodynamic quasi equilibrium dissociation constants (K_Dm0g^#) from which are calculated the apparent gas phase Gibbs energies of activation over temperature (ΔG_m0g^#T). The order of the apparent gas phase dissociation constants of four antibody - epitope peptide pairs matched well with those obtained from in-solution measurements. The determined gas phase values for antibody affinities are independent from the source of the investigated peptides and from the applied instrument. Data are available via ProteomeXchange with identifier PXD016024. SIGNIFICANCE: ITEM - TWO enables rapid epitope mapping and determination of apparent dissociation energies of immune complexes with minimal in-solution handling. Mixing of antibody and antigen peptide solutions initiates immune complex formation in solution. Epitope binding strengths are determined in the gas phase after electrospraying the antibody / antigen peptide mixtures and mass spectrometric analysis of immune complexes under different collision induced dissociation conditions. Since the order of binding strengths in the gas phase is the same as that in solution, ITEM - TWO characterizes two most important antibody properties, specificity and affinity.

Leaving Group Effects in a Series of Electrosprayed CcHhN1 Anthracene Derivatives

We investigate the gas-phase structures and fragmentation pathways of model compounds of anthracene derivatives of the general formula C_cH_hN₁ utilizing tandem mass spectrometry and computational methods. We vary the substituent alkyl chain length, composition, and degree of branching. We find substantial experimental and theoretical differences between the linear and branched congeners in terms of fragmentation thresholds, available pathways, and distribution of products. Our calculations predict that the linear substituents initially isomerize to form lower energy branched isomers prior to loss of the alkyl substituents as alkenes. The rate-determining chemistry underlying these related processes is dominated by the ability to stabilize the alkene loss transition structures. This task is more effectively undertaken by branched substituents. Consequently, analyte lability systematically increased with degree of branching (linear < secondary < tertiary). The resulting anthracen-9-ylmethaniminium ion generated from these alkene loss reactions undergoes rate-limiting proton transfer to enable expulsion of either hydrogen cyanide or CNH. The combination of the differences in primary fragmentation thresholds and degree of radical-based fragmentation processes provide a potential means of distinguishing compounds that contain branched alkyl chain substituents from those with linear ones.

Thermodynamics and Reaction Mechanisms for Decomposition of a Simple Protonated Tripeptide, H+GAG: a Guided Ion Beam and Computational Study

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated glycylalanylglycine (H⁺GAG) with Xe using a guided ion beam tandem mass spectrometer. Kinetic energy dependent cross sections for nine ionic products were observed and analyzed to provide 0 K barriers for the six primary products: [b₂]⁺, [y₁ + 2H]⁺, [b₃]⁺, CO loss, [y₂ + 2H]⁺, and [a₁]⁺; and three secondary products: [a₂]⁺, [a₃]⁺, and CH₃CHNH₂⁺, after accounting for multiple ion-molecule collisions, internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-GD3BJ/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the six primary and one secondary products (where the other two secondary products have mechanisms previously established). Geometry optimizations and single-point energy calculations were performed at several levels of theory. These theoretical energies are compared with experimental threshold energies and are found to give reasonably good agreement, with B3LYP-GD3BJ and M06-2X levels of theory performing better than other levels. The results obtained here are also compared with previous results for decomposition of H⁺GGG. The primary difference observed is a lowering of the threshold for the [b₂]⁺ product ion and a concomitant suppression of the directly competing [y₁ + 2H]⁺ product, the result of specific methylation of the [b₂]⁺ product ion.

Extended kinetic method and RRKM modeling to reinvestigate proline's proton affinity and approach the meaning of effective temperature

Proline proton affinity PA(Pro) was previously measured by extended kinetic methods with several amines as reference bases using a triple quadrupole mass spectrometer ( J Mass Spectrom 2005; 40: 1300). The measured value of 947.5 ± 5 kJ.mol^-1 differs by more than 10 kJ.mol^-1 from previous reported experimental or calculated values. This difference may be explained in part by the existence of relatively large entropy difference between the two dissociation channels (ΔΔS^‡_avg = 31 ± 10 J.mol^-1.K^-1) and by the inaccuracy of the amines proton affinity used as reference bases. In the present work, these experimental measurements were reinvestigated by RRKM modeling using MassKinetics software. From this modeling, a new PA value of 944.5 ± 5 kJ.mol^-1 and a ΔΔS^‡_avg(600K) value of 33 ± 10 J.mol^-1.K^-1 are determined. However, the difference between experiment and recent theoretical calculations remains large (10 kJ.mol^-1). These RRKM simulations allow also accessing to the effective temperature parameter (T _eff) and to discuss the meaning of this term. As previously reported, T _eff mainly depends on the internal energy and on the decomposition time as well. It also depends on the critical energies and on the transition state. Considering the entrance of the collision cell as a new ion source, T _eff is finally shown to be close to a characteristic temperature (T _char).

Anti-cancer organoruthenium(ii) complexes and their interactions with cysteine and its analogues. A mass-spectrometric study

The ruthenium complexes [Ru(CYM)(p-Cl-dkt)(Cl)] (1), [Ru(CYM)(pta)(p-Cl-dkt)]PF₆ (2), and [Ru(CYM)(pta)Cl₂] (3, RAPTA-C) (CYM = para-cymene, p-Cl-dkt = 1-(4-chlorophenyl)-4,4,4-trifluorobutane-1,3-dione, pta = 1,3,5-triaza-7-phosphaadamantane) are biologically active and show anti-cancer activities, albeit with different mechanisms. To further understand these mechanisms, we compared their speciation in aqueous solutions with an amino acid (cysteine), with an amino acid derivative (N-acetylcysteine) and with a tripeptide (glutathione) by Mass Spectrometry (MS). Here, we show that all ruthenium complexes have high selectivity for cysteine and cysteine-derived molecules. On one hand, [Ru(CYM)(p-Cl-dkt)(Cl)] undergoes solvolysis in water and forms [Ru₂(CYM)₂(OH)₃]⁺. Subsequently, all hydroxyl anions are exchanged by deprotonated cysteine. Infrared Photodissociation Spectroscopy (IRPD) showed that cysteine binds to the ruthenium atoms via the deprotonated thiol group and that sulfur bridges the ruthenium centers. On the other hand, the pta-bearing complexes remain monometallic and undergo only slow Cl or p-Cl-dkt exchange by deprotonated cysteine. Therefore, the pta ligand protects the ruthenium complexes from ligand exchange with water and from the formation of biruthenium clusters, possibly explaining why the mechanism of pta-bearing ruthenium complexes is not based on ROS production but on their reactivity as monometallic complexes.

ESI-MS study of ruthenium complexes shows their high selectivity toward thiol containing molecules and formation of larger thiolate-bound clusters in absence of a protecting ligand such as pta.

Protonated Asparaginyl-Alanine Decomposition: a TCID, SORI-CID, and Computational Analysis

Deamidation of asparagine residues, one of the fastest known post-translational modifications in proteins, plays a significant role in various biological functions and degenerative, aging diseases. Here, we present a full description of deamidation (as well as other key dissociation processes) from protonated asparaginyl-alanine, H⁺(AsnAla), by studying its kinetic energy-dependent threshold collision-induced dissociation (TCID) with Xe using a guided ion beam tandem mass spectrometer. Relative thresholds compare favorably with those acquired by sustained off-resonance irradiation-CID of H⁺(AsnAla) with Ar in a Fourier transform ion cyclotron resonance mass spectrometer. Absolute threshold energies from the TCID studies are compared to relative single point energies of major reaction species calculated at the B3LYP, B3LYP-GD3BJ, B3P86, MP2(full), and M06-2X levels of theory. Relative energies of key TSs and products allow for the characterization of the important rate-limiting steps involved in H⁺(AsnAla) decomposition. The influence of water solvation on key TSs is also explored computationally, where bridging the gap between gas-phase and solvated studies is an important aspect of the biological relevance of this analysis. The comprehensive results presented (in addition to complementary studies discussed herein) allow for an insightful comparison to previous deamidation studies such that effects of the C-terminal residue side chain can be elucidated. Graphical abstract ᅟ.

Integrated approach for confidence-enhanced quantitative analysis of herbal medicines, Cistanche salsa as a case

Although far away from perfect, it is practical to assess the quality of a given herbal medicine (HM) through simultaneous determination of a panel of components. However, the confidences of the quantitative outcomes from LC-MS/MS platform risk several technical barriers, such as chemical degradation, polarity range, concentration span, and identity misrecognition. Herein, we made an attempt to circumvent these obstacles by integrating several fit-for-purpose techniques, including online extraction (OLE), serially coupled reversed phase LC-hydrophilic interaction liquid chromatography (RPLC-HILIC), tailored multiple reaction monitoring (MRM), and relative response vs. collision energy curve (RRCEC) matching. Confidence-enhanced quantitative analysis of Cistanche salsa (Csa), a well-known psammophytic species and tonic herbal medicine, was conducted as a proof-of-concept. OLE module was deployed to prohibit chemical degradation, in particular E/Z-configuration transformation for phenylethanoid glycosides. Satisfactory retention took place for each analyte regardless of polarity because of successive passing through RPLC and HILIC columns. Optimum parameters for the minor components, at the meanwhile of inferior ones for the abundant ingredients, ensured the locations of all contents in the linear ranges. The unequivocal assignment of the captured signals was achieved by matching retention times, ion transitions, and more importantly, RRCECs between authentic compounds and suspect peaks. Diverse validation assays demonstrated the newly developed method to be reliable. Particularly, the distribution of mannitol rather than galactitol was disclosed although these isomers showed identical retention time and ion transitions. The contents of 21 compounds-of-interest were definitively determined in Csa as well as two analogous species, and the quantitative patterns exerted great variations among not only different species but different Csa samples. Together, the fortification of OLE-RPLC-HILIC-tailored MRM with RRCEC matching could fully address the demands from confidence-enhanced quantitative analysis of HMs.

Calculation of Ligand Dissociation Energies in Large Transition-Metal Complexes

The accurate calculation of ligand dissociation (or equivalently, ligand binding) energies is crucial for computational coordination chemistry. Despite its importance, obtaining accurate ab initio reference data is difficult, and density-functional methods of uncertain reliability are chosen for feasibility reasons. Here, we consider advanced coupled-cluster and multiconfigurational approaches to reinvestigate our WCCR10 set of 10 gas-phase ligand dissociation energies [ J. Chem. Theory Comput. 2014, 10, 3092]. We assess the potential multiconfigurational character of all molecules involved in these reactions with a multireference diagnostic [ Mol. Phys. 2017, 115, 2110] in order to determine where single-reference coupled-cluster approaches can be applied. For some reactions of the WCCR10 set, large deviations of density-functional results including semiclassical dispersion corrections from experimental reference data had been observed. This puzzling observation deserves special attention here, and we tackle the issue (i) by comparing to ab initio data that comprise dispersion effects on a rigorous first-principles footing and (ii) by a comparison of density-functional approaches that model dispersion interactions in various ways. For two reactions, species exhibiting nonnegligible static electron correlation were identified. These two reactions represent hard problems for electronic structure methods and also for multireference perturbation theories. However, most of the ligand dissociation reactions in WCCR10 do not exhibit static electron correlation effects, and hence, we may choose standard single-reference coupled-cluster approaches to compare with density-functional methods. For WCCR10, the Minnesota M06-L functional yielded the smallest mean absolute deviation of 13.2 kJ mol^-1 out of all density functionals considered (PBE, BP86, BLYP, TPSS, M06-L, PBE0, B3LYP, TPSSh, and M06-2X) without additional dispersion corrections in comparison to the coupled-cluster results, and the PBE0-D3 functional produced the overall smallest mean absolute deviation of 4.3 kJ mol^-1. The agreement of density-functional results with coupled-cluster data increases significantly upon inclusion of any type of dispersion correction. It is important to emphasize that different density-functional schemes available for this purpose perform equally well. The coupled-cluster dissociation energies, however, deviate from experimental results on average by 30.3 kJ mol^-1. Possible reasons for these deviations are discussed.

Deamidation of Protonated Asparagine-Valine Investigated by a Combined Spectroscopic, Guided Ion Beam, and Theoretical Study

Peptide deamidation of asparaginyl residues is a spontaneous post-translational modification that is believed to play a role in aging and several diseases. It is also a well-known small-molecule loss channel in the MS/MS spectra of protonated peptides. Here we investigate the deamidation reaction, as well as other decomposition pathways, of the protonated dipeptide asparagine-valine ([AsnVal + H]+) upon low-energy activation in a mass spectrometer. Using a combination of infrared ion spectroscopy, guided ion beam tandem mass spectrometry, and theoretical calculations, we have been able to identify product ion structures and determine the energetics and mechanisms for decomposition. Deamidation proceeds via ammonia loss from the asparagine side chain, initiated by a nucleophilic attack of the peptide bond oxygen on the γ-carbon of the Asn side chain. This leads to the formation of a furanone ring containing product ion characterized by a threshold energy of 129 ± 5 kJ/mol (15 kJ/mol higher in energy than dehydration of [AsnVal + H]+, the lowest energy dissociation channel available to the system). Competing formation of a succinimide ring containing product, as has been observed for protonated asparagine-glycine ([AsnGly + H]+) and asparagine-alanine ([AsnAla + H]+), was not observed here. Quantum-chemical modeling of the reaction pathways confirms these subtle differences in dissociation behavior. Measured reaction thresholds are in agreement with predicted theoretical reaction energies computed at several levels of theory.

Binding energies of hydrated cobalt(ii) by collision-induced dissociation and theoretical studies: evidence for a new critical size

The experimental sequential bond energies for loss of water from Co²⁺(H₂O)_x complexes, x = 5-11, are determined by threshold collision-induced dissociation (TCID) using a guided ion beam tandem mass spectrometer with a thermal electrospray ionization source. Kinetic energy dependent TCID cross sections are analyzed to yield 0 K thresholds for sequential loss of neutral water molecules. The thresholds are converted from 0 to 298 K values to give hydration enthalpies and free energies. Theoretical geometry optimizations and single point energy calculations at several levels of theory are performed for the reactant and product ion complexes. Theoretical bond energies for ground structures are used for direct comparison with experimental values to obtain structural information on these complexes. In addition, the dissociative charge separation process, Co²⁺(H₂O)_x → CoOH⁺(H₂O)_m + H⁺(H₂O)_x-m-1, is observed at x = 4, 6, and 7 in competition with primary water loss products. Energies for the charge separation rate-limiting transition states are calculated and compared to experimental threshold measurements. Results suggest that the critical size for which charge separation is energetically favored over water loss is x_crit = 6, in contrast to lower values in previous literature reports.

Internal Energy Deposition in Dielectric Barrier Discharge Ionization is Significantly Lower than in Direct Analysis in Real-Time Mass Spectrometry

The extent of internal energy deposition using three different plasma-based ionization mass spectrometry (MS) methods, atmospheric pressure chemical ionization (APCI), direct analysis in real time (DART), and active capillary dielectric barrier discharge ionization (DBDI), was investigated using benzylammonium ‘thermometer’ ions. Ions formed by DBDI were activated significantly less than those that were formed by DART and APCI under these conditions. Thermal ion activation by DART can be reduced slightly by positioning the DART source further from the capillary entrance to the MS and reducing the heat that is applied to metastable atoms exiting the DART source. For example, the average ion internal energy distribution decreased by less than 10 % (166.9 ± 0.3 to 152.2 ± 1.0 kJ mol−1) when the distance between the DART source and the MS was increased by 250 % (10 to 25 mm). By lowering the DART temperature from 350 to 150°C, the internal energy distributions of the thermometer ions decreased by ~15 % (169.93 ± 0.83 to 150.21 ± 0.52 kJ mol−1). Positioning the DART source nozzle more than 25 mm from the entrance to the MS and decreasing the DART temperature further resulted in a significant decrease in ion signal. Thus, varying the major DART ion source parameters had minimal impact on the ‘softness’ of the DART ion source under these conditions. Overall, these data indicate that DBDI can be a significantly ‘softer’ ion source than two of the most widely used plasma-based ion sources that are commercially available.