Going beyond electrospray: mass spectrometric studies of chemical reactions in and on liquids

Revisiting Biginelli-like reactions: solvent effects, mechanisms, biological applications and correction of several literature reports

This study critically reevaluates reported Biginelli-like reactions using a Kamlet-Abboud-Taft-based solvent effect model. Surprisingly, structural misassignments were discovered in certain multicomponent reactions, leading to the identification of pseudo three-component derivatives instead of the expected MCR adducts. Attempts to replicate literature conditions failed, prompting reconsideration of the described MCRs and proposed mechanisms. Electrospray ionization (tandem) mass spectrometry, NMR, melting points, elemental analyses and single-crystal X-ray analysis exposed inaccuracies in reported MCRs and allowed for the proposition of a complete catalytic cycle. Biological investigations using both pure and "contaminated" derivatives revealed distinctive features in assessed bioassays. A new cellular action mechanism was unveiled for a one obtained pseudo three-component adduct, suggesting similarity with the known dihydropyrimidinone Monastrol as Eg5 inhibitors, disrupting mitosis by forming monoastral mitotic spindles. Docking studies and RMSD analyses supported this hypothesis. The findings described herein underscore the necessity for a critical reexamination and potential corrections of structural assignments in several reports. This work emphasizes the significance of rigorous characterization and critical evaluation in synthetic chemistry, urging a careful reassessment of reported synthesis and biological activities associated with these compounds.

SAM-SFM: High-Efficiency and High-Resolution Tandem Mass Spectrometry Enabled by Sinusoidal Amplitude Modulation of Multiple Sinusoidal Frequency-Modulated Waveforms

In miniature ion trap mass spectrometry, achieving a balance between isolation resolution and efficiency is a formidable challenge. The presence of absorption curves causes target ions to inadvertently absorb energy from AC signal components near their resonant frequencies. To mitigate this issue, SAM-SFM waveforms introduce a parameter known as the decreasing factor. Unlike SWIFT waveforms, SAM-SFM's spectral profile intentionally departs from a rectangular window, adopting an arch-shaped excitation window to minimize the impact on target ions and improve ion isolation efficiency. SAM-SFM waveforms have the advantage of low computational complexity, enabling real-time computation using an embedded FPGA technology. Regardless of any parameter changes, the FPGA can consistently guarantee waveform output within 1 μs. This not only enhances throughput but also eliminates the need for a PC in miniature mass spectrometry devices. The performance of SAM-SFM has been validated on an improved "Brick" miniature ion trap mass spectrometer. Comparative experiments with SWIFT waveforms confirm the lossless unit-mass isolation capabilities of SAM-SFM. This waveform has the capability to simultaneously isolate multiple target ions, even allowing for the lossless isolation of ions with lower abundance within isotopic clusters, albeit at the cost of requiring extended isolation durations.

Limitations of Atomistic Molecular Dynamics to Reveal Ejection of Proteins from Charged Nanodroplets

Atomistic molecular dynamics (MD) is frequently used to unravel the mechanisms of macroion release from electrosprayed droplets. However, atomistic MD is currently feasible for only the smallest window of droplet sizes appearing at the end steps of a droplet's lifetime. The relevance of the observations made to the actual droplet evolution, which is much longer than the simulated sizes, has not been addressed yet in the literature. Here, we perform a systematic study of the desolvation mechanisms of poly(ethylene glycol) (PEG), protonated peptides of different compositions, and proteins, to (a) obtain insight into the charging mechanism of macromolecules in larger droplets than those that are currently amenable to atomistic MD and (b) examine whether currently used atomistic MD modeling can establish the extrusion mechanism of proteins from droplets. To mimic larger droplets that are not amenable to MD modeling, we scale down the systems, by simulating a large droplet size relative to the macromolecule. MD of PEG charging reveals that, above a critical droplet size, ions are available near the backbone of the macromolecule, but charging occurs only transiently by transfer of ions from the solvent to the macroion, while below the critical size, the capture of the ion from PEG has a lifetime sufficiently long for the extrusion of a charged PEG from the aqueous droplet. This is the first report of the role of droplet curvature in the relation between macroion conformation and charging. Simulations of protonated peptides with a high degree of hydrophobicity show that partial extrusion of a peptide from the droplet surface is rare relative to desolvation by drying-out. Different from what has been presented in the literature, we argue that atomistic MD simulations have not sufficiently established the extrusion mechanism of proteins from droplets and their charging mechanism. We also argue that release of highly charged proteins can occur at an earlier stage of a droplet's lifetime than predicted by atomistic MD. In this earlier stage, we emphasize the key role of jets emanating from a droplet at the point of charge-induced instability in the release of proteins.

Immuno-Desorption Electrospray Ionization Mass Spectrometry Imaging Identifies Functional Macromolecules by Using Microdroplet-Cleavable Mass Tags

We present immunoassay-based desorption electrospray ionization mass spectrometry imaging (immuno-DESI-MSI) to visualize functional macromolecules such as drug targets and cascade signaling factors. A set of boronic acid mass tags (BMTs) were synthesized to label antibodies as MSI probes. The boronic ester bond is employed to cross-link the BMT with the galactosamine-modified antibody. The BMT can be released from its tethered antibody by ultrafast cleavage of the boronic ester bond caused by the acidic condition of sprayed DESI microdroplets containing water. The fluorescent moiety enables the BMT to work in both optical and MS imaging modes. The positively charged quaternary ammonium group enhances the ionization efficiency. The introduction of the boron element also makes mass tags readily identified because of its unique isotope pattern. Immuno-DESI-MSI provides an appealing strategy to spatially map macromolecules beyond what can be observed by conventional DESI-MSI, provided antibodies are available to the targeted molecules of interest.

In-Situ Analysis of Anionic Coordination Polymerizations by Electrospray-Ionization Mass Spectrometry

Anionic coordination polymerizations proceed via highly reactive intermediates, whose in situ analysis has remained difficult. Here, we show that electrospray-ionization mass spectrometry is a promising method to obtain detailed information on the polymerization process. Focusing on polymerization reactions of 1,3-dienes initiated by CoCl₂ /RLi (R=Me, nBu, tBu, Ph), we directly observe the growing polymer chains and characterize the active anionic cobalt centers by gas-phase fragmentation experiments. On the basis of these results, we suggest a plausible mechanism for the polymerization reaction. Moreover, the ESI mass spectra permit the determination of molecular weight distributions, which are in good agreement with those derived from NMR-spectroscopic as well as MALDI mass-spectrometric measurements, and afford a wealth of kinetic data.

Characterization of selected organometallic compounds by electrospray ionization- and matrix-assisted laser desorption/ionization-mass spectrometry using different types of instruments: Possibilities and limitations

RATIONALE

Organometallic compounds are becoming increasingly important in their industrial application as catalysts. Mass spectrometry is an essential tool for the structural confirmation of such organometallics. Because the analysis of this class of molecules can be challenging, the ionization behavior and structural confirmation of selected transition metal catalysts are described in this work.

METHODS

The transition metal catalysts investigated were analyzed using classical vacuum MALDI reflectron TOF-MS as well as intermediate pressure matrix-assisted laser desorption/ionization quadrupole time-of-flight mass spectrometry (MALDI QTOF-MS). Obtained mass spectra were compared with electrospray ionization MS (ESI-MS) already established for organometallic compounds, utilizing a QTOF mass spectrometer here. In addition, various sample preparations, including two selected MALDI matrices (trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile and 2,2':5',2″-terthiophene) with different solvent combinations for MALDI-MS measurements, were investigated in detail with respect to their correct isotope distribution of the molecular ions observed.

RESULTS

All investigated organometallic compounds were successfully identified by vacuum and intermediate pressure MALDI-MS. Accurate masses of ions related to molecular ion species (e.g., [M-Cl]⁺ , [M]⁺ , and [M + Na]⁺ ) could be determined by MALDI QTOF-MS measurements with a mass error of less than ±5 ppm for all compounds. Both investigated MALDI matrices performed equally on both instruments. The impact of the analyte/matrix solvent mixtures turned out to be crucial for a successful analysis of the investigated compounds. In contrast, ESI QTOF-MS yielded masses of ions related to molecular ion species in favorable cases.

CONCLUSIONS

The use of MALDI-MS for the structural confirmation of organometallic compounds is still not widely used. Nevertheless, this work showed that this analytical technique does have some benefits. The analysis of neutral catalysts proves to be quite useful, concluding that this technique provides a complement and/or an alternative to ESI-MS.

On-Water Selectivity Switch in Microdroplets in the 1,2,3-Triazole Synthesis from Bromoethenesulfonyl Fluoride

Water profoundly affects many organic reactions by accelerating them or changing their selectivity. Performing reactions "on-water" offers an intriguing opportunity to influence chemical reactivity. A nebulizer plume is an efficient way of generating microdroplets─the uniquely complex reaction environment which opens alternative possibilities that are not readily accessible in bulk emulsions. We describe the on-water switch of chemoselectivity in the formation of triazoles controlled by the on-water environment in dual spray. These conditions facilitate elimination of H-SO₂F from the triazoline intermediate, whereas the reaction in organic solvents results in the exclusive HBr elimination. The influence of two-phase conditions was investigated to obtain the best reaction efficiency, and the crucial importance of the water/organic interface interactions was verified by pH variation and D₂O use.

p-Aminobenzoic acid protonation dynamics in an evaporating droplet by ab initio molecular dynamics

Protonation equilibria are known to vary from the bulk to microdroplet conditions, which could induce many chemical and physical phenomena. Protonated p-aminobenzoic acid (PABA + H⁺) can be considered a model for probing the protonation dynamics in an evaporating droplet, as its protonation equilibrium is highly dependent on the formation conditions from solution via atmospheric pressure ionization sources. Experiments using diverse experimental techniques have shown that protic solvents allow formation of the O-protomer (PABA protonated in the carboxylic acid group) stable in the gas phase, while aprotic solvents yield the N-protomer (protonated in the amino group) that is the most stable protomer in solution. In this work, we explore the protonation equilibrium of PABA solvated by different numbers of water molecules (n = 0 to 32) using ab initio molecular dynamics. For n = 8-32, the protonation is either at the NH₂ group or in the solvent network. The solvent network interacts with the carboxylic acid group, but there is no complete proton transfer to form the O-protomer. For smaller clusters, however, solvent-mediated proton transfers to the carboxylic acid were observed, both via the Grotthuss mechanism and the vehicle or shuttle mechanism (for n = 1 and 2). Thermodynamic considerations allowed a description of the origins of the kinetic trapping effect, which explains the observation of the solution structure in the gas phase. This effect likely occurs in the final evaporation steps, which are outside the droplet size range covered by previous classical molecular dynamics simulations of charged droplets. These results may be considered relevant in determining the nature of the species observed in the ubiquitous ESI based mass spectrometry analysis, and in general for droplet chemistry, explaining how protonation equilibria are drastically changed from bulk to microdroplet conditions.

Measurement of the Population of Electrosprayed Deprotomers of Coumaric Acids Using UV-Vis Laser Photodissociation Spectroscopy

The measurement of deprotonation sites in multifunctional molecules following electrospray ionization is important to better inform a wide range of spectroscopic and photophysical studies that use electrospray to prepare molecular species for study in the gas phase. We demonstrate that low-resolution UV-vis laser photodissociation spectroscopy can be applied in situ to identify the deprotomers of three coumaric acids, trans-para-coumaric acid (CMA), trans-caffeic acid (CA), and trans-ferulic acid (FA), formed via electrospray. Electronic absorption spectra of the deprotonated coumaric acids are recorded via photodepletion and photofragmentation following electrospray from solutions of ethanol and acetonitrile. By comparing the experimental spectra to wave function theory calculations, we are able to confirm the presence of phenoxide and carboxylate deprotomers upon electrospray for all three coumaric acids, when sprayed from both protic and aprotic solvents. Ratios of the phenoxide:carboxylate deprotomers are obtained by generating summed theoretical absorption spectra that reproduce the experimental spectra. We find that choice of electrospray solvent has little effect on the ratio of deprotomers obtained for deprotonated CMA and CA but has a greater impact for FA. Our results are in excellent agreement with previous work conducted on deprotonated CMA using IR spectroscopy and demonstrate that UV photodissociation spectroscopy of electrosprayed ions has potential as a diagnostic tool for identifying deprotomeric species.

Low Density Interior in Supercooled Aqueous Nanodroplets Expels Ions to the Subsurface

The interaction between water and ions within droplets plays a key role in the chemical reactivity of atmospheric and man-made aerosols. Here we report direct computational evidence that in supercooled aqueous nanodroplets a lower density core of tetrahedrally coordinated water expels the cosmotropic ions to the denser and more disordered subsurface. In contrast, at room temperature, depending on the nature of the ion, the radial distribution in the droplet core is nearly uniform or elevated toward the center. We analyze the spatial distribution of a single ion in terms of a reference electrostatic model. The energy of the system in the analytical model is expressed as the sum of the electrostatic and surface energy of a deformable droplet. The model predicts that the ion is subject to a harmonic potential centered at the droplet's center of mass. We name this effect "electrostatic confinement". The model's predictions are consistent with the simulation findings for a single ion at room temperature but not at supercooling. We anticipate this study to be the starting point for investigating the structure of supercooled (electro)sprayed droplets that are used to preserve the conformations of macromolecules originating from the bulk solution.

DESI-MS imaging to visualize spatial distribution of xenobiotics and endogenous lipids in the skin

The cutaneous biodistribution method (CBM) yields a high-resolution quantitative profile of drug deposition as a function of skin depth. However, it provides limited details about drug spatial distribution or penetration pathways. Mass spectrometry imaging (MSI) can complement the detailed quantitative data generated by CBM studies. The objectives of this work were to use desorption electrospray ionization (DESI)-MSI to (i) investigate the spatial cutaneous distributions of a topically applied drug and excipient and relate them to skin structures and (ii) image endogenous skin components and combine these results to gain insight into drug penetration routes. Porcine skin was used to compare two bioequivalent creams of econazole nitrate (ECZ) and a micelle formulation based on D-α-tocopheryl succinate polyethylene glycol 1000 (TPGS). DESI-MSI successfully imaged the cutaneous spatial distribution of ECZ and TPGS in 40 µm-thick horizontal sections and vertical cross-sections of the skin. Interestingly, clinically bioequivalent formulations did not appear to exhibit the same molecular distribution of ECZ in XY-horizontal sections. DESI-MSI also enabled visualization of TPGS (m/z 772.4706), mainly in the upper epidermis (≤80 µm). In conclusion, through co-localization of drugs and excipients with endogenous elements of the skin, DESI-MSI could further our understanding of the cutaneous penetration pathways of xenobiotics.

Direct Liquid Extraction and Ionization Techniques for Understanding Multimolecular Environments in Biological Systems (Secondary Publication)

A combination of direct liquid extraction using a small volume of solvent and electrospray ionization allows the rapid measurement of complex chemical components in biological samples and visualization of their distribution in tissue sections. This review describes the development of such techniques and their application to biological research since the first reports in the early 2000s. An overview of electrospray ionization, ion suppression in samples, and the acceleration of specific chemical reactions in charged droplets is also presented. Potential future applications for visualizing multimolecular environments in biological systems are discussed.

Mass spectrometry-based chemical mapping and profiling toward molecular understanding of diseases in precision medicine

Precision medicine has been strongly promoted in recent years. It is used in clinical management for classifying diseases at the molecular level and for selecting the most appropriate drugs or treatments to maximize efficacy and minimize adverse effects. In precision medicine, an in-depth molecular understanding of diseases is of great importance. Therefore, in the last few years, much attention has been given to translating data generated at the molecular level into clinically relevant information. However, current developments in this field lack orderly implementation. For example, high-quality chemical research is not well integrated into clinical practice, especially in the early phase, leading to a lack of understanding in the clinic of the chemistry underlying diseases. In recent years, mass spectrometry (MS) has enabled significant innovations and advances in chemical research. As reported, this technique has shown promise in chemical mapping and profiling for answering "what", "where", "how many" and "whose" chemicals underlie the clinical phenotypes, which are assessed by biochemical profiling, MS imaging, molecular targeting and probing, biomarker grading disease classification, etc. These features can potentially enhance the precision of disease diagnosis, monitoring and treatment and thus further transform medicine. For instance, comprehensive MS-based biochemical profiling of ovarian tumors was performed, and the results revealed a number of molecular insights into the pathways and processes that drive ovarian cancer biology and the ways that these pathways are altered in correspondence with clinical phenotypes. Another study demonstrated that quantitative biomarker mapping can be predictive of responses to immunotherapy and of survival in the supposedly homogeneous group of breast cancer patients, allowing for stratification of patients. In this context, our article attempts to provide an overview of MS-based chemical mapping and profiling, and a perspective on their clinical utility to improve the molecular understanding of diseases for advancing precision medicine.

Simulation Verification of Barrierless HONO Formation from the Oxidation Reaction System of NO, Cl, and Water in the Atmosphere

Nitrous acid (HONO) is a major source of hydroxyl (OH) radicals, and identifying its source is crucial to atmospheric chemistry. Here, a new formation route of HONO from the reaction of NO with Cl radicals with the aid of one or two water molecules [(Cl) (NO) (H₂O)_n (n = 1-2)] as well as on the droplet surface was found by Born-Oppenheimer molecular dynamic simulation and metadynamic simulation. The (Cl) (NO) (H₂O)₁ (monohydrate) system exhibited a free-energy barrier of approximately 0.95 kcal mol^-1, whereas the (Cl) (NO) (H₂O)₂ (dihydrate) system was barrierless. For the dihydrate system and the reaction of NO with Cl radicals on the droplet surface, only one water molecule participated in the reaction and the other acted as the "solvent" molecule. The production rates of HONO suggested that the monohydrate system ([NO] = 8.56 × 10¹² molecule cm^-3, [Cl] = 8.00 × 10⁶ molecule cm^-3, [H₂O] = 5.18 × 10¹⁷ molecule cm^-3) could account for 40.3% of the unknown HONO production rate (P_unknown) at site 1 and 53.8% of P_unknown at site 2 in the East China Sea. This study identified the importance of the reaction system of NO, Cl, and water molecules in the formation of HONO in the marine boundary layer region.

How and Why to Investigate Multicomponent Reactions Mechanisms? A Critical Review

We review the most innovative efforts and greatest challenges faced when elucidating multicomponent reactions (MCRs) mechanisms. When compared to traditional reactions, the often two or more concurrent reactions pathways and the greater number of possible intermediates in MCRs turn their mechanistic investigation both a harder and trickier task. The common approaches used to investigate reaction mechanisms are often unable to clarify MCRs mechanisms; hence few but clever approaches are currently used to determine these mechanisms and to depict their key transformations. Their complexity has required most innovative approaches and the use of a number of unique techniques that have shed light over the favored pathway selected from the myriad of alternatives theoretically available for MCRs. This review focuses on the most successful efforts applied by a few leading groups to perform these puzzlingly investigations.

On-chip mass spectrometric analysis in non-polar solvents by liquid beam infrared matrix-assisted laser dispersion/ionization

By the on-chip integration of a droplet generator in front of an emitter tip, droplets of non-polar solvents are generated in a free jet of an aqueous matrix. When an IR laser irradiates this free liquid jet consisting of water as the continuous phase and the non-polar solvent as the dispersed droplet phase, the solutes in the droplets are ionized. This ionization at atmospheric pressure enables the mass spectrometric analysis of non-polar compounds with the aid of a surrounding aqueous matrix that absorbs IR light. This works both for non-polar solvents such as n-heptane and for water non-miscible solvents like chloroform. In a proof of concept study, this approach is applied to monitor a photooxidation of N-phenyl-1,2,3,4-tetrahydroisoquinoline. By using water as an infrared absorbing matrix, analytes, dissolved in non-polar solvents from reactions carried out on a microchip, can be desorbed and ionized for investigation by mass spectrometry.

A Domino Fusion of an Organic Ligand Depended on Metal-Induced and Oxygen Insertion, Unraveled by Crystallography, Mass Spectrometry, and DFT Calculations

Herein, the reaction of (1-methyl-1 H-benzo[d]imidazol-2-yl)methanamine (L1) with Co(H₂ O)₆ Cl₂ , in CH₃ CN at 120 °C, leading to the 2,3,5,6-tetrakis(1-methyl-1 H-benzo[d]imidazol-2-yl)pyrazine (3), isolated as a dimeric cluster {[Co^II ₂ (3)Cl₄ ]⋅2 CH₃ CN} (2), is reported. When O₂ and H₂ O are present, (1-methyl-1 H-benzo[d]imidazole-2-carbonyl)amide (HL1') is first formed and crystallized as [Co^III (L1)₂ (L1')]Cl₂ ⋅2 H₂ O (4) before fusion of HL1' with L1, giving 1-methyl-N-(1-methyl-1 H-benzo[d]imidazol-2-carbonyl)-1 H-benzo[d]imidazol-2-carboxamide (HL2'') forming a one-dimensional (1D) chain of [Co^II ₃ (L2'')₂ Cl₄ ]_n (5). The combination of crystallography and mass spectrometry (ESI-MS) of isolated crystals and the solutions taken from the reaction as a function time reveal seven intermediate steps leading to 2, but six steps for 5, for which a different sequence takes place. Control and isotope labeling experiments confirm the two carbonyl oxygen atoms in 5 originate from both air and water. The dependence on the metals, compared with FeCl₃ ⋅6 H₂ O leading to a stable triheteroarylmethyl radical, is quite astounding, which could be attributed to the different oxidation states of the metals and coordination modes confirmed by DFT calculations. This metal and valence dependent process is a very useful way for selectively obtaining these large molecules, which are unachievable by common organic synthesis.

Molecular Characterization of the Surface Excess Charge Layer in Droplets

The surface excess charge layer (SECL) in droplets has often been associated with distinct chemistry. We examine the effect of the nature of ions in the composition and structure of SECL by using molecular dynamics. We find that in the presence of simple ions the thickness of SECL is invariant not only with respect to droplet size but also with respect to the nature of the ions. In the presence of simple ions, this layer has a thickness of ∼1.5-1.7 nm but in the presence of macroions it may extend to ∼2.0 nm. The proportion of ions contained in SECL depends on the nature of the ions and the droplet size. For the same droplet size, I^- and model H₃O⁺ ions show considerably higher concentration than Na⁺ and Cl^- ions. We identify the maximum ion concentration region, which, in nanodrops, may partially overlap with SECL. As the relative shape fluctuations decrease when microdrop size is approached, the overlap between SECL and maximum ion concentration region increases. We suggest the extension of the bilayer droplet structure assumed in the equilibrium partitioning model of Enke to include the maximum ion concentration region that may not coincide with SECL in nanodrops. We compute the ion concentrations in SECL, which are those that should enter the kinetic equation in the ion-evaporation mechanism, instead of the overall drop ion concentration that has been used.

Exploring the Molecular Conformation Space by Soft Molecule-Surface Collision

Biomolecules function by adopting multiple conformations. Such dynamics are governed by the conformation landscape whose study requires characterization of the ground and excited conformation states. Here, the conformational landscape of a molecule is sampled by exciting an initial gas-phase molecular conformer into diverse conformation states, using soft molecule-surface collision (0.5-5.0 eV). The resulting ground and excited molecular conformations, adsorbed on the surface, are imaged at the single-molecule level. This technique permits the exploration of oligosaccharide conformations, until now, limited by the high flexibility of oligosaccharides and ensemble-averaged analytical methods. As a model for cellulose, cellohexaose chains are observed in two conformational extremes, the typical "extended" chain and the atypical "coiled" chain-the latter identified as the gas-phase conformer preserved on the surface. Observing conformations between these two extremes reveals the physical properties of cellohexaose, behaving as a rigid ribbon that becomes flexible when twisted. The conformation space of any molecule that can be electrosprayed can now be explored.

Accelerated reactions of amines with carbon dioxide driven by superacid at the microdroplet interface

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk. Carbon dioxide undergoes C–N bond formation reactions with amines at the interface of droplets to form carbamic acids. Electrospray ionization mass spectrometry displays the reaction products in the form of the protonated and deprotonated carbamic acid. Electrosonic spray ionization (ESSI) utilizing carbon dioxide as nebulization gas, confines reaction to the gas–liquid interface where it proceeds much faster than in the bulk. Intriguingly, trace amounts of water accelerate the reaction, presumably by formation of superacid or superbase at the water interface. The suggested mechanism of protonation of CO₂ followed by nucleophilic attack by the amine is analogous to that previously advanced for imidazole formation from carboxylic acids and diamines.

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk.

Identifying reactive intermediates by mass spectrometry

Development of new reactions requires finding and understanding of novel reaction pathways. In challenging reactions such as C-H activations, these pathways often involve highly reactive intermediates which are the key to our understanding, but difficult to study. Mass spectrometry has a unique sensitivity for detecting low abundant charged species; therefore it is increasingly used for detection of such intermediates in metal catalysed- and organometallic reactions. This perspective shows recent developments in the field of mass spectrometric research of reaction mechanisms with a special focus on going beyond mass-detection. Chapters discuss the advantages of collision-induced dissociation, ion mobility and ion spectroscopy for characterization of structures of the detected intermediates. In addition, we discuss the relationship between the condensed phase chemistry and mass spectrometric detection of species from solution.

Facile Strategy to Generate Charged Droplets with Desired Polarities

Water droplets are usually charged positively via either electrospray or contact electrification at the solid/liquid interface. Herein, we describe a facile two-step strategy to generate charged droplets with desired polarities. In particular, negatively charged droplets can be generated via electrostatic induction using a precharged superamphiphobic substrate as an electret. The interplay of repulsive and attractive interactions between like- and unlike-charged droplets or electret leads to rapid droplet transport and self-assembly of specific highly ordered arrays.

Accelerated Reaction Kinetics in Microdroplets: Overview and Recent Developments

Various organic reactions, including important synthetic reactions involving C-C, C-N, and C-O bond formation as well as reactions of biomolecules, are accelerated when the reagents are present in sprayed or levitated microdroplets or in thin films. The reaction rates increase by orders of magnitude with decreasing droplet size or film thickness. The effect is associated with reactions at the solution-air interface. A key factor is partial solvation of the reagents at the interface, which reduces the critical energy for reaction. This phenomenon is of intrinsic interest and potentially of practical value as a simple, rapid method of performing small-scale synthesis.

Accelerated Formation of H2 Nanobubbles from a Surface Nanodroplet Reaction

The compartmentalization of chemical reactions within droplets has advantages in low costs, reduced consumption of reagents, and increased throughput. Reactions in small droplets have also been shown to greatly accelerate the rate of many chemical reactions. The accelerated growth rate of nanobubbles from nanodroplet reactions is demonstrated in this work. The gaseous products from the reaction at the nanodroplet surface promoted nucleation of hydrogen nanobubbles within multiple organic liquid nanodroplets. The nanobubbles were confined within the droplets and selectively grew and collapsed at the droplet perimeter, as visualized by microscopy with high spatial and temporal resolutions. The growth rate of the bubbles was significantly accelerated within small droplets and scaled inversely with droplet radius. The acceleration was attributed to confinement from the droplet volume and effect from the surface area on the interfacial chemical reaction for gas production. The results of this study provide further understanding for applications in droplet enhanced production of nanobubbles and the on-demand liberation of hydrogen.

A Gas-Phase Reaction Accelerator Using Vortex Flows

Gas-phase microdroplets have been increasingly used for reaction acceleration. Here, we report the development of a vortex tube as a reaction accelerator. Three types of reactions, viz., aromatization, amination isomerization, and acid-induced cytochrome c unfolding were used to characterize the performance of the vortex tube. During ion transfer from a nanoelectrospray ionization (nanoESI) source to the mass spectrometry (MS) inlet, the generated vortex flows helped droplet desolvation and ion confinement and thus improved the MS intensity by 2-3 orders of magnitude compared with that when the vortex tube was not applied. Like the stirring effect in the bulk phase, the reactants were more sufficiently mixed and reacted in vortices. Therefore, with the same reaction distance, a 2-3-fold improvement of conversion ratios was observed by using the vortices. Notably, the vortex tube enabled the use of flow rate to control the reaction time for ∼60 μs, which was useful for precise control of reaction progress. As a demonstration, the intermediates of the amination isomerization were tracked and the equilibrium constant and rate constant of the cytochrome c unfolding were determined.

Mechanistic Study of Isotactic Poly(propylene oxide) Synthesis using a Tethered Bimetallic Chromium Salen Catalyst

Initial catalyst dormancy has been mitigated for the enantioselective polymerization of propylene oxide using a tethered bimetallic chromium(III) salen complex. A detailed mechanistic study provided insight into the species responsible for this induction period and guided efforts to remove them. High-resolution electrospray ionization-mass spectrometry and density functional theory computations revealed that a μ-hydroxide and a bridged 1,2-hydroxypropanolate complex are present during the induction period. Kinetic studies and additional computation indicated that the μ-hydroxide complex is a short-lived catalyst arrest state, where hydroxide dissociation from one metal allows for epoxide enchainment to form the 1,2-hydroxypropanolate arrest state. While investigating anion dependence on the induction period, it became apparent that catalyst activation was the main contributor for dormancy. Using a 1,2-diol or water as chain transfer agents (CTAs) led to longer induction periods as a result of increased 1,2-hydroxyalkanolate complex formation. With a minor catalyst modification, rigorous drying conditions, and avoiding 1,2-diols as CTAs, the induction period was essentially removed.

Nebulization prior to ionization for mechanistic studies of chemical reactions

Many important chemical transformations proceed by way of ionic and/or neutral intermediates. Great effort has been expended to understand the mechanism, with only minimum attention given to separate associated ionic and neutral intermediates. Herein, we present a nebulization method followed by on-line ionization to isolate and characterize the ionic and neutral intermediates. The separation of nebulization and ionization and electrical deflection of ionic species guarantee that only neutrals undergo the subsequent on-line ionization. We present data that show the formation of neutral intermediates and iminium ions with short lifetime in Eschweiler-Clarke methylation of di-n-butylamine, as well as data that provide evidence for the formation of carbocation and its isomer lactone products resulting from copper-mediated oxidative cyclization of 4-phenylbutyric acid. Experiments in which dissociation behavior of these two isomers varied at the same collision energy confirmed the carbocation during the cyclization. The nature of this process, which online isolates the ionic and neutral intermediates prior to ionization, greatly advances in mechanistic studies.

Degradation mechanism study of fluoroquinolones in UV/Fe2+/peroxydisulfate by on-line mass spectrometry

Antibiotics are of concern due to their prevalent detection in aquatic environment. Sulfate radical based advanced oxidation processes show a great capacity to degrade antibiotics, but the mechanisms are still unclear. In this work, the degradation mechanism of fluoroquinolones (FQs), a major group of antibiotics, in UV/Fe²⁺/PMS was deeply investigated. The degradation process was in-situ and real-time monitoring by illumination-assisted droplet spray ionization mass spectrometry. A series of reactive intermediates were captured, and further characterized by high-resolution mass spectrometry (HRMS) and tandem MS. About 50 different transformation products have been identified for ciprofloxacin and norfloxacin. More than 15 products were the first time reported. Taking into consideration of the sequential formation and intensity change of intermediates, the feasible and complete transformation pathways of FQs were proposed. Compared with the photolysis process, the defluorination of FQs was not observed in this system. This work provided abundant information of FQs degradation by persulfate advanced oxidation processes (AOPs) and meanwhile demonstrated the importance of HRMS and on-line MS in mechanism research of AOPs.

In Situ Metal-Assisted Ligand Modification Induces Mn4 Cluster-to-Cluster Transformation: A Crystallography, Mass Spectrometry, and DFT Study

Dehydration of (S,S)-1,2-bis(1H-benzo[d]imidazol-2-yl)ethane-1,2-diol (H₄ L) to (Z)-1,2-bis(1H-benzo[d]imidazol-2-yl)ethenol) (H₃ L') was found to be metal-assisted, occurs under solvothermal conditions (H₂ O/CH₃ OH), and leads to [Mn^II ₄ (H₃ L)₄ Cl₂ ]Cl₂ ⋅5 H₂ O⋅5 CH₃ OH (Mn₄ L₄ ) and [Mn^II ₄ (H₂ L')₆ (μ₃ -OH)]Cl⋅4 CH₃ OH⋅H₂ O (Mn₄ L'₆ ), respectively. Their structures were determined by single-crystal XRD. Extensive ESI-MS studies on solutions and solids of the reaction led to the proposal consisting of an initial stepwise assembly of Mn₄ L₄ from the reactants via [MnL] and [Mn₂ L₂ ] below 80 °C, and then disassembly to [MnL] and [MnL₂ ] followed by ligand modification before reassembly to Mn₄ L'₆ via [MnL'], [MnL'₂ ], and [Mn₂ L'₃ ] with increasing solvothermal temperature up to 140 °C. Identification of intermediates [Mn₄ L_x L'_6-x ] (x=5, 4, 3, 2, 1) in the process further suggested an assembly/disassembly/in situ reaction/reassembly transformation mechanism. These results not only reveal that multiple phase transformations are possible even though they were not realized in the crystalline state, but also help to better understand the complex transformation process between coordination clusters during "black-box" reactions.

Structure, Anion, and Solvent Effects on Cation Response in ESI-MS

The abundance of an ion in an electrospray ionization mass spectrum is dependent on many factors beyond just solution concentration. Even in cases where the analytes of interest are permanently charged (under study here are ammonium and phosphonium ions) and do not rely on protonation or other chemical processes to acquire the necessary charge, factors such as cation structure, molecular weight, solvent, and the identity of the anion can affect results. Screening of a variety of combinations of cations, anions, and solvents provided insight into some of the more important factors. Rigid cations and anions that conferred high conductivity tended to provide the highest responses. The solvent that most closely reflected actual solution composition was acetonitrile, while methanol, acetonitrile/water, and dichloromethane produced a higher degree of discrimination between different ions. Functional groups that had affinity for the solvent tended to depress response. These observations will provide predictive power when accounting for analytes that for reasons of high reactivity can not be isolated.

N-Substituted Auxiliaries for Aerobic Dehydrogenation of Tetrahydro-isoquinoline: A Theory-Guided Photo-Catalytic Design

Visible-light mediated aerobic dehydrogenation of N-heterocyclic compounds is a reaction with enormous potential for application. Herein, we report the first complete aerobic dehydrogenation pathway to large-scale production of isoquinolines. The discovery of this visible light photoredox reaction was enabled through the combination of mathematical simulations and real-time quantitative mass spectrometry screening. The theoretical calculations showed that hyper-conjugation, the main underlying factor hindering the aerobic oxidation of tetrahydroisoquinolines, is relieved both by π- and σ-donating substituents. This mechanistic insight provided a novel photocatalytic route based on N-substituted auxiliaries that facilitated the conversion of tetrahydroisoquinolines into the corresponding isoquinolines in just three simple steps (yield 71.7% in bulk-solution phase), using unmodified Ru(bpy)₃Cl₂ photocatalyst, sun energy, atmospheric O₂, and at ambient temperature.

Reply to the 'Comment on "The chemical reactions in electrosprays of water do not always correspond to those at the pristine air-water interface"' by A. J. Colussi and S. Enami, Chem. Sci., 2019, , DOI: 10.1039/c9sc00991d

We explain why chemical reactions in/on electrosprays of water may not always represent those at the air–water interface. Thus, electrospray-based techniques cannot be relied upon as generalized “surface-specific” platforms for water.

The air–water interface serves as a crucial site for numerous chemical and physical processes in environmental science and engineering, such as cloud chemistry, ocean-atmosphere exchange, and wastewater treatment. The development of “surface-selective” techniques for probing interfacial properties of water therefore lies at the forefront of research in chemical science. Recently, researchers have adapted electrospray ionization mass spectrometry (ESIMS) to generate microdroplets of water to investigate interfacial phenomena at thermodynamic equilibrium. In contrast, using a broad set of experimental and theoretical techniques, we found that electrosprays of water could facilitate partially hydrated (gas-phase) ions (e.g., H₃O⁺·(H₂O)₂) to drive/catalyze chemical reactions that are otherwise not possible to accomplish by purely interfacial effects (e.g., enhanced water–hydrophobe surface area) (Chem. Sci., 2019, 10, 2566). Thus, techniques exploiting electrosprays of water cannot be relied upon as generalized surface-selective platforms. Here, we respond to the comments raised by Colussi & Enami (Chem. Sci., 2019, 10, DOI: ; 10.1039/c9sc00991d) on our paper.

Iodide Accelerates the Processing of Biogenic Monoterpene Emissions on Marine Aerosols

Marine photosynthetic organisms emit organic gases, including the polyolefins isoprene (C₅H₈) and monoterpenes (MTPs, C₁₀H₁₆), into the boundary layer. Their atmospheric processing produces particles that influence cloud formation and growth and, as a result, the Earth's radiation balance. Here, we report that the heterogeneous ozonolysis of dissolved α-pinene by O₃(g) on aqueous surfaces is dramatically accelerated by I^-, an anion enriched in the ocean upper microlayer and sea spray aerosols (SSAs). In our experiments, liquid microjets of α-pinene solutions, with and without added I^-, are dosed with O₃(g) for τ < 10 μs and analyzed online by pneumatic ionization mass spectrometry. In the absence of I^-, α-pinene does not detectably react with O₃(g) under present conditions. In the presence of ≥ 0.01 mM I^-, in contrast, new signals appear at m/z = 169 (C₉H₁₃O₃ ^-), m/z = 183 (C₁₀H₁₅O₃ ^-), m/z = 199 (C₁₀H₁₅O₄ ^-), m/z = 311 (C₁₀H₁₆IO₃ ^-), and m/z = 461 (C₂₀H₃₀IO₄ ^-), plus m/z = 175 (IO₃ ^-), and m/z = 381 (I₃ ^-). Collisional fragmentation splits CO₂ from C₉H₁₃O₃ ^-, C₁₀H₁₅O₃ ^- and C₁₀H₁₅O₄ ^-, and I^- plus IO^- from C₁₀H₁₆IO₃ ^- as expected from a trioxide IOOO^•C₁₀H₁₆ ^- structure. We infer that the oxidative processing of α-pinene on aqueous surfaces is significantly accelerated by I^- via the formation of IOOO^- intermediates that are more reactive than O₃. A mechanism in which IOOO^- reacts with α-pinene (and likely with other unsaturated species) in competition with its isomerization to IO₃ ^- accounts for present results and the fact that soluble iodine in SSA is mostly present as iodine-containing organic species rather than the thermodynamically more stable iodate. By this process, a significant fraction of biogenic MTPs and other unsaturated gases may be converted to water-soluble species rather than emitted to the atmosphere.

Detecting Intermediates and Products of Fast Heterogeneous Reactions on Liquid Surfaces via Online Mass Spectrometry

One of the research priorities in atmospheric chemistry is to advance our understanding of heterogeneous reactions and their effect on the composition of the troposphere. Chemistry on aqueous surfaces is particularly important because of their ubiquity and expanse. They range from the surfaces of oceans (360 million km2), cloud and aerosol drops (estimated at ~10 trillion km2) to the fluid lining the human lung (~150 m2). Typically, ambient air contains reactive gases that may affect human health, influence climate and participate in biogeochemical cycles. Despite their importance, atmospheric reactions between gases and solutes on aqueous surfaces are not well understood and, as a result, generally overlooked. New, surface-specific techniques are required that detect and identify the intermediates and products of such reactions as they happen on liquids. This is a tall order because genuine interfacial reactions are faster than mass diffusion into bulk liquids, and may produce novel species in low concentrations. Herein, we review evidence that validates online pneumatic ionization mass spectrometry of liquid microjets exposed to reactive gases as a technique that meets such requirements. Next, we call attention to results obtained by this approach on reactions of gas-phase ozone, nitrogen dioxide and hydroxyl radicals with various solutes on aqueous surfaces. The overarching conclusion is that the outermost layers of aqueous solutions are unique media, where most equilibria shift and reactions usually proceed along new pathways, and generally faster than in bulk water. That the rates and mechanisms of reactions at air-aqueous interfaces may be different from those in bulk water opens new conceptual frameworks and lines of research, and adds a missing dimension to atmospheric chemistry.

Chemical signatures of surface microheterogeneity on liquid mixtures

Many chemical reactions in Nature, the laboratory, and chemical industry occur in solvent mixtures that bring together species of dissimilar solubilities. Solvent mixtures are visually homogeneous, but are not randomly mixed at the molecular scale. In the all-important binary water-hydrotrope mixtures, small-angle neutron and dynamic light scattering experiments reveal the existence of short-lived (<50 ps), short-ranged (∼1 nm) concentration fluctuations. The presence of hydrophobic solutes stabilizes and extends such fluctuations into persistent, mesoscopic (10-100 nm) inhomogeneities. While the existence of inhomogeneities is well established, their impacts on reactivity are not fully understood. Here, we search for chemical signatures of inhomogeneities on the surfaces of W:X mixtures (W = water; X = acetonitrile, tetrahydrofuran, or 1,4-dioxane) by studying the reactions of Criegee intermediates (CIs) generated in situ from O₃(g) addition to a hydrophobic olefin (OL) solute. Once formed, CIs isomerize to functionalized carboxylic acids (FC) or add water to produce α-hydroxy-hydroperoxides (HH), as detected by surface-specific, online pneumatic ionization mass spectrometry. Since only the formation of HH requires the presence of water, the dependence of the R = HH/FC ratio on water molar fraction x w expresses the accessibility of water to CIs on the surfaces of mixtures. The finding that R increases quasi-exponentially with x w in all solvent mixtures is consistent with CIs being preferentially produced (from their OL hydrophobic precursor) in X-rich, long-lived OL:X _m W _n interfacial clusters, rather than randomly dispersed on W:X surfaces. R vs x w dependences therefore reflect the average ⟨m, n⟩ composition of OL:X _m W _n interfacial clusters, as weighted by cluster reorganization dynamics. Water in large, rigid clusters could be less accessible to CIs than in smaller but more flexible clusters of lower water content. Since mesoscale inhomogeneities are intrinsic to most solvent mixtures, these phenomena should be quite general.

Mechanistic understanding of catalysis by combining mass spectrometry and computation

The combination of mass spectrometry and computational chemistry has been proven to be powerful for exploring reaction mechanisms. The former provides information of reaction intermediates, while the latter gives detailed reaction energy profiles.

The chemical reactions in electrosprays of water do not always correspond to those at the pristine air-water interface

The recent application of electrosprays to characterize the air-water interface, along with the reports on dramatically accelerated chemical reactions in aqueous electrosprays, have sparked a broad interest. Herein, we report on complementary laboratory and in silico experiments tracking the oligomerization of isoprene, an important biogenic gas, in electrosprays and isoprene-water emulsions to differentiate the contributions of interfacial effects from those of high voltages leading to charge-separation and concentration of reactants in the electrosprays. To this end, we employed electrospray ionization mass spectrometry, proton nuclear magnetic resonance, ab initio calculations and molecular dynamics simulations. We found that the oligomerization of isoprene in aqueous electrosprays involved minimally hydrated and highly reactive hydronium ions. Those conditions, however, are non-existent at pristine air-water interfaces and oil-water emulsions under normal temperature and pressure. Thus, electrosprays should be complemented with surface-specific platforms and theoretical methods to reliably investigate chemistries at the pristine air-water interface.

Probing surfaces of atmospherically relevant organic particles by easy ambient sonic-spray ionization mass spectrometry (EASI-MS)

EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Both ambient and laboratory-generated particles can have a surface composition different from the bulk, but there are currently few analytical techniques available to probe these differences. Easy ambient sonic-spray ionization mass spectrometry (EASI-MS) was applied to solid, laboratory-generated particles with core–shell morphologies formed from a variety of dicarboxylic acids. The soft ionization facilitated parent peak detection for the two compounds, from which the depth probed could be determined from the relative signal intensities. Two different configurations of a custom-made nebulizer are reported that yield different probe depths. In the “orthogonal mode,” with the nebulizer ∼10 centimeters away from the particle stream and at a 90° angle to the MS inlet, evaporation of the nebulizer droplets forms ions before interaction with the particles. The probe depth for orthogonal mode EASI-MS is shown to be 2–4 nm in these particle systems. In the “droplet mode”, the nebulizer and particle streams are in close proximity to each other and the MS inlet so that the particles interact with charged liquid droplets. This configuration resulted in full dissolution of the particles and gives particle composition similar to that from collection on filters and extraction of the particles (bulk). These studies establish that EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Coaxial Electrospray Ionization for the Study of Rapid In-source Chemistry

Coaxial electrospray has been used effectively for several dual-emitter applications, but has not been utilized for the study of rapid in-source chemistry. In this paper, we report the fabrication of a coaxial, micro-volume dual-emitter through the modification of a manufacturer's standard electrospray probe. This modification creates rapid mixing inside the Taylor cone and the ability to manipulate fast reactions using a variety of solvents and analytes. We demonstrate its potential as a low-cost, dual-emitter assembly for diverse applications through three examples: relative ionization in a biphasic electrospray, hydrogen-deuterium exchange, and protein supercharging. Graphical Abstract.

Reactive Laser Ablation Electrospray Ionization Time-Resolved Mass Spectrometry of Click Reactions

Reactions in confined compartments like charged microdroplets are of increasing interest, notably because of their substantially increased reaction rates. When combined with ambient ionization mass spectrometry (MS), reactions in charged microdroplets can be used to improve the detection of analytes or to study the molecular details of the reactions in real time. Here, we introduce a reactive laser ablation electrospray ionization (reactive LAESI) time-resolved mass spectrometry (TRMS) method to perform and study reactions in charged microdroplets. We demonstrate this approach with a class of reactions new to reactive ambient ionization MS: so-called click chemistry reactions. Click reactions are high-yielding reactions with a high atom efficiency, and are currently drawing significant attention from fields ranging from bioconjugation to polymer modification. Although click reactions are typically at least moderately fast (time scale of minutes to a few hours), in a reactive LAESI approach a substantial increase of reaction time is required for these reactions to occur. This increase was achieved using microdroplet chemistry and followed by MS using the insertion of a reaction tube-up to 1 m in length-between the LAESI source and the MS inlet, leading to near complete conversions due to significantly extended microdroplet lifetime. This novel approach allowed for the collection of kinetic data for a model (strain-promoted) click reaction between a substituted tetrazine and a strained alkyne and showed in addition excellent instrument stability, improved sensitivity, and applicability to other click reactions. Finally, the methodology was also demonstrated in a mass spectrometry imaging setting to show its feasibility in future imaging experiments.