Paper Spray Mass Spectrometry with On-Paper Electrokinetic Manipulations: Part-Per-Trillion Detection of Per/Polyfluoroalkyl Substances in Water and Opioids in Urine

We developed a simple, paper-based device that enables sensitive detection by mass spectrometry (MS) without solid phase extraction or other sample preparation. Using glass fiber filter papers within a 3D printed holder, the device employs electrokinetic manipulations to stack, separate, and desalt charged molecules on paper prior to spray into the MS. Due to counter-balanced electroosmotic flow and electrophoresis, charged analytes stack on the paper and desalting occurs in minutes. One end of the paper strip was cut into a sharp point and positioned near the inlet of a MS. The stacked analyte bands move toward the paper tip with the EOF where they are ionized by paper spray. The device was applied to analysis of PFAS in tap water with sub part-per-trillion detection limits in less than ten minutes with no sample pretreatment. Analysis of opioids in urine also occurs in minutes. The crucial parameters to enable stacking, separation, and MS ionization of both positively and negatively charged analytes were determined and optimized. Experimental and computational modeling studies confirm the electrokinetic stacking and analyte transport mechanisms. On-paper separations were carried out by stacking analyte bands at different locations depending on their electrophoretic mobility, achieving baseline separation in some cases.

Enhanced Compound Analysis Using Reactive Paper Spray Mass Spectrometry: Leveraging Schiff Base Reaction for Amino Acid Detection

Paper spray mass spectrometry (PS-MS) has evolved into a promising tool for monitoring reactions in thin films and microdroplets, known as reactive PS, alongside its established role in ambient and direct ionization. This study addresses the need for rapid, cost-effective methods to improve analyte identification in biofluids by leveraging reactive PS-MS in clinical chemistry environments. The technique has proven effective in derivatizing target analytes, altering hydrophobicity to enhance elution and ionization efficiency, and refining detection through thin-film reactions on paper, significantly expediting reaction rates by using amino acids (AAs) as model analytes. These molecules are prone to interacting with substrates like paper, impeding elution and detection. Additionally, highly abundant species in biofluids, such as lipids, often suppress AA ionization. This study employs the Schiff base (SB) reaction utilizing aromatic aldehydes for AA derivatization to optimize reaction conditions time, temperature, and catalyst presence and dramatically increasing the conversion ratio (CR) of formed SB. For instance, using leucine as a model AA, the CR surged from 57% at room temperature to 89% at 70 °C, with added pyridine during and after 7.5 min, displaying a 43% CR compared to the bulk reaction. Evaluation of various aromatic aldehydes as derivatization agents highlighted the importance of specific oxygen substituents for achieving higher conversion rates. Furthermore, diverse derivatization agents unveiled unique fragmentation pathways, aiding in-depth annotation of the target analyte. Successfully applied to quantify AAs in human and rat plasma, this reactive PS-MS approach showcases promising potential in efficiently detecting conventionally challenging compounds in PS-MS analysis.

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.

Solvent-pumped evaporation concentration on paper in linear and radial geometries

Solvent-pumped evaporation-driven concentration of an initial distribution of solutes on a porous substrate is considered in one and two dimensions. Approximate analytic solutions to the isotropic advection-dispersion equations are first found for a Gaussian kernel and an infinite domain, following the smoothed particle approximation. Analytic solutions for more general initial distributions are then found as sums of Gaussians, and comparison is made with numerical solutions. In each case, initial distributions are advected toward the stagnation point and concentrated. Two-dimensional analysis is then extended to describe anisotropy in permeability and diffusion, and hydrodynamic dispersion. Radial-flow experiments are performed using filter papers and water-soluble dyes. Diffusion coefficients, temperature and humidity profiles, and the evolution of spot distributions are measured. The results confirm minor anisotropy in permeability and diffusion, limited hydrodynamic dispersion, and largely uniform evaporation. Péclet numbers over 2500 are demonstrated. Evaporation-driven concentration provides a mechanism for solute transport over long timescales. Potential applications lie in the design of paper spray microanalytical devices operating by solvent pumping rather than capillary flow.

Suzuki C-C Coupling in Paper Spray Ionization: Microsynthesis of Biaryls and High-Sensitivity MS Detection of Aryl Bromides

Suzuki-Miyaura cross-coupling is one of the most powerful strategies for constructing biaryl compounds. However, classic Suzuki-Miyaura coupling suffers from hour-scale reaction time and competitive protodeboronation. To address these problems, a mild nonaqueous potassium trimethylsilanolate (TMSOK)-assisted Suzuki-Miyaura coupling strategy was designed for the microsynthesis of biaryls in paper spray ionization (PSI). Due to the acceleration power facilitated by microdroplet chemistry in reactive PSI, the microsynthesis of biaryls by reactive PSI was accomplished within minutes with comparable yields to the bulk, showing good substrate applicability from 32 Suzuki-Miyaura reactions of aryl bromides and aryl boronic acid/borates bearing different substituents. Based on the above TMSOK-assisted Suzuki-Miyaura coupling strategy, we further developed a high-sensitivity and selective PSI mass spectrometry (MS) method for quantitative analysis of aryl bromides, a class of environmentally persistent organic pollutants that cannot be directly detected by ambient mass spectrometry due to their low ionization efficiency. In situ derivatization of aryl bromides was achieved with aryl borates bearing quaternary ammonium groups in PSI. The proposed PSI-MS method shows good linearity over the 0.01-10 μmol L^-1 range with low detection limits of 1.8-4.8 nmol L^-1 as well as good applicability to the rapid determination of six aryl bromides in three environmental water samples. The proposed PSI-MS method also shows good applicability to brominated flame retardants (polybrominated diphenyls/diphenyl esters). Overall, this study provides a simple, rapid, low-cost, high-sensitivity, and high-selectivity strategy for trace aryl bromides and other brominated pollutants in real samples with minimal/no sample pretreatment.

Can electric fields drive chemistry for an aqueous microdroplet?

Reaction rates of common organic reactions have been reported to increase by one to six orders of magnitude in aqueous microdroplets compared to bulk solution, but the reasons for the rate acceleration are poorly understood. Using a coarse-grained electron model that describes structural organization and electron densities for water droplets without the expense of ab initio methods, we investigate the electric field distributions at the air-water interface to understand the origin of surface reactivity. We find that electric field alignments along free O-H bonds at the surface are ~16 MV/cm larger on average than that found for O-H bonds in the interior of the water droplet. Furthermore, electric field distributions can be an order of magnitude larger than the average due to non-linear coupling of intramolecular solvent polarization with intermolecular solvent modes which may contribute to even greater surface reactivity for weakening or breaking chemical bonds at the droplet surface.

A direct mass spectrometry method for cannabinoid quantitation in urine and oral fluid utilizing reactive paper spray ionization

Reactive paper spray mass spectrometry mitigates many of the challenges associated with direct cannabinoid measurements, allowing for rapid, reliable and quantitative measurements in oral fluid and in urine at clinically relevant levels.

CHEMICAL REACTION MONITORING BY AMBIENT MASS SPECTROMETRY

Chemical reactions conducted in different media (liquid phase, gas phase, or surface) drive developments of versatile techniques for the detection of intermediates and prediction of reasonable reaction pathways. Without sample pretreatment, ambient mass spectrometry (AMS) has been applied to obtain structural information of reactive molecules that differ in polarity and molecular weight. Commercial ion sources (e.g., electrospray ionization, atmospheric pressure chemical ionization, and direct analysis in real-time) have been reported to monitor substrates and products by offline reaction examination. While the interception or characterization of reactive intermediates with short lifetime are still limited by the offline modes. Notably, online ionization technologies, with high tolerance to salt, buffer, and pH, can achieve direct sampling and ionization of on-going reactions conducted in different media (e.g., liquid phase, gas phase, or surface). Therefore, short-lived intermediates could be captured at unprecedented timescales, and the reaction dynamics could be studied for mechanism examinations without sample pretreatments. In this review, via various AMS methods, chemical reaction monitoring and mechanism elucidation for different classifications of reactions have been reviewed. The developments and advances of common ionization methods for offline reaction monitoring will also be highlighted.

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.

Combining the Katritzky Reaction and Paper Spray Ionization Mass Spectrometry for Enhanced Detection of Amino Acid Neurotransmitters in Mouse Brain Sections

This work describes the development of a system that combines a derivatization protocol based on the Katritzky reaction with paper spray ionization mass spectrometry (PSI-MS) for the analysis of amino acid neurotransmitters in mouse brain tissues. The system is relatively simple, consisting of spraying the derivatization solution onto a mouse brain section mounted on a glass slide, applying a small volume of solvent to moisten the sample, pressing a triangular paper onto the sample surface to transfer the sample constituents to the paper surface, and using the paper as a substrate for PSI-MS analysis. The Katritzky reaction facilitated the ionization of the amino acids by reacting a pyrylium salt with the amino group of the analytes, forming very stable pyridinium cations, which greatly increased the sensitivity of the PSI-MS analysis. Most of the intensities of the amino acids modified by the Katritzky reaction were more than 10 times greater than the nonderivatized ones. The system was applied for the analysis of brain sections obtained from mice with Parkinson's disease, and the amino acids gamma-aminobutyric acid (GABA) and glycine (Gly), two compounds very well-known in studies of Parkinson's disease, were readily detected. The results suggest that the Katritzky reaction combined with PSI-MS might offer a significant advance in the knowledge on protocols that improve the sensitivity of detection of crucial biological compounds.

Point-of-Care Test Paper for Exhaled Breath Aldehyde Analysis via Mass Spectrometry

Volatile organic compounds (VOCs) from exhaled breath (EB) are considered to be promising biomarkers for lung diseases. A convenient and sensitive point-of-care (POC) testing method for EB VOCs is essential. Here, we developed a POC test paper for the analysis of EB aldehydes, which are potential biomarkers for lung cancer. A probe molecule, 4-aminothiophenol (4-ATP), was anchored on a paper substrate to specifically capture gas-phase aldehydes through the Schiff base reaction. Meanwhile, thin-film reaction acceleration was utilized to increase capture efficiency. By directly coupling the test paper to a mass spectrometer through paper spray, high sensitivity (0.1 ppt) and a wide quantification linear range (from 10 ppt to 1 ppm) were obtained. Analysis of EB from lung cancer patients with the test paper showed a significant increase in several reported aldehyde markers compared to EB from healthy volunteers, indicating the potential of this method for sensitive, low-cost, and convenient lung cancer screening and diagnosis.

Accelerated five-component spiro-pyrrolidine construction at the air-liquid interface

Multi-component reactions assemble complex molecules in a highly effective way, however, they often suffer from long reaction times. We demonstrate that acceleration of a five-component spiro-pyrrolidine construction can be achieved in microdroplets and thin films. The deposition method and mild heating are crucial factors for product formation. Three key intermediates were captured by mass spectrometry to elucidate the tandem reaction mechanism. We also found that hydrogen bonding can significantly flatten the energy barrier at the air-liquid interface.

Reaction acceleration at air-solution interfaces: Anisotropic rate constants for Katritzky transamination

To disentangle the factors controlling the rates of accelerated reactions in droplets, we used mass spectrometry to study the Katritzky transamination in levitated Leidenfrost droplets of different yet constant volumes over a range of concentrations while holding concentration constant by adding back the evaporated solvent. The set of concentration and droplet volume data indicates that the reaction rate in the surface region is much higher than that in the interior. These same effects of concentration and volume were also seen in bulk solutions. Three pyrylium reagents with different surface activity showed differences in transamination reactivity. The conclusion is drawn that reactions with surface-active reactants are subject to greater acceleration, as seen particularly at lower concentrations in systems of higher surface-to-volume ratios. These results highlight the key role that air-solution interfaces play in Katritzky reaction acceleration. They are also consistent with the view that reaction-increased rate constant is at least in part due to limited solvation of reagents at the interface.

Reaction Kinetics of Organic Aerosol Studied by Droplet Assisted Ionization: Enhanced Reactivity in Droplets Relative to Bulk Solution

Droplet Assisted Ionization (DAI) is a relatively new method for online analysis of aerosol droplets that enables measurement of the rate of an aerosol reaction. Here, we used DAI to study the reaction of carbonyl functionalities in secondary organic aerosol (SOA) with Girard's T (GT) reagent, a reaction that can potentially be used to enhance the detection of SOA in online measurements. SOA was produced by α-pinene ozonolysis. Particulate matter was collected on a filter, extracted, and mixed with GT reagent in water. While the reaction hardly proceeded at all in bulk solution, products were readily observed with DAI when the solution was atomized to produce micron-size droplets. Varying the droplet transit time between the atomizer and mass spectrometer allowed the reaction rate constant to be determined, which was found to be 4 orders of magnitude faster than what would be expected from bulk solution kinetics. Decreasing the water content of the droplets, either by heating the capillary inlet to the mass spectrometer or by decreasing the relative humidity of the air surrounding the droplets in the transit line from the atomizer to the mass spectrometer, enhanced product formation. The results suggest that reaction enhancement occurs at the droplet surface, which is consistent with previous reports of reaction acceleration during mass spectrometric analysis, where a bulk solution is analyzed with an ionization method that produces aerosol droplets.

Vibrating Sharp-edge Spray Ionization (VSSI) for voltage-free direct analysis of samples using mass spectrometry

RATIONALE

The development of miniaturized and field portable mass spectrometers could not succeed without a simple, compact, and robust ionization source. Here we present a voltage-free ionization method, Vibrating Sharp-edge Spray Ionization (VSSI), which can generate a spray of liquid samples using only one standard microscope glass slide to which a piezoelectric transducer is attached. Compared with existing ambient ionization methods, VSSI eliminates the need for a high electric field (~5000 V·cm-1 ) for spray generation, while sharing a similar level of simplicity and flexibility with the simplest direct ionization techniques currently available such as paper spray ionization (PSI) and other solid substrate-based electrospray ionization methods.

METHODS

The VSSI device was fabricated by attaching a piezoelectric transducer onto a standard glass microscope slide using epoxy glue. Liquid sample was aerosolized by either placing a droplet onto the vibrating edge of the glass slide or touching a wet surface with the glass edge. Mass spectrometric detection was achieved by placing the VSSI device 0.5-1 cm from the inlet of the mass spectrometer (Q-Exactive, ThermoScientific).

RESULTS

VSSI is demonstrated to ionize a diverse array of chemical species, including small organic molecules, carbohydrates, peptides, proteins, and nucleic acids. Preliminary sensitivity experiments show that high-quality mass spectra of acetaminophen can be obtained by consuming 100 femtomoles of the target. The dual spray of VSSI was also demonstrated by performing in-droplet denaturation of ubiquitin. Finally, due to the voltage-free nature and the direct-contact working mode of VSSI, it has been successfully applied for the detection of chemicals directly from human fingertips.

CONCLUSIONS

Overall, we report a compact ionization method based on vibrating sharp-edges. The simplicity and voltage-free nature of VSSI make it an attractive option for field portable applications or analyzing biological samples that are sensitive to high voltage or difficult to access by conventional ionization methods.

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.

High-Throughput Mass Spectrometry Screening Platform for Discovering New Chemical Reactions under Uncatalyzed, Solvent-Free Experimental Conditions

A gas-phase high-throughput reaction screening platform was developed for the first time to study chemical structures of closely related functional groups and for the discovery of novel organic reaction pathways. Experiments were performed using the contained atmospheric pressure chemical ionization (APCI) source that enabled nonthermal, nonequilibrium plasma chemistry to be monitored by mass spectrometry (MS) in real time. This contained-APCI MS platform allowed an array of reagents to be tested, resulting in the studies of multiple gas-phase reactions in parallel. By exposing headspace vapor of the selected reagents to corona discharge, solvent-free Borsche-Drecsel cyclization reaction, Katritzky chemistry, and Paal-Knorr pyrrole synthesis were examined in the gas phase, outside the high vacuum environment of the mass spectrometer. A new radical-mediated hydrazine coupling reaction was also discovered, which provided a selective pathway to synthesize secondary amines without using a catalyst. The mechanisms of these atmospheric pressure gas-phase reactions were explored through the direct capture of intermediates and via comparison with the corresponding bulk solution and droplet-phase reactions.

A High-Throughput Screening Method for Determining the Optimized Synthesis Conditions of Quinoxaline Derivatives Using Microdroplet Reaction

Quinoxaline derivatives demonstrate many distinguished chemical, biological, and physical properties and have a wide application in dyes, electroluminescent material, organic semiconductors, biological agents, etc. However, the synthesis of quinoxaline still suffers from several drawbacks, for instance, longer reaction time, unsatisfactory yields, and use of metal catalysts. Here, utilizing microdroplet-assisted reaction, we demonstrate the successive synthesis of several important quinoxaline derivatives. For case studies of 1H-indeno [1, 2-b] quinoxaline and 3,5-dimethyl-2-phenylquinoxaline, the present microdroplet approach can complete in milliseconds and the conversion rate reached 90% without adding any catalyst, which is considerably quicker and higher than conversional bulk-phase reactions. When combined with MS detection, high-throughput screening of the optimal reaction conditions can be achieved. Several impacts of droplet volume, reaction flow rate, distance from the MS inlet, spray voltage, and flow rate of the auxiliary gas can be screened on-site quickly for enhanced reaction speed and yields. More importantly, this platform is capable to be used for the scaled-up microdroplet synthesis of quinoxaline diversities. Considering the facile, economic, and environmentally friendly features of the microdroplet approach, we sincerely hope that the current strategy can effectively promote the academic research and industrial fabrications of functional quinoxaline substances for chemical, biological, and pharmaceutical application developments.

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.

Accelerating Electrochemical Reactions in a Voltage-Controlled Interfacial Microreactor

Microdroplet chemistry is attracting increasing attention for accelerated reactions at the solution-air interface. We report herein a voltage-controlled interfacial microreactor that enables acceleration of electrochemical reactions which are not observed in bulk or conventional electrochemical cells. The microreactor is formed at the interface of the Taylor cone in an electrospray emitter with a large orifice, thus allowing continuous contact of the electrode and the reactants at/near the interface. As a proof-of-concept, electrooxidative C-H/N-H coupling and electrooxidation of benzyl alcohol were shown to be accelerated by more than an order of magnitude as compared to the corresponding bulk reactions. The new electrochemical microreactor has unique features that allow i) voltage-controlled acceleration of electrochemical reactions by voltage-dependent formation of the interfacial microreactor; ii) "reversible" electrochemical derivatization; and iii) in situ mechanistic study and capture of key radical intermediates when coupled with mass spectrometry.

Emerging trends in paper spray mass spectrometry: Microsampling, storage, direct analysis, and applications

Recent advancements in the sensitivity of chemical instrumentation have led to increased interest in the use of microsamples for translational and biomedical research. Paper substrates are by far the most widely used media for biofluid collection, and mass spectrometry is the preferred method of analysis of the resultant dried blood spot (DBS) samples. Although there have been a variety of review papers published on DBS, there has been no attempt to unify the century old DBS methodology with modern applications utilizing modified paper and paper-based microfluidics for sampling, storage, processing, and analysis. This critical review will discuss how mass spectrometry has expanded the utility of paper substrates from sample collection and storage, to direct complex mixture analysis to on-surface reaction monitoring.

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.

Quantum Mechanical Modeling of Reaction Rate Acceleration in Microdroplets

Organic reactions in microdroplets can be orders of magnitude faster than their bulk counterparts. We hypothesize that solvation energy differences between bulk and interface play a key role in the intrinsic rate constant increase and test the hypothesis with explicit solvent calculations. We demonstrate for both the protonated phenylhydrazine reagent and the hydrazone transition state (TSB) that molecular orientations which place the charge sites at the surface confer high energy. A pathway in which this high-energy form transforms into a fully solvated TSB has a lower activation energy than bulk by some 59 kJ/mol, a result that is consistent with experimental rate acceleration studies.

Ultrafast enzymatic digestion of proteins by microdroplet mass spectrometry

Enzymatic digestion for protein sequencing usually requires much time, and does not always result in high sequence coverage. Here we report the use of aqueous microdroplets to accelerate enzymatic reactions and, in particular, to improve protein sequencing. When a room temperature aqueous solution containing 10 µM myoglobin and 5 µg mL⁻¹ trypsin is electrosonically sprayed (−3 kV) from a homemade setup to produce tiny (∼9 µm) microdroplets, we obtain 100% sequence coverage in less than 1 ms of digestion time, in sharp contrast to 60% coverage achieved by incubating the same solution at 37 °C for 14 h followed by analysis with a commercial electrospray ionization source that produces larger (∼60 µm) droplets. We also confirm the sequence of the therapeutic antibody trastuzumab (∼148 kDa), with a sequence coverage of 100% for light chains and 85% for heavy chains, demonstrating the practical utility of microdroplets in drug development.

Mass spectrometry (MS)-based protein sequencing usually relies on in-solution proteolytic digestion, which is time-consuming and inefficient for certain proteins. Here, the authors achieve full protein sequence coverage in less than 1 ms by subjecting protein-protease mixtures to electrosonic spray ionization-MS.

High-yield gram-scale organic synthesis using accelerated microdroplet/thin film reactions with solvent recycling

A closed system has been designed to perform microdroplet/thin film reactions with solvent recycling capabilities for gram-scale chemical synthesis. Claisen-Schmidt, Schiff base, Katritzky and Suzuki coupling reactions show acceleration factors relative to bulk of 15 to 7700 times in this droplet spray system. These values are much larger than those reported previously for the same reactions in microdroplet/thin film reaction systems. The solvent recycling mode of the new system significantly improves the reaction yield, especially for reactions with smaller reaction acceleration factors. The microdroplet/thin film reaction yield improved on recycling from 33% to 86% and from 32% to 72% for the Katritzky and Suzuki coupling reactions, respectively. The Claisen-Schmidt reaction was chosen to test the capability of this system in gram scale syntheses and rates of 3.18 g per h and an isolated yield of 87% were achieved.

Covalent organic frameworks-based paper solid phase microextraction combined with paper spray mass spectrometry for highly enhanced analysis of tetrabromobisphenol A

COFs-based paper solid phase microextraction-paper spray mass spectrometry was developed for tetrabromobisphenol A detection with enhanced analysis performance.

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.

High-throughput, low-cost reaction screening using a modified 3D printer

We describe a reaction screening system, based on a 96-well array, and scaled to suit use on the individual scientist's bench. The system was built by modifying a desktop 3D printer and fitting it with a glass syringe and microtiter plate. The effects of experimental variables were characterized, and the performance of the system was optimized. Precise volumes of reaction mixtures (<3% CV) were dispensed into the 96-well array in ca. 40 minutes. The system was used to screen reagents and solvents for the N-alkylation, Katritzky transamination, and Suzuki cross-coupling reactions. Product distributions derived from electrospray mass spectra and represented as heat maps facilitated recognition of optimum conditions. Screening of 96 reaction mixtures was completed in the modest time of approximately 105 minutes (∼65 seconds per reaction mixture). The system is constructed from open-source software and inexpensive 3D printer hardware.

Accelerated reactions in field desorption mass spectrometry

Field desorption mass spectrometry under ambient conditions is used to study solution-phase organic reactions in micro-volumes. Reagent solution is transferred onto the microdendrites of the field emitter, and reaction products are examined online by mass spectrometry. Three reactions, hydrazone formation by phenyl hydrazine and indoline-2,3-dione, the Katritzky reaction between a pyrylium salt and anisidine, and the Hantzsch synthesis of 1,4-dihydropyridine, were investigated, and reaction acceleration was observed to different extents. The increase in rate relative to the corresponding bulk reactions is attributed to solvent evaporation (simple concentration effect) and to the increase of surface-to-volume ratio (enhanced interfacial reactions). A distinguishing feature of this method of reaction acceleration, relative to that based on nano electrospray ionization, is the observation of radical cations and the formation of radical cation products. The study also breaks new ground in using field emitters at atmospheric pressure.

Elucidating the Reaction Mechanisms between Triazine and Hydrogen Sulfide with pH Variation Using Mass Spectrometry

Triazine is one of the most economical and effective scavengers for hydrogen sulfide (H₂S) removal, but the reaction mechanisms between triazine and H₂S with pH variation in solution are still poorly understood. Herein, we show that the reaction process can be directly probed by means of paper spray mass spectrometry, in which an aprotic solvent (e.g., acetonitrile) is more favorable to the observation of reaction intermediates than a protic solvent (e.g., methanol), because of hydrogen bond interaction. Varying the pH of the reaction leads to completely different reaction pathways. With the pH in the range of 5.58 to 7.73, the major product was thiadiazine. With a pH of 3.02-3.69, thiadiazine is converted to 2-(5-(2-hydroxyethyl)-1,3,5-thiadiazinan-3-yl)acetaldehyde, which differs from the traditional pathway of analogous reactions. However, as ammonia was added into the reaction and the pH was adjusted to the range 8.45-9.43, triazine readily undergoes hydrolysis, and the formed intermediate reacts with ammonia and formaldehyde generated in situ from triazine to produce 1-(2-hydroxyethyl)-3,5,7-triaza-1-azoniatricyclo [3.3.1.1^3,7]decane (HTAD). Further increasing the pH up to 10.27-11.21 leads to the decomposition of HTAD. Based on the experimental observation and evidence from high-resolution and tandem mass spectrometry, we propose the plausible reaction mechanisms between triazine and H₂S, as well as the derived reaction from triazine under different pH conditions.

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.

On-surface Fenton and Fenton-like reactions appraised by paper spray ionization mass spectrometry

On-surface degradation of sildenafil (an adequate substrate as it contains assorted functional groups in its structure) promoted by the Fenton (Fe²⁺ /H₂ O₂ ) and Fenton-like (Mⁿ⁺ /H₂ O₂ ; Mⁿ⁺  = Fe³⁺ , Co²⁺ , Cu²⁺ , Mn²⁺ ) systems was investigated by using paper spray ionization mass spectrometry (PS-MS). The performance of each system was compared by measuring the ratio between the relative intensities of the ions of m/z 475 (protonated sildenafil) and m/z 235 (protonated lidocaine, used as a convenient internal standard and added to the paper just before the PS-MS analyzes). The results indicated the following order in the rates of such reactions: Fe²⁺ /H₂ O₂  ≫ H₂ O₂  ≫ Cu²⁺ /H₂ O₂  > Mⁿ⁺ /H₂ O₂ (Mⁿ⁺  = Fe³⁺ , Co²⁺ , Mn²⁺ ) ~ Mⁿ⁺ (Mⁿ⁺  = Fe²⁺ , Fe³⁺ , Co²⁺ , Cu²⁺ , Mn² ). The superior capability of Fe²⁺ /H₂ O₂ in causing the degradation of sildenafil indicates that Fe²⁺ efficiently decomposes H₂ O₂ to yield hydroxyl radicals, quite reactive species that cause the substrate oxidation. The results also indicate that H₂ O₂ can spontaneously decompose likely to yield hydroxyl radicals, although in a much smaller extension than the Fenton system. This effect, however, is strongly inhibited by the presence of the other cations, ie, Fe³⁺ , Co²⁺ , Cu²⁺ , and Mn²⁺ . A unique oxidation by-product was detected in the reaction between Fe²⁺ /H₂ O₂ with sildenafil, and a possible structure for it was proposed based on the MS/MS data. The on-surface reaction of other substrates (trimethoprim and tamoxifen) with the Fenton system was also investigated. In conclusion, PS-MS shows to be a convenient platform to promptly monitor on-surface oxidation reactions.

Preparative microdroplet synthesis of carboxylic acids from aerobic oxidation of aldehydes

Single liquid-phase and liquid-liquid phase reactions in microdroplets have shown much faster kinetics than that in the bulk phase. This work extends the scope of microdroplet reactions to gas-liquid reactions and achieves preparative synthesis. We report highly efficient aerobic oxidation of aldehydes to carboxylic acids in microdroplets. Molecular oxygen plays two roles: (1) as the sheath gas to shear the aldehyde solution into microdroplets, and (2) as the sole oxidant. The dramatic increase of the surface-area-to-volume ratio of microdroplets compared to bulk solution, and the efficient mixing of gas and liquid phases using spray nozzles allow effective mass transfer between aldehydes and molecular oxygen. The addition of catalytic nickel(ii) acetate is shown to accelerate further microdroplet reactions of this kind. We show that aliphatic, aromatic, and heterocyclic aldehydes can be oxidized to the corresponding carboxylic acids in a mixture of water and ethanol using the nickel(ii) acetate catalyst, in moderate to excellent yields (62-91%). The microdroplet synthesis is scaled up to make it preparative. For example, aerobic oxidation of 4-tert-butylbenzaldehyde to 4-tert-butylbenzoic acid was achieved at a rate of 10.5 mg min-1 with an isolated product yield of 66%.

Mass Spectrometry for Synthesis and Analysis

Mass spectrometry is the science and technology of ions. As such, it is concerned with generating ions, measuring their properties, following their reactions, isolating them, and using them to build and transform materials. Instrumentation is an essential element of these activities, and analytical applications are one driving force. Work from the Aston Laboratories at Purdue University's Department of Chemistry is described here, with an emphasis on accelerated reactions of ions in solution and small-scale synthesis; ion/surface collision processes, including surface-induced dissociation (SID) and ion soft landing; and applications to tissue imaging. Our special interest in chirality and the chemistry behind the origins of life is also featured together with the exciting area of tissue diagnostics.

Reaction Acceleration in Thin Films with Continuous Product Deposition for Organic Synthesis

Thin film formats are used to study the Claisen-Schmidt base-catalyzed condensation of 6-hydroxy-1-indanone with substituted benzaldehydes and to compare the reaction acceleration relative to the bulk. Relative acceleration factors initially exceeded 10³ and were on the order of 10² at steady state, although the confined volume reaction was not electrostatically driven. Substituent effects were muted compared to those in the corresponding bulk and microdroplet reactions and it is concluded that the rate-limiting step at steady state is reagent transport to the interface. Conditions were found that allowed product deposition from the thin film to occur continuously as the reaction mixture was added and as the solvent evaporated. Yields of 74 % and production rates of 98 mg h^-1 were reached in a very simple experimental system that could be multiplexed to greater scales.

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] -->.

Mass spectrometric snapshots for electrochemical reactions

A hybrid ultramicroelectrode containing one micro-carbon electrode and one empty micro-channel was employed to be a micro-electrochemical cell and a mass spectrometric nanospray emitter. This setup can combine MS with an electrode directly and provide in situ information about an electrochemical reaction. The mechanisms proposed by Bard et al. for a Ru(bpy)32+ (bpy = 2,2'-bipyridine) electrochemiluminescence (ECL) system were confirmed by the MS detection of key intermediates. The short-lived diimine intermediate of electrochemical oxidation of uric acid was also detected, which affirms that the novel technique is able to catch fleeting intermediates. These experimental results demonstrate that this new method is simple, easy to implement and can be coupled with many commercial mass spectrometric instruments to provide very useful information about electrochemical reactions.

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.