Beyond the flask: Reactions on the fly in ambient mass spectrometry

Recent developments and applications of ambient mass spectrometry imaging in pharmaceutical research: an overview

The application of ambient mass spectrometry imaging "MSI" is expanding in the areas of fundamental research on drug delivery and multiple phases of the process of identifying and developing drugs. Precise monitoring of a drug's pharmacological workflows, such as intake, distribution, metabolism, and discharge, is made easier by MSI's ability to determine the concentrations of the initiating drug and its metabolites across dosed samples without losing spatial data. Lipids, glycans, and proteins are just a few of the many phenotypes that MSI may be used to concurrently examine. Each of these substances has a particular distribution pattern and biological function throughout the body. MSI offers the perfect analytical tool for examining a drug's pharmacological features, especially in vitro and in vivo effectiveness, security, probable toxic effects, and putative molecular pathways, because of its high responsiveness in chemical and physical environments. The utilization of MSI in the field of pharmacy has further extended from the traditional tissue examination to the early stages of drug discovery and development, including examining the structure-function connection, high-throughput capabilities in vitro examination, and ex vivo research on individual cells or tumor spheroids. Additionally, an enormous array of endogenous substances that may function as tissue diagnostics can be scanned simultaneously, giving the specimen a highly thorough characterization. Ambient MSI techniques are soft enough to allow for easy examination of the native sample to gather data on exterior chemical compositions. This paper provides a scientific and methodological overview of ambient MSI utilization in research on pharmaceuticals.

Reactive Paper Spray Ionization Mass Spectrometry for Rapid Detection of Estrogens in Cosmetics

Rapid detection of harmful estrogens in cosmetics is essential in protecting public health. To reduce time-consuming pretreatment and analytical procedures, a novel reactive paper spray ionization mass spectrometry (RPSI-MS) methodology was developed. RPSI-MS is suitable for quantitatively analyzing estrogens in cosmetics by utilizing an online derivatization reaction between estrogens and 2-fluoro-1-methyl-pyridinium-p-toluene-sulfonate (FluMP). Using estradiol valerate as the internal standard (I.S.), three estrogens, estradiol, estriol, and ethinyloestradiol, in cosmetics were quantitatively characterized within minutes. Multiple parameters were optimized including FluMP concentration and volume, triethylamine amount as well as the drying time. The three estrogens displayed good linearity ranging from 0.002 to 1 μg/mL, with R2 above 0.99. The recovery results of all the estrogens were within 80~111%. The limit of detection (LOD) was 0.001 μg/mL for the three estrogens. Compared to conventional paper spray ionization mass spectrometry (PSI-MS), extraction is not required and the detection sensitivity of RPSI-MS was improved by 34,000, 80,000, and 1400 times for estradiol, estriol, and ethinyloestradiol, respectively. The protocol established in this paper is sensitive, eco-friendly, and suitable for rapid testing of estrogens in cosmetics.

Accelerated synthesis of phthalimide derivatives: Intrinsic reactivity of diamines towards phthalic anhydride evaluated by paper spray ionization mass spectrometry

RATIONALE

Paper spray (PS) is a simple and innovative ambient ionization technique for mass spectrometry (MS) analysis. Under PS-MS conditions, chemical reactions, which usually occur slowly on a bulk scale, are accelerated. Moreover, the formation of products and transient species can be easily monitored. In this manuscript, reactions between phthalic anhydride and diamines were conducted and monitored using a PS-MS platform. The reaction products (phthalimides) have many pharmaceutical applications, but their traditional syntheses can take hours under reflux, requiring laborious purification steps.

METHODS

In situ reactions were performed by dropping methanolic solutions of phthalic anhydride and diamines on a triangular paper. The analyses were achieved by positioning the triangle tip in front of the mass spectrometer entrance, whereas a metal clip was attached to the triangle base. After adding methanol to the paper, a high voltage was applied across the metal clip, and the mass spectra were acquired.

RESULTS

The intrinsic reactivity of alkyl and aromatic diamines was evaluated. The carbon chain remarkably influenced the reactivity of aliphatic diamines. For aryl diamines, the ortho isomer was the most reactive. Moreover, for aryl amines with electron-withdrawing substituents, no reaction was noticed.

CONCLUSIONS

Taking advantage of the unique characteristics of PS-MS, it was possible to investigate the intrinsic reactivity of model alkyl (ethylene versus propylene) and aryl (o-phenylene versus m-phenylene and p-phenylene) diamines towards phthalic anhydride. Some crucial parameters that affect the intrinsic reactivity of organic molecules, such as isomerism, intramolecular interaction, and conformation, were easily explored.

Implementing reactive secondary electrospray ionization based on gas–droplet reactions for VOC analysis

A reactive secondary electrospray ionization method is proposed based on accelerated gas–liquid reactions in microdroplets. It enables online derivatization of volatile organic compounds and can facilitate rapid analysis of these samples.

Accelerated synthesis of energetic precursor cage compounds using confined volume systems

Confined volume systems, such as microdroplets, Leidenfrost droplets, or thin films, can accelerate chemical reactions. Acceleration occurs due to the evaporation of solvent, the increase in reactant concentration, and the higher surface-to-volume ratios amongst other phenomena. Performing reactions in confined volume systems derived from mass spectrometry ionization sources or Leidenfrost droplets allows for reaction conditions to be changed quickly for rapid screening in a time efficient and cost-saving manner. Compared to solution phase reactions, confined volume systems also reduce waste by screening reaction conditions in smaller volumes prior to scaling. Herein, the condensation of glyoxal with benzylamine (BA) to form hexabenzylhexaazaisowurtzitane (HBIW), an intermediate to the highly desired energetic compound 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), was explored. Five confined volume systems were compared to evaluate which technique was ideal for forming this complex cage structure. Substituted amines were also explored as BA replacements to screen alternative cage structure intermediates and evaluate how these accelerated techniques could apply to novel reactions, discover alternative reagents to form the cage compound, and improve synthetic routes for the preparation of CL-20. Ultimately, reaction acceleration is ideal for predicting the success of novel reactions prior to scaling up and determining if the expected products form, all while saving time and reducing costs. Acceleration factors and conversion ratios for each reaction were assessed by comparing the amount of product formed to the traditional bulk solution phase synthesis.

Microdroplet mass spectrometry: Accelerating reaction and application

Electrospray ionization (ESI) and desorption electrospray ionization (DESI) are common soft ionization method of mass spectrometry (MS). However, recent studies revealed that some chemical reactions can be induced or greatly accelerated in the sprayed microdroplets compared to the same reaction in the bulk. These open a new area in using microdroplet MS to explore new chemistry and develop new applications. This minireview will introduce microdroplet chemistries and explore various microdroplet techniques most of which are ESI- or DESI-based extensions by incorporating transfer tube, supersonic nebulizing gas, droplet fusion, spray extraction, laser irradiation, or laser ablation for online/offline MS analysis. Potential applications associated with new techniques, including real-time reaction monitoring, high-throughput reaction screening, protein identification, and protein characterization, are also described. Future outlook, such as coupling microdroplet MS with separation techniques, is proposed and discussed.

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.

Cationization of neutral small molecules by site-specific carboxylation of 10-phenyl-10H-phenothiazine in laser desorption/ionization

It is a pre-requisite to ionize analyte molecules efficiently for detection by laser desorption/ionization mass spectrometry. Here, we report a conceptual demonstration of cationizing neutral small molecules which are typically difficult to be ionized with the traditional organic matrices due to their low proton/cation affinity values. Our strategy features generating radical cations from site-specifically carboxylated 10-(4-carboxyphenyl)-10H-phenothiazine-3,7-dicarboxylic acid (PTZ(A)₂-Ph(A)) with a laser, and anchoring the chlorine ion from NaCl through covalent bond-like bridging interactions with the N/S atoms in the heterocyclic structure. This "Maverick" design allows a dramatic change of the energy landscape of analyte sodiation with an enhanced efficiency. We have synthesized two families of compounds based on the model structures of phenothiazine (PTZ) and phenoxazine (PXZ) and their carboxylated derivatives, and performed comparison between them or against the traditional organic matrices in a systematic format. We have demonstrated that PTZ(A)₂-Ph(A) is outstanding as a novel MALDI matrix for the detection of oligosaccharides and amino acids, with an ultra-clean background baseline and high signal-to-noise ratios (up to dozens of times better than the traditional matrices). This work provides a new method for the cationization of neutral small molecules in a distinct mechanism, inspiring the development of next-generation matrices for sensitive detection of hard-to-be-ionized molecules by MALDI MS.

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.

Field induced displacement reactions with proton bound dimers of organophosphorus compounds in a tandem differential mobility spectrometer

Endothermic displacement reactions between proton bound dimers of organophosphorus compounds (OPCs) and isopropanol (IPA) were enabled in air at ambient pressure with tandem differential mobility spectrometry (DMS). Proton bound dimers (M2H+) were mobility isolated in purified air with a first DMS stage, mixed with IPA at ≥100 ppm in a middle reactive stage at 106 to 160 Td from a symmetrical 4 MHz waveform, and mobility analyzed in a second DMS stage. Although the enthalpy for displacement of M by IPA in M2H+ is unfavorable by +44 to 50 kJ mol-1, formation of the heterogenous proton bound dimer, MH+(IPA) arises from field induced dissociation of M2H+ to MH+ and addition of IPA. While peak dispersion for M2H+ of OPCs is limited to -2.25 to -0.5 V compensation voltage, peaks for MH+(IPA) were located at -10.5 to -8.25 V through a combination of ion transformation and mobility-based vapor modification. This inaugural use of ion reactions in air at ambient pressure demonstrates that multi-stage sequential processing of ions can improve significantly the analytical performance in a mobility spectrometer.

Microdroplet Ultrafast Reactions Speed Antibody Characterization

Recently, microdroplet reactions have aroused much interest because the microdroplet provides a unique medium where organic reactions could be accelerated by a factor of 10³ or more. However, microdroplet reactions of proteins have been rarely studied. We report the occurrence of multiple-step reactions of a large protein, specifically, the digestion, reduction, and deglycosylation of an intact antibody, which can take place in microseconds with high reaction yields in aqueous microdroplets at room temperature. As a result, fast structural characterization of a monoclonal antibody, essential for assessing its quality as a therapeutic drug, can be enabled. We found that the IgG1 antibody can be digested completely by the IdeS protease in aqueous microdroplets in 250 microseconds, a 7.5 million-fold improvement in speed in comparison to traditional digestion in bulk solution (>30 min). Strikingly, inclusion of the reductant tris(2-carboxyethyl)phosphine in the spray solution caused simultaneous antibody digestion and disulfide bond reduction. Digested and reduced antibody fragments were either collected or analyzed online by mass spectrometry. Further addition of PNGase F glycosylase into the spray solution led to antibody deglycosylation, thereby producing reduced and deglycosylated fragments of analytical importance. In addition, glycated fragments of IgG1 derived from glucose modification were identified rapidly with this ultrafast digestion/reduction technique. We suggest that microdroplets can serve as powerful microreactors for both exploring large-molecule reactions and speeding their structural analyses.

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.

Accelerated nucleophilic substitution reactions of dansyl chloride with aniline under ambient conditions via dual-tip reactive paper spray

Paper spray ionization (PSI) mass spectrometry (MS) is an emerging tool for ambient reaction monitoring via microdroplet reaction acceleration. PSI-MS was used to accelerate and monitor the time course of the reaction of dansyl chloride with aniline, in acetonitrile, to produce dansyl aniline. Three distinct PSI arrangements were explored in this study representing alternative approaches for sample loading and interaction; conventional single tip as well as two novel setups, a dual-tip and a co-axial arrangement were designed so as to limit any on-paper interaction between reagents. The effect on product abundance was investigated using these different paper configurations as it relates to the time course and distance of microdroplet travel. It was observed that product yield increases at a given distance and then decreases thereafter for all PSI configurations. The fluorescent property of the product (dansyl aniline) was used to visually inspect the reaction progress on the paper substrate during the spraying process. Amongst the variety of sample loading methods the novel dual-tip arrangement showed an increased product yield and microdroplet density, whilst avoiding any on-paper interaction between the reagents.

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets

Electrospray and Electrosonic Spray Ionization Mass Spectrometry (ESI-MS and ESSI-MS) have been widely used to report evidence that many chemical reactions in micro- and nano-droplets are dramatically accelerated by factors of ∼102 to 106 relative to macroscale bulk solutions. Despite electrospray's relative simplicity to both generate and detect reaction products in charged droplets using mass spectrometry, substantial complexity exists in how the electrospray process itself impacts the interpretation of the mechanism of these observed accelerated rates. ESI and ESSI are both coupled multi-phase processes, in which analytes in small charged droplets are transferred and detected as gas-phase ions with a mass spectrometer. As such, quantitative examination is needed to evaluate the impact of multiple experimental factors on the magnitude and mechanisms of reaction acceleration. These include: (1) evaporative concentration of reactants as a function of droplet size and initial concentration, (2) competition from gas-phase chemistry and reactions on experimental surfaces, (3) differences in ionization efficiency and ion transmission and (4) droplet charge. We examine (1-4) using numerical models, new ESI/ESSI-MS experimental data, and prior literature to assess the limitations of these approaches and the experimental best practices required to robustly interpret acceleration factors in micro- and nano-droplets produced by ESI and ESSI.

Accelerated microdroplet synthesis of benzimidazoles by nucleophilic addition to protonated carboxylic acids

We report a metal-free novel route for the accelerated synthesis of benzimidazole and its derivatives in the ambient atmosphere. The synthetic procedure involves 1,2-aromatic diamines and alkyl or aryl carboxylic acids reacting in electrostatically charged microdroplets generated using a nano-electrospray (nESI) ion source. The reactions are accelerated by orders of magnitude in comparison to the bulk. No other acid, base or catalyst is used. Online analysis of the microdroplet accelerated reaction products is performed by mass spectrometry. We provide evidence for an acid catalyzed reaction mechanism based on identification of the intermediate arylamides. Their dehydration to give benzimidazoles occurs in a subsequent thermally enhanced step. It is suggested that the extraordinary acidity at the droplet surface allows the carboxylic acid to function as a C-centered electrophile. Comparisons of this methodology with data from thin film and bulk synthesis lead to the proposal of three key steps in the reaction: (i) formation of an unusual reagent (protonated carboxylic acid) because of the extraordinary conditions at the droplet interface, (ii) accelerated bimolecular reaction because of limited solvation at the interface and (iii) thermally assisted elimination of water. Eleven examples are shown as evidence of the scope of this chemistry. The accelerated synthesis has been scaled-up to establish the substituent-dependence and to isolate products for NMR characterization.

Reactive carbon fiber ionization-mass spectrometry for characterization of unsaturated hydrocarbons from plant aroma

Carbon fiber ionization (CFI)-mass spectrometry (MS) is an ambient technique that can be used to detect samples in gas, liquid, and solid forms simply by using a piece of carbon fiber as the ionization emitter. Reactive MS can be performed to selectively detect target analytes by conducting fast reactions during ionization. Most ambient ionization MS techniques used to monitor chemical reactions are limited to liquid-phase reactions. Herein, we develop reactive CFI-MS to be a suitable tool for monitoring of reaction products derived from volatile unsaturated hydrocarbons in the gas phase. Hydroamination is a fast reaction that can form a carbon-nitrogen bond through the addition of an amine to unsaturated hydrocarbons. In this study, reactive CFI-MS was used to selectively characterize aroma molecules, which are unsaturated hydrocarbons derived from plants, through hydroamination. A piece of carbon fiber was placed close (~ 1 mm) to the inlet of the mass spectrometer and deposited with dried methylamine. The sample in either liquid or solid form was placed underneath the carbon fiber. The volatiles derived from the sample reacted with amine on the carbon fiber were simultaneously determined once the mass spectrometer was switched on. For proof of concept, ethylene glycol dimethacrylate, which has double bonds and is highly volatile, was initially selected as the model sample to demonstrate the feasibility of using reactive CFI-MS to detect its hydroamination derivative. Banana, garlic, and ginger, which possess aroma molecules with unsaturated hydrocarbons, were selected as real-world samples. Graphical abstract.

Facile and efficient preparation of organoimido derivatives of [Mo6O19]2- using accelerated reactions in Leidenfrost droplets

Reaction acceleration is a hot topic in recent years since it is very useful for rapid reaction screening and small-scale synthesis on a short timescale. It is known that the rates of chemical reactions are often accelerated in confined volumes (small droplets or thin films) where the unique chemical reactivities of molecules at the air-droplet/thin film interface, usually different from that in the bulk and gas phases, play a dominant role in acceleration. The Leidenfrost effect was employed to create small levitated droplets with no net charge. These droplets can accelerate many kinds of organic reactions. Our first accelerated synthesis of a series of organoimido-functionalized polyoxometalate (POM) clusters using Leidenfrost droplets with product analysis by electrospray ionization mass spectrometry (ESI-MS) demonstrated that this method can be successfully extended to the synthesis of inorganic/organic hybrids, a very promising area for developing POM-based functional materials. Comparable amounts of synthetic products [Mo₆O₁₈(NC₆H₄R)]^2- (R = H (6), m/z 477; p-i-C₃H₇ (7), m/z 498; p-OCH₃ (8), m/z 492; p-NO₂ (9), m/z 500) were prepared within minutes in Leidenfrost droplets versus in hours in the corresponding bulk reactions under the same reaction conditions in the presence of the DCC catalyst, suggesting that both concentration and interfacial effects are pivotal in causing reaction acceleration in the Leidenfrost droplet. Compared to the conventional bulk reactions, the acceleration factors (AFs) were 92, 136, 126, and 89 for the four model reactions (1)-(4), respectively. We also found out that substitution affects the rate of reactions occurring in droplets, and hence the magnitude of AF. The rates increase in the order of R = NO₂ < H < i-C₃H₇ < OCH₃, in which the electron-donating groups (i.e., R = OCH₃, i-C₃H₇) on the benzene ring are more favorable to the reaction than the electron-withdrawing group (i.e., R = NO₂). This experimental result is in good agreement with the DFT calculation which indicates that the free-energy barriers for the direct imidoylization of POM with RNH₂ are linearly correlated with the basicity constants (pK_b) of amines.

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.

Screening of the Suzuki Cross-Coupling Reaction Using Desorption Electrospray Ionization in High-Throughput and in Leidenfrost Droplet Experiments

Suzuki cross-coupling is a widely performed reaction, typically using metal catalysts under heated conditions. Acceleration of the Suzuki cross-coupling reaction has been previously explored in microdroplets using desorption electrospray ionization mass spectrometry (DESI-MS). Building upon previous work, presented here is the use of a high-throughput DESI-MS screening system to identify optimal reaction conditions. Multiple reagents, bases, and stoichiometries were screened using the automated system at rates that approach 10,000 reaction mixture systems per hour. The DESI-MS system utilizes reaction acceleration in microdroplets to allow rapid screening. The results of screening of an array of reaction mixtures using this technique are presented as product ion images via standard MS imaging software, facilitating quick readout. Instructive comparisons are provided with another method of generating droplets for reaction acceleration-the Leidenfrost technique. Acceleration factors greater than 200 were measured for brominated substrates, paralleling the DESI-MS results. Acceleration factors dropped to near unity with highly substituted pyridines, attributable to a steric effect. The reaction proceeded in the absence of a base in Leidenfrost droplets although no product formation was seen without base in the bulk or in the DESI-MS screening experiments. These differences between Leidenfrost chemistry and the bulk and in droplets formed in high-throughput DESI are tentatively attributed to extremes of pH associated with the surfaces of Leidenfrost droplets.

Reaction Acceleration in Electrospray Droplets: Size, Distance, and Surfactant Effects

Phenylhydrazone formation from isatin is used to examine the effects on the reaction rate of (i) electrospray emitter distance from the mass spectrometer (MS) inlet, (ii) emitter tip diameter, and (iii) presence of surfactant. Reaction rates are characterized through measurement of conversion ratios. It is found that there is an increase in the conversion ratio as (i) the electrospray source is moved further from the inlet of the mass spectrometer, (ii) smaller sprayer diameters are used, and (iii) when surfactants are present. Each of these experimental operations is associated with an increase in reaction rate and with a decrease in average droplet sizes. The size measurements are made using super resolution microscopy from the "splash" on a collector surface produced by a fluorescent marker sprayed using conditions similar to those used for the reaction mixture. This measurement showed that droplets undergo significant evaporation as a function of distance of flight, thereby increasing their surface to volume ratios. Similarly, the effect of nanoelectrospray emitter size on conversion ratio is also found to be associated with changes in droplet size for which a 4 to 10 times increase in reaction rate is seen using tip diameters ranging from 20 μm down to 1 μm. Finally, the effects of surfactants in producing smaller droplets with corresponding large increases in reaction rate are demonstrated by splash microscopy. These findings point to reaction acceleration being strongly associated with reactions at the surfaces of microdroplets.

Microdroplet Fusion Chemistry for Charge State Reduction of Synthetic Polymers via Bipolar Dual Spray with Anionic Reagents

Microdroplet fusion chemistry is an emerging area of analyte manipulation that utilizes the ion source region of a mass spectrometer to covalently derivatize or manipulate the charge state distribution. This technique utilizes two electrospray emitters in close proximity, where the droplets from each electrospray plume fuse and undergo the subsequent chemistry. In this study, microdroplet fusion chemistry via bipolar dual spray has demonstrated the ability to reduce the average charge state of polyethylene glycol (PEG) cations using anionic reagents. Bipolar dual spray was implemented on a commercial mass spectrometer with limited hardware modifications to the ion source. Reagents including ammonium hydroxide, formic acid, and lithium chloride showed dramatic shifts in the average charge state of PEG 3.8 K cations (e.g., 5.0+ to 2.5+) along with the emergence of newly detected charge states. An organic base, tributylamine, had no effect on the charge state distribution of PEG 3.8 K cations. These results were consistent with an ion-pairing mechanism, where reagent anions destabilized ammonium cation interactions with PEG 3.8 K upon droplet fusion from the negative and positive emitters. Additional bipolar dual spray experiments with PEG 12.6 K demonstrated the ability to transform uninterpretable mass information into distinct charge states ranging from [M+8NH₄]⁺ to [M+3NH₄]⁺. Overall, this study provides insight into the nature of dual spray chemistry and will aid future experimental design in microdroplet covalent chemistry.

Fundamental Studies of New Ionization Technologies and Insights from IMS-MS

Exceptional ion mobility spectrometry mass spectrometry (IMS-MS) developments by von Helden, Jarrold, and Clemmer provided technology that gives a view of chemical/biological compositions previously not achievable. The ionization method of choice used with IMS-MS has been electrospray ionization (ESI). In this special issue contribution, we focus on fundamentals of heretofore unprecedented means for transferring volatile and nonvolatile compounds into gas-phase ions singly and multiply charged. These newer ionization processes frequently lead to different selectivity relative to ESI and, together with IMS-MS, may provide a more comprehensive view of chemical compositions directly from their original environment such as surfaces, e.g., tissue. Similarities of results using solvent- and matrix-assisted ionization are highlighted, as are differences between ESI and the inlet ionization methods, especially with mixtures such as bacterial extracts. Selectivity using different matrices is discussed, as are results which add to our fundamental knowledge of inlet ionization as well as pose additional avenues for inquiry. IMS-MS provides an opportunity for comparison studies relative to ESI and will prove valuable using the new ionization technologies for direct analyses. Our hypothesis is that some ESI-IMS-MS applications will be replaced by the new ionization processes and by understanding mechanistic aspects to aid enhanced source and method developments this will be hastened.

Organic Reactions in Microdroplets: Reaction Acceleration Revealed by Mass Spectrometry

The striking finding that reaction acceleration occurs in confined-volume solutions sets up an apparent conundrum: Microdroplets formed by spray ionization can be used to monitor the course of bulk-phase reactions and also to accelerate reactions between the reagents in such a reaction. This Minireview introduces droplet and thin-film acceleration phenomena and summarizes recent methods applied to study accelerated reactions in confined-volume, high-surface-area solutions. Conditions that dictate either simple monitoring or acceleration are reconciled in the occurrence of discontinuous and complete desolvation as the endpoint of droplet evolution. The contrasting features of microdroplet and bulk-solution reactions are described together with possible mechanisms that drive reaction acceleration in microdroplets. Current applications of droplet microreactors are noted as is reaction acceleration in confined volumes and possible future scale-up.

A Mass Spectrometer in Every Fume Hood

Since their inception, mass spectrometers have played a pivotal role in the direction and application of synthetic chemical research. The ability to develop new instrumentation to solve current analytical challenges in this area has always been at the heart of mass spectrometry, although progress has been slow at times. Herein, we briefly review the history of how mass spectrometry has been used to approach challenges in organic chemistry, how new developments in portable instrumentation and ambient ionization have been used to open novel areas of research, and how current techniques have the ability to expand on our knowledge of synthetic mechanisms and kinetics. Lastly, we discuss the relative paucity of work done in recent years to embrace the concept of improving benchtop synthetic chemistry with mass spectrometry, the disconnect between applications and fundamentals within these studies, and what hurdles still need to be overcome. Graphical Abstract.

Reaction screening and optimization of continuous-flow atropine synthesis by preparative electrospray mass spectrometry

Preparative electrospray (ES) exploits the acceleration of reactions in charged microdroplets to perform a small scale chemical synthesis. In combination with on-line mass spectrometric (MS) analysis, it constitutes a rapid screening tool to select reagents to generate specific products. A successful reaction in preparative ES triggers a refined microfluidic reaction screening procedure which includes the optimization for stoichiometry, temperature and residence time. We apply this combined approach for refining a flow synthesis of atropine. A successful preparative ES pathway for the synthesis of the phenylacetyl ester intermediate, using tropine/HCl/phenylacetyl chloride, was optimized for solvent in both the preparative ES and microfluidics flow systems and a base screening was conducted by both methods to increase atropine yield, increase percentage conversion and reduce byproducts. In preparative ES, the first step yielded 55% conversion (judged using MS) to intermediate and the second step yielded 47% conversion to atropine. When combined in two discrete steps in continuous-flow microfluidics, a 44% conversion of the starting material and a 30% actual yield of atropine were achieved. When the reactions were continuously telescoped in a new form of preparative reactive extractive electrospray (EES), atropine was synthesized with a 24% conversion. The corresponding continuous-flow microfluidics experiment gave a 55% conversion with an average of 34% yield in 8 min residence time. This is the first in depth study to utilize telescoped preparative ES and the first use of dual ESI emitters for multistep synthesis.

High throughput reaction screening using desorption electrospray ionization mass spectrometry

We report the high throughput analysis of reaction mixture arrays using methods and data handling routines that were originally developed for biological tissue imaging.

We report the high throughput analysis of reaction mixture arrays using methods and data handling routines that were originally developed for biological tissue imaging. Desorption electrospray ionization (DESI) mass spectrometry (MS) is applied in a continuous on-line process at rates that approach 10⁴ reactions per h at area densities of up to 1 spot per mm² (6144 spots per standard microtiter plate) with the sprayer moving at ca. 10⁴ microns per s. Data are analyzed automatically by MS using in-house software to create ion images of selected reagents and products as intensity plots in standard array format. Amine alkylation reactions were used to optimize the system performance on PTFE membrane substrates using methanol as the DESI spray/analysis solvent. Reaction times can be <100 μs when reaction acceleration occurs in microdroplets, enabling the rapid screening of processes like N-alkylation and Suzuki coupling reactions as reported herein. Products and by-products were confirmed by on-line MS/MS upon rescanning of the array.

Substrate-Coated Illumination Droplet Spray Ionization: Real-Time Monitoring of Photocatalytic Reactions

Real-time monitoring of photocatalytic reactions facilitates the elucidation of the mechanisms of the reactions. However, suitable tools for real-time monitoring are lacking. Herein, a novel method based on droplet spray ionization named substrate-coated illumination droplet spray ionization (SCI-DSI) for direct analysis of photocatalytic reaction solution is reported. SCI-DSI addresses many of the analytical limitations of electrospray ionization (ESI) for analysis of photocatalytic-reaction intermediates, and has potential for both in situ analysis and real-time monitoring of photocatalytic reactions. In SCI-DSI-mass spectrometry (MS), a photocatalytic reaction occurs by loading sample solutions onto the substrate-coated cover slip and by applying UV light above the modified slip; one corner of this slip adjacent to the inlet of a mass spectrometer is the high-electric-field location for launching a charged-droplet spray. After both testing and optimizing the performance of SCI-DSI, the value of this method for in situ analysis and real-time monitoring of photocatalytic reactions was demonstrated by the removal of cyclophosphamide (CP) in TiO₂/UV. Reaction times ranged from seconds to minutes, and the proposed reaction intermediates were captured and identified by tandem mass spectrometry. Moreover, the free hydroxyl radical (·OH) was identified as the main radicals for CP removal. These results show that SCI-DSI is suitable for in situ analysis and real-time monitoring of CP removal under TiO₂-based photocatalytic reactions. SCI-DSI is also a potential tool for in situ analysis and real-time assessment of the roles of radicals during CP removal under TiO₂-based photocatalytic reactions.Graphical Abstract.

Fischer Indole Synthesis in the Gas Phase, the Solution Phase, and at the Electrospray Droplet Interface

Previous reports have shown that reactions occurring in the microdroplets formed during electrospray ionization can, under the right conditions, exhibit significantly greater rates than the corresponding bulk solution-phase reactions. The observed acceleration under electrospray ionization could result from a solution-phase, a gas-phase, or an interfacial reaction. This study shows that a gas-phase ion/molecule (or ion/ion) reaction is not responsible for the observed rate enhancement in the particular case of the Fischer indole synthesis. The results show that the accelerated reaction proceeds in the microdroplets, and evidence is provided that an interfacial process is involved. Graphical Abstract <!-- [INSERT GRAPHICAL ABSTRACT TEXT HERE] -->.

Ambient Ionization Mass Spectrometry Measurement of Aminotransferase Activity

A change in enzyme activity has been used as a clinical biomarker for diagnosis and is useful in evaluating patient prognosis. Current laboratory measurements of enzyme activity involve multi-step derivatization of the reaction products followed by quantitative analysis of these derivatives. This study simplified the reaction systems by using only the target enzymatic reaction and directly detecting its product. A protocol using paper spray mass spectrometry for identifying and quantifying the reaction product has been developed. Evaluation of the activity of aspartate aminotransferase (AST) was chosen as a proof-of-principle. The volume of sample needed is greatly reduced compared with the traditional method. Paper spray has a desalting effect that avoids sprayer clogging problems seen when examining serum samples by nanoESI. This very simple method does not require sample pretreatment and additional derivatization reactions, yet it gives high quality kinetic data, excellent limits of detection (60 ppb from serum), and coefficients of variation <10% in quantitation. Graphical Abstract ᅟ.

Molecular labels for analysis of amines and diols by spray based ionization-mass spectrometry

A family of imidazolium and pyridinium salts was synthesized for the purpose of labeling amines and 1,2-diols for highly sensitivite analysis by mass spectrometry. The chosen mass labels are shown to serve as effective reporters when bound to particles functionalized with amines or 1,2-diols and the binding is reversible. The straightforward synthetic route allows analogous internal standards to be generated quickly. Solvents amenable to electrospray ionization facilitate the rapid liberation of imidazolium and pyridinium mass labels from particles by hydrolysis in aqueous acid, while the acetal bond remains stable in anhydrous or buffered aqueous solution. Detection of the labels is demonstrated at 1 nM and quantitation of mass labels released from particles is also demonstrated.

Accelerated hydrazone formation in charged microdroplets

RATIONALE

Electrospray ionization-mass spectrometry (ESI-MS) is an emerging tool for reaction monitoring. It can be accompanied by reaction acceleration in charged droplets.

METHODS

The time course of the bulk reaction of indoline-2,3-dione with phenylhydrazine in methanol to produce 3-(2- phenylhydrazono)indolin-2-one was monitored by ESI. Both nanoESI and electrosonic spray ionization (ESSI) were used for this study as representing two common forms of ionization for reaction monitoring. The effect on product yield of the distance the droplets travel between the source and the MS inlet was varied and product/starting material ratios were examined.

RESULTS

Product yield is dramatically increased by increasing the distance. At short distances reaction monitoring can be performed without acceleration and at greater distances reaction acceleration occurs. This distance effect over the course of the reaction roughly parallels the time dependence of the bulk-phase reaction.

CONCLUSIONS

Reaction acceleration in droplets is attributed to solvent evaporation leading to increased surface to volume ratios. An acceleration factor of 10(4) , measured relative to the bulk reaction at short times, is readily achieved by simply increasing the droplet distance of flight showing that the same ionization source can be used to monitor reactions with or without acceleration. Copyright © 2016 John Wiley & Sons, Ltd.

Ambient Ionization Mass Spectrometry for Point-of-Care Diagnostics and Other Clinical Measurements

BACKGROUND

One driving motivation in the development of point-of-care (POC) diagnostics is to conveniently and immediately provide information upon which healthcare decisions can be based, while the patient is on site. Ambient ionization mass spectrometry (MS) allows direct chemical analysis of unmodified and complex biological samples. This suite of ionization techniques was introduced a decade ago and now includes a number of techniques, all seeking to minimize or eliminate sample preparation. Such approaches provide new opportunities for POC diagnostics and rapid measurements of exogenous and endogenous molecules (e.g., drugs, proteins, hormones) in small volumes of biological samples, especially when coupled with miniature mass spectrometers.

CONTENT

Ambient MS-based techniques are applied in diverse fields such as forensics, pharmaceutical development, reaction monitoring, and food analysis. Clinical applications of ambient MS are at an early stage but show promise for POC diagnostics. This review provides a brief overview of various ambient ionization techniques providing background, examples of applications, and the current state of translation to clinical practice. The primary focus is on paper spray (PS) ionization, which allows quantification of analytes in complex biofluids. Current developments in the miniaturization of mass spectrometers are discussed.

SUMMARY

Ambient ionization MS is an emerging technology in analytical and clinical chemistry. With appropriate MS instrumentation and user-friendly interfaces for automated analysis, ambient ionization techniques can provide quantitative POC measurements. Most significantly, the implementation of PS could improve the quality and lower the cost of POC testing in a variety of clinical settings.

Emerging flow injection mass spectrometry methods for high-throughput quantitative analysis

Where does flow injection analysis mass spectrometry (FIA-MS) stand relative to ambient mass spectrometry (MS) and chromatography-MS? Improvements in FIA-MS methods have resulted in fast-expanding uses of this technique. Key advantages of FIA-MS over chromatography-MS are fast analysis (typical run time <60 s) and method simplicity, and FIA-MS offers high-throughput without compromising sensitivity, precision and accuracy as much as ambient MS techniques. Consequently, FIA-MS is increasingly becoming recognized as a suitable technique for applications where quantitative screening of chemicals needs to be performed rapidly and reliably. The FIA-MS methods discussed herein have demonstrated quantitation of diverse analytes, including pharmaceuticals, pesticides, environmental contaminants, and endogenous compounds, at levels ranging from parts-per-billion (ppb) to parts-per-million (ppm) in very complex matrices (such as blood, urine, and a variety of foods of plant and animal origin), allowing successful applications of the technique in clinical diagnostics, metabolomics, environmental sciences, toxicology, and detection of adulterated/counterfeited goods. The recent boom in applications of FIA-MS for high-throughput quantitative analysis has been driven in part by (1) the continuous improvements in sensitivity and selectivity of MS instrumentation, (2) the introduction of novel sample preparation procedures compatible with standalone mass spectrometric analysis such as salting out assisted liquid-liquid extraction (SALLE) with volatile solutes and NH4(+) QuEChERS, and (3) the need to improve efficiency of laboratories to satisfy increasing analytical demand while lowering operational cost. The advantages and drawbacks of quantitative analysis by FIA-MS are discussed in comparison to chromatography-MS and ambient MS (e.g., DESI, LAESI, DART). Generally, FIA-MS sits 'in the middle' between ambient MS and chromatography-MS, offering a balance between analytical capability and sample analysis throughput suitable for broad applications in life sciences, agricultural chemistry, consumer safety, and beyond.

Observation and confirmation of oxidation reactions occurring on ultra-high-performance liquid chromatography columns

RATIONALE

Ultra-high-performance liquid chromatography coupled with electrospray ionization mass spectrometry (UPLC/ESI-MS) has been frequently used for chemical analysis. A redox reaction in the ESI source has been observed during the ionization process. However, it is still unclear whether this redox reaction can take place on UPLC columns.

METHODS

In this study, the oxidation reactions potentially occurring on UPLC columns were investigated using polyphenols including baicalin, baicalein, propyl gallate (PG), quercetin-3-rhamnoside (QR), rutin, naringin and 2,3,5,4'-tetrahydroxystilbene-2-Ο-β-D-glucoside (THS-G) as model compounds. The on-column oxidation reaction was ascertained by post-column infusion of antioxidants such as ammonium sulfide ((NH4)2S). The oxidized products were reduced to their parent forms in the ESI source. This on-column oxidation reaction was further confirmed by means of post-column infusion of baicalin solution.

RESULTS

On-column oxidation reactions were observed and confirmed for baicalin, baicalein, PG, rutin, and QR. The exact reaction site was located at the outlet frits of the UPLC columns. (NH4)2S was proved to be the most suitable reducing agent among the tested antioxidants for eliminating negative effects caused by on-column oxidation reaction. It was subsequently proposed to be an efficient additive to suppress oxidation reactions in the ESI source.

CONCLUSIONS

Oxidation reactions can take place at the outlet frits of UPLC columns. Ascertaining on-column oxidation reactions and consequently eliminating relevant negative effects are of great interest for determination of oxidation-sensitive compounds such as polyphenols.

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

Recently developed and applied ionization techniques have brought mass spectrometry to bear on previously inaccessible chemistry. We offer our perspective on this field and its application for studying reaction mechanisms.

There has been a burst in the number and variety of available ionization techniques to use mass spectrometry to monitor chemical reactions in and on liquids. Chemists have gained the capability to access chemistry at unprecedented timescales, and monitor reactions and detect intermediates under almost any set of conditions. Herein, recently developed ionization techniques that facilitate mechanistic studies of chemical processes are reviewed. This is followed by a discussion of our perspective on the judicious application of these and similar techniques in order to study reaction mechanisms.