Venturi easy ambient sonic-spray ionization

Advancing mass spectrometry-based chemical imaging: A noncontact continuous flow surface probe in mass spectrometry for enhanced signal detection and spatial resolution

A new method has been developed for mass spectrometric imaging of small molecules and proteins on tissue or in thinly sliced materials. A laser desorption Venturi electrospray ionization-mass spectrometer was developed for molecular imaging. This method combines laser desorption (LD) and electrospray ionization (ESI) systems before a mass spectrometer (MS). To carry out laser desorption, samples are excited with a laser from the back side of a glass substrate. The desorbed molecules or particles are then captured by a solvent flow. In the ESI system, these desorbed particles and molecules are ionized. The spray part of the solvent system consists of two capillaries: one delivers solvent to the sample plate sides to capture desorbed molecules and particles, and the other carries the solution to the mass spectrometry side using the Venturi effect. A 2D stage facilitates sampling. The system is designed to minimize the sample size after desorption using a 355 nm diode laser, and it is optimized for molecules of various sizes, including organic molecules, amino acids, and proteins. Despite challenging atmospheric conditions for protein desorption, this specialized design enables the collection of protein spectra. The amino acids and other small molecules showed high sensitivity in the MSI measurements. This innovative MS imaging system can be directly applied to real tissue systems and other plant samples to visualize the molecular level distributions.

Caffeine quantification in dietary supplements using high-throughput on-line solid phase extraction coupled to Venturi easy ambient sonic-spray ionization mass spectrometry

Caffeine is present in a large number of beverages and is an additive used in dietary supplements. Therefore, the concern about its quality and safety for consumers has been increasing and hence requires faster and simpler analytical methods to determine the caffeine amount. The high-throughput analysis is an appropriate solution to pharmaceuticals, bioanalysis, forensic and food laboratory routines. In this sense, Venturi easy ambient sonic-spray ionization mass spectrometry (V-EASI-MS), a specific ambient ionization source, is suitable to enable direct analysis of sample solutions in real time and is appropriate to be coupled to liquid chromatography (LC). The development of an on-line solid phase extraction system coupled to V-EASI-MS optimizes the advantages of LC-MS hyphenation by enhancing the figures of merit of the analytical method according to AOAC guidelines and simultaneously minimizing the runtime analysis to 1.5 min per sample, as well as sample preparation steps and solvent consumption, which is currently a challenge for quantitative applications of ambient ionization MS.

Capillary self-aspirating electrospray ionization (CSESI) for convenient and versatile mass spectrometry analysis

Electrospray ionization (ESI) is one of the most widely used ionization techniques, and its simplification can benefit many interested users. In this study, sample introduction by capillary action was studied and used to develop a simple ESI source called capillary self-aspirating electrospray ionization (CSESI). A conventional CSESI source requires only a common capillary of appropriate diameter in addition to the support of high voltage (HV). No pumps and sample loading is needed because the solution can spontaneously climb across the capillary by capillary action. With the proper modification of the glass capillary, the operation of CSESI can be further simplified and efficient. Specifically, cold plasma processing of the capillary creates a more hydrophilic surface that can facilitate sample introduction. Moreover, sputtering a thin platinum layer on the capillary tip makes the application of HV more convenient, and it also eliminates the influence of air bubbles in the capillary to ensure a sustained and stable electrospray. Overall, CSESI exhibits multiple desirable features such as simple structure, self-aspiration ability, low sample consumption, and inherent physical filtration capability. Apart from the routine ESI-MS analysis, it has also been applied in real-time monitoring of the oxidative dimerization of 8-methyl-1,2,3,4-tetrahydroquinoline, as well as direct analysis of muddy soil solutions without pretreatment.

Matrix Effects Free Imaging of Thin Tissue Sections Using Pneumatically Assisted Nano-DESI MSI

Mass spectrometry imaging has the potential to reveal important molecular interaction in morphological regions in tissue. However, the simultaneous ionization of the continuously altered and complex chemistry in each pixel can introduce artifacts that result in skewed molecular distributions in the compiled ion images. These artifacts are known as matrix effects. Mass spectrometry imaging using nanospray desorption electrospray ionization (nano-DESI MSI) enables the elimination of matrix effects by doping the nano-DESI solvent with internal standards. Carefully selected internal standards ionize similarly and simultaneously with the extracted analytes from thin tissue sections, and the matrix effects are eliminated through a robust data normalization method. Herein we describe the setup and use of pneumatically assisted (PA) nano-DESI MSI with standards doped in the solvent for elimination of matrix effects in ion images.

Ambient Ionization Mass Spectrometry: Application and Prospective

Mass spectrometry (MS) is a formidable analytical tool for the analysis of non-polar to polar compounds individually and/or from mixtures, providing information on the molecular weights and chemical structures of the analytes. During the last more than one-decade, ambient ionization mass spectrometry (AIMS) has developed quickly, producing a wide range of platforms and proving scientific improvements in a variety of domains, from biological imaging to quick quality control. These methods have made it possible to detect target analytes in real time without sample preparation in an open environment, and they can be connected to any MS system with an atmospheric pressure interface. They also have the ability to analyze explosives, illicit drugs, disease diagnostics, drugs in biological samples, adulterants in food and agricultural products, reaction progress, and environmental monitoring. The development of novel ambient ionization techniques, such as probe electrospray ionization, paper spray ionization, and fiber spray ionization, employed even at picolitre to femtolitre solution levels to provide femtogram to attogram levels of the target analytes. The special characteristic of this ambient ion source, which has been extensively used, is the noninvasive property of PESI of examination of biological real samples. The results in the current review supports the idea that AIMS has emerged as a pioneer in MS-based approaches and that methods will continue to be developed along with improvements to existing ones in the near future.

A Direct Infusion Probe for Rapid Metabolomics of Low-Volume Samples

Targeted and nontargeted metabolomics has the potential to evaluate and detect global metabolite changes in biological systems. Direct infusion mass spectrometric analysis enables detection of all ionizable small molecules, thus simultaneously providing information on both metabolites and lipids in chemically complex samples. However, to unravel the heterogeneity of the metabolic status of cells in culture and tissue a low number of cells per sample should be analyzed with high sensitivity, which requires low sample volumes. Here, we present the design and characterization of the direct infusion probe, DIP. The DIP is simple to build and position directly in front of a mass spectrometer for rapid metabolomics of chemically complex biological samples using pneumatically assisted electrospray ionization at 1 μL/min flow rate. The resulting data is acquired in a square wave profile with minimal carryover between samples that enhances throughput and enables several minutes of uniform MS signal from 5 μL sample volumes. The DIP was applied to study the intracellular metabolism of insulin secreting INS-1 cells and the results show that exposure to 20 mM glucose for 15 min significantly alters the abundance of several small metabolites, amino acids, and lipids.

High-Throughput Screening Using a Synchronized Pulsed Self-aspiration Vacuum Electrospray Ionization Miniature Mass Spectrometer

With the advantages of rapid analysis, high sensitivity, and multicomponent identification, mass spectrometry (MS) is recognized as an appealing choice for high-throughput screening (HTS) analysis. Aiming at the small size, simple operation, and adequate performance, the development of miniature mass spectrometers has made great progress over the last 2 decades. Besides the essential analytical performance, simple operation and HTS capability are two other crucial features desired in miniature MS instruments. In this paper, an induced self-aspiration vacuum electrospray ionization source (ISA-VESI) was developed and coupled to a miniature ion trap mass spectrometer. A special timing sequence was designed to synchronize all the operation steps in each measurement, including dual-pulse sample injection, multipulse gas injection, MS analysis, and the movement of the homemade HTS platform used as the sampler. Then, the automatic high-throughput analysis of multiple samples can be accomplished with close coordination among the sample delivery, the sample introduction and ionization, and the ion trap operation. The measurement time of each ISA-VESI-MS analysis was about 7 s, with a sample consumption of less than 100 nL.

Optimizing Coaxial Sonic Spray Geometry for Generating Water Microdroplets

Sonic spray creates a stream of neutral and charged microdroplets without application of voltage, heating, laser irradiation, or corona discharge. The solvent of interest flows through an inner capillary (usually constructed of fused silica) that is surrounded by an outer stainless-steel tube through which a nebulizing gas flows under pressure. This technique has been widely used as the interface in mass spectrometric studies for chemical analysis and for understanding microdroplet chemistry. We have used light scattering to characterize the size distribution and density for water microdroplets as a function of several parameters, such as water quality, water flow rate, nebulizing gas pressure, and sonic sprayer geometry. We find that the size distribution of the microdroplets, which is critical to many applications, depends most sensitively on the distance between the inner and outer capillary outlets and the gas flow pressure. The best performance as measured by the smallness of the microdroplet diameters is obtained when the gas flow pressure is the highest and there is no separation distance, d, between the two capillary outlets. In addition, at d = 0 mm, the microdroplet diameter distribution is nearly independent of the water flow rate, indicating that studies under these conditions can be scaled up.

Cell-Based Ambient Venturi Autosampling and Matrix-Assisted Laser Desorption Ionization Mass Spectrometric Imaging of Secretory Products

A cell-based ambient Venturi autosampling device was established for the monitoring of dynamic cell secretions in response to chemical stimulations in real time with temporal resolution on the order of a second. Detection of secretory products of cells and screening of bioactive compounds are primarily performed on an ambient autosampling probe and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. It takes advantage of the Venturi effect in which the fluid flowing through an inlet capillary tube is automatically fed into a parallel array of multiple outlet capillaries. Cells are incubated inside the inlet capillary tube that is connected with either a syringe pump or liquid chromatography (LC) for the transfer of single compounds or mixtures, respectively. Secretory products were continuously pushed into the outlet capillaries and then spotted into a compressed thin film of the matrix material 9-aminoacridine for MALDI mass spectrometric imaging. In physiological pH, without the use of high voltages and without the use of chemical derivatizations, this platform can be applied to the direct assay of neurotransmitters or other secretory products released from cells in response to the stimulation of individual compounds or LC-separated eluates of natural mixtures. It provides a new way to identify bioactive compounds with a detection limit down to 0.04 fmol/pixel.

Liquid chromatography coupled to Venturi easy ambient sonic spray ionization mass spectrometry

Venturi easy ambient sonic spray ionization (V-EASI) is a soft ambient ionization (AI) source that has the advantages of being suitable to the analysis of samples in solution (differently from the majority of AI sources), performing self-pumping, voltage- and heat-free ionization, and requiring minimum or no sample preparation. Since this ionization technique has not been fully explored, the present study provides a proof of principle of the coupling of liquid chromatography to mass spectrometry (LC-MS) using V-EASI as the interface. In order to test the performance of the developed LC-V-EASI-MS system, a quantification method for bixin, a natural dye from annatto (Bixa Orellana L.), which is known to be sensitive to the high voltage applied for electrospray ionization mass spectrometry (ESI-MS) analysis, was validated according to FDA criteria and tested in real samples. The analytical method was successfully applied and met the validation criteria, providing a detectability 10 times better than methods already reported to the quantification of bixin and no matrix effect was observed. Therefore, this proof of principle contributes to the continuous development of AI sources that represents the last great technological advance in MS towards becoming a miniaturized technique able to analyze samples closer to their actual state.

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.

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

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

MASS SPECTROMETRY ANALYSIS OF DRUGS OF ABUSE: CHALLENGES AND EMERGING STRATEGIES

Mass spectrometry has been the "gold standard" for drugs of abuse (DoA) analysis for many decades because of the selectivity and sensitivity it affords. Recent progress in all aspects of mass spectrometry has seen significant developments in the field of DoA analysis. Mass spectrometry is particularly well suited to address the rapidly proliferating number of very high potency, novel psychoactive substances that are causing an alarming number of fatalities worldwide. This review surveys advancements in the areas of sample preparation, gas and liquid chromatography-mass spectrometry, as well as the rapidly emerging field of ambient ionization mass spectrometry. We have predominantly targeted literature progress over the past ten years and present our outlook for the future. © 2020 Periodicals, Inc. Mass Spec Rev.

Facile Production of Large-Area Cell Arrays Using Surface-Assembled Microdroplets

Techniques that enable the spatial arrangement of living cells into defined patterns are broadly applicable to tissue engineering, drug screening, and cell-cell investigations. Achieving large-scale patterning with single-cell resolution while minimizing cell stress/damage is, however, technically challenging using existing methods. Here, a facile and highly scalable technique for the rational design of reconfigurable arrays of cells is reported. Specifically, microdroplets of cell suspensions are assembled using stretchable surface-chemical patterns which, following incubation, yield ordered arrays of cells. The microdroplets are generated using a microfluidic-based aerosol spray nozzle that enables control of the volume/size of the droplets delivered to the surface. Assembly of the cell-loaded microdroplets is achieved via mechanically induced coalescence using substrates with engineered surface-wettability patterns based on extracellular matrices. Robust cell proliferation inside the patterned areas is demonstrated using standard culture techniques. By combining the scalability of aerosol-based delivery and microdroplet surface assembly with user-defined chemical patterns of controlled functionality, the technique reported here provides an innovative methodology for the scalable generation of large-area cell arrays with flexible geometries and tunable resolution.

Wire Desorption Combined with Electrospray Ionization Mass Spectrometry: Direct Analysis of Small Organic and Large Biological Compounds

A novel atmospheric pressure ionization mass spectrometry based on wire desorption and electrospray ionization (WD-ESI) for direct analysis was developed to characterize chemical compounds with different polarities and thermal stabilities at atmospheric pressure. This technique is a variant of the thermal desorption electrospray ion source developed by Shiea et al. One large improvement is that the heating speed (>500 °C/s) of the thermal desorption in this work is extremely fast, using a self-heating metal wire, with which sample solution can splash from the surface to form small droplets and thus the analytes can be protected from thermal decomposition. With this feature, we have successfully achieved soft ionization of highly polar organic and biological compounds such as aflatoxin, small peptides, and even large proteins from complex matrices. The simple structure and self-cleaning capability of the WD-ESI source make it ideal for on-site screening in various applications such as food safety and biodrug testing, especially when coupled with a transportable mass spectrometer.

Study of the noncovalent interactions between phenolic acid and lysozyme by cold spray ionization mass spectrometry (CSI-MS), multi-spectroscopic and molecular docking approaches

Elucidating the recognition mechanisms of the noncovalent interactions between pharmaceutical molecules and proteins is important for understanding drug delivery in vivo, and for the further rapid screening of clinical drug candidates and biomarkers. In this work, a strategy based on cold spray ionization mass spectrometry (CSI-MS), combined with fluorescence, circular dichroism (CD), Fourier transform infrared spectroscopy (FTIR), and molecular docking methods, was developed and applied to the study of the noncovalent interactions between phenolic acid and lysozyme (Lys). Based on the real characterization of noncovalent complex, the detailed binding parameters, as well as the protein conformational changes and specific binding sites could be obtained. CSI-MS and tandem mass spectrometry (MS/MS) technique were used to investigate the phenolic acid-Lys complexes and the structure-affinity relationship, and to assess their structural composition and gas phase stability. The binding affinity was obtained by direct and indirect MS methods. The fluorescence spectra showed that the intrinsic fluorescence quenching of Lys in solution was a static quenching mechanism caused by complex formation, which supported the MS results. The CD and FTIR spectra revealed that phenolic acid changed the secondary structure of Lys and increased the α-helix content, indicating an increase in the tryptophan (W) hydrophobicity near the protein binding site resulting in a conformational alteration of the protein. In addition, molecular docking studies were performed to investigate the binding sites and binding modes of phenolic acid on Lys. This strategy can more comprehensively and truly characterize the noncovalent interactions and can guide further research on the interactions of phenolic acid with other proteins.

Induced Self-aspiration Electrospray Ionization Mass Spectrometry for Flexible Sampling and Analysis

Electrospray ionization (ESI) operating in pulse mode can enhance the utilization efficiency of the electrospray ions by a mass spectrometer. Herein, a novel ionization technique called induced self-aspiration-electrospray ionization (ISA-ESI) was developed based on self-aspiration sampling and capacitive induction. The sample solution polarized in a strong electric field was pulsed drawn into a capillary that was connected to a subambient chamber. The sample solution with polarized ions forms a charged liquid column, which can initiate an electrospray when reaching the capillary outlet. In addition to the self-aspiration ability, the use of a constant high voltage supply and no electrical contact with the solution can also simplify the sampling and ionization operation, enabling a convenient ESI mass spectrometry analysis. The developed ISA-ESI source has been used for multidimensional monitoring of chemical reactions as well as liquid extraction surface analysis of plant tissues. It was expected that this special ionization method could be extended to automated high-throughput ESI-MS analysis.

A "Soft" and "Hard" Ionization Method for Comprehensive Studies of Molecules

Ambient mass spectrometry can be rapidly and directly effective for molecular studies, while there still seems to be a gap between two major groups of electrospray ionization (ESI)- and atmospheric pressure chemical ionization (APCI)-related techniques, for detection of moderately polar to polar and low polar to nonpolar molecules in a relatively low mass range, respectively. Here, an extensively applicable "soft" and "hard" ionization method, spray-dependent plasma mass spectrometry (SDP MS), was established for detecting various molecules with diverse polarities or molecular weights. By SDP MS, both fragment ions and intact molecular ions can be obtained. Significantly, cluster ions of aggregates in high mass range formed by weak molecular interactions can also be well recorded, much softer than traditional ESI MS. By filling the gap between ESI-based and APCI-based ionization techniques, SDP MS would enhance MS performance for comprehensive molecular studies and be extensively applicable in fields of organic synthesis, biological chemistry, medical chemistry, and clinical diagnosis.

Transferring Ions from Solution to the Gas Phase: The Two Basic Principles

The efficient formation of gaseous ions is the crucial step in all successful mass spectrometric experiments. The invention of electrospray ionization (ESI) has strongly facilitated this step by transferring preformed ions directly from solution to the gas phase - thereby circumventing the need to first convert analytes to the gas phase and then ionize them - and therefore ESI has become an extremely useful and widely applied MS technique. The invention of sonic spray ionization (SSI) has also allowed for the transfer of ions from solution into the gas phase, but without the assistance of a voltage or heating. Numerous ionization techniques, using similar principles to those applied in either ESI or SSI, have subsequently been developed. Although experimental conditions used in such techniques vary markedly, herein we argue that they are all based on either one of two basic principles by which ions can be transferred from solution to the gas phase, that is: via (1) neutralizing the counter ion, or (2) separating the ions. We have selected 35 such techniques and categorized them accordingly. This article thereby aims to establish the basic principles by which gaseous ions can be obtained from solvated ions. We further propose that any new ionization technique used to transfer solvated ions to the gas phase will similarly fall into one of these two mechanistic categories. Graphical abstract.

FAD roles in glucose catalytic oxidation studied by multiphase flow of extractive electrospray ionization (MF-EESI) mass spectrometry

The role of the coenzyme flavin adenine dinucleotide (FAD) in the catalytic oxidation of glucose was elucidated by MS using a new extraction and ionization method.

The role of the coenzyme flavin adenine dinucleotide (FAD) in the catalytic oxidation of glucose was elucidated by MS using a new extraction and ionization method. By a multiphase flow of liquid–gas, extractive electrospray ionization was achieved, and this technique (MF-EESI) decreased the salt-matrix interference effectively, avoided salt crystallizations at the capillary tip and increased ionization efficiency by a concentric-sprayed solvent. Notably, two intermediate complexes of FAD–glucose have been observed and differentiated for the first time using this MF-EESI technique. These intermediate complexes were demonstrated to be responsible for the hydride abstraction from glucose, as well as the cyclic coenzyme conversion of FAD during glucose oxidation. Online monitoring was also employed in MF-EESI, thereby providing a potential and informative tool to scrutinize enzymatic catalytic reactions.

Determination of chlorate, perchlorate and bromate anions in water samples by microbore reversed-phase liquid chromatography coupled to sonic-spray ionization mass spectrometry

RATIONALE

Sonic-spray ionization mass spectrometry (SSI-MS) has recently been shown to provide similar mass spectra to those generated by electrospray ionization mass spectrometry for a wide range of compounds, i.e. from small inorganic species to peptides, proteins and numerous other biomolecules. However, limited information about this new ionization technique, such as sensitivity, limit of detection and quantification accuracy, has been reported. In particular, its coupling to liquid chromatography needs further development and assessment, along with the introduction of a broad range of applications.

METHODS

A high-efficiency glass pneumatic nebulizer, used for decades for sample introduction in atomic spectrometry, was used for the SSI-MS analysis of chlorate (ClO₃^- ), perchlorate (ClO₄^- ) and bromate (BrO₃^- ) anions, following their separation using reversed-phase microbore high-performance liquid chromatography and tandem mass spectrometry (MS/MS) operated in selected reaction monitoring mode.

RESULTS

The developed and optimized microbore HPLC/SSI-MS/MS technique exhibited low limits of detection: 5.3 ng L^-1 for chlorate, 10 ng L^-1 for perchlorate and 33.7 ng L^-1 for bromate, and provided reliable and accurate measurements of chlorate concentrations in water samples as demonstrated when comparing it with Ion Chromatography-Conductivity Detection (IC-CD), the benchmark technique for ion quantitation.

CONCLUSIONS

This is the first time that the use of HPLC/SSI-MS/MS has been reported for the detection and quantitation of chlorate, perchlorate and bromate in water samples. In addition, the exceptionally low LODs achieved using SSI render the technique competitive with the established and dominating electrospray ionization technique. Here, we have demonstrated that a commercially available high-efficiency glass pneumatic nebulizer can also be used, without any further modification, as an efficient gas-phase ion source. Copyright © 2017 John Wiley & Sons, Ltd.

Characterization and application of a self-aspirating electrospray source with pneumatic-assisted ionization

A single gas-assisted electrospray ion source developed for ambient mass spectrometry is introduced in this paper. Simultaneous self-aspiration and electrospray could be achieved by using a constant sheath gas flow supplied from a mini air pump. A gas dynamic study of the spray module is carried out for structural optimization. The entire device exhibits a simplified design and has been systematically characterized through both simulated and experimental investigations. According to the results, the ion source exhibited satisfactory stability and the ability for quantitative operation in routine electrospray ionization mass spectrometry. Furthermore, the ion source can be operated as a desorption electrospray ionization source to perform direct desorption/ionization of the solid samples. The versatile source described here appears to provide a practical approach to perform ambient mass spectrometry analysis with unrestricted sampling operation, and the extensive gas dynamic studies together with the experimental characterization are believed to be helpful in building self-aspirating spray devices. Copyright © 2017 John Wiley & Sons, Ltd.

Enhanced aerodynamic reach of vapor and aerosol sampling for real-time mass spectrometric detection using Venturi-assisted entrainment and ionization

Venturi-assisted ENTrainment and Ionization (VENTI) was developed, demonstrating efficient entrainment, collection, and transport of remotely sampled vapors, aerosols, and dust particulate for real-time mass spectrometry (MS) detection. Integrating the Venturi and Coandă effects at multiple locations generated flow and analyte transport from non-proximate locations and more importantly enhanced the aerodynamic reach at the point of collection. Transport through remote sampling probes up to 2.5 m in length was achieved with residence times on the order of 10^-2 s to 10^-1 s and Reynolds numbers on the order of 10³ to 10⁴. The Venturi-assisted entrainment successfully enhanced vapor collection and detection by greater than an order of magnitude at 20 cm stand-off (limit of simple suction). This enhancement is imperative, as simple suction restricts sampling to the immediate vicinity, requiring close proximity to the vapor source. In addition, the overall aerodynamic reach distance was increased by approximately 3-fold over simple suction under the investigated conditions. Enhanced aerodynamic reach was corroborated and observed with laser-light sheet flow visualization and schlieren imaging. Coupled with atmospheric pressure chemical ionization (APCI), the detection of a range of volatile chemical vapors; explosive vapors; explosive, narcotic, and mustard gas surrogate (methyl salicylate) aerosols; and explosive dust particulate was demonstrated. Continuous real-time Venturi-assisted monitoring of a large room (approximately 90 m² area, 570 m³ volume) was demonstrated for a 60-min period without the remote sampling probe, exhibiting detection of chemical vapors and methyl salicylate at approximately 3 m stand-off distances within 2 min of exposure.

A pneumatically assisted nanospray desorption electrospray ionization source for increased solvent versatility and enhanced metabolite detection from tissue

Pneumatically assisted nano-DESI offers improved sensitivity for metabolite species by 1–3 orders of magnitude through more complete desolvation and reduced ionization suppression.

Self-aspiration sampling extractive electrospray ionization mass spectrometry (EESI-MS) for high-throughput analysis of liquid samples

RATIONALE

Extractive electrospray ionization mass spectrometry (EESI-MS) was invented as a typical ambient mass spectrometry method (AMS) and has been used for analyzing complex liquid samples. Here, we designed a Venturi effect-based self-aspiration sampling device and applied it to the EESI-MS for high-throughput analysis of liquid sample.

METHODS

A special concentric nebulizer was designed and employed to produce a suction force for the direct aspiration of liquid samples, followed by ionization and detection. This sample aspiration process was explained and optimized using computational fluid dynamics (CFD) analysis. Experiment data were recorded to exhibit the sensitivity, memory effect, inter-day reproducibility, throughput, and applicability of the self-aspiration sampling EESI-MS.

RESULTS

The limit of detection (LOD) of this method was determined as 4.5 × 10(-10)  g/mL (S/N = 3) for caffeine, and the sample throughput and relative standard deviation (RSD) for full scan mode can reach 0.67 samples/s and 4.76%, respectively. Even for MS/MS mode, the frequency can still be kept at 0.4 samples/s (RSD = 4.71%). Inter-day RSD examined in 1 week was below 10% for the signal of characteristic fragment ions of reserpine. Moreover, based on this method, the amount of caffeine in instant coffee was determined as 4.7%. This device was also proven to be suitable for the protein/peptide analysis.

CONCLUSIONS

These experiment results, especially the amazing results on sample throughput and inter-day RSD, suggest that we provide a valuable device which can be used for the direct high-throughput qualitative/quantitative mass spectrometry analysis of real liquid samples in ambient. Copyright © 2016 John Wiley & Sons, Ltd.

Monitoring Enzymatic Reactions in Real Time Using Venturi Easy Ambient Sonic-Spray Ionization Mass Spectrometry

We developed a technique to monitor spatially confined surface reactions with mass spectrometry under ambient conditions, without the need for voltage or organic solvents. Fused-silica capillaries immersed in an aqueous solution, positioned in close proximity to each other and the functionalized surface, created a laminar flow junction with a resulting reaction volume of ∼5 pL. The setup was operated with a syringe pump, delivering reagents to the surface through a fused-silica capillary. The other fused-silica capillary was connected to a Venturi easy ambient sonic-spray ionization source, sampling the resulting analytes at a slightly higher flow rate compared to the feeding capillary. The combined effects of the inflow and outflow maintains a chemical microenvironment, where the rate of advective transport overcomes diffusion. We show proof-of-concept where acetylcholinesterase was immobilized on an organosiloxane polymer through electrostatic interactions. The hydrolysis of acetylcholine by acetylcholinesterase into choline was monitored in real-time for a range of acetylcholine concentrations, fused-silica capillary geometries, and operating flow rates. Higher reaction rates and conversion yields were observed with increasing acetylcholine concentrations, as would be expected.

Mass spectrometry-guided refinement of chemical energy buffers

Biocatalytic reactions often require supplying chemical energy and phosphate groups in the form of adenosine triphosphate (ATP). Auxiliary enzymes can be used to convert a reaction by-product-adenosine diphosphate (ADP)-back to ATP. By employing real-time mass spectrometry (RTMS), one can gain an insight into inter-conversions of reactants in multi-enzyme reaction systems and optimize the reaction conditions. In this study, temporal traces of ions corresponding to adenosine monophosphate (AMP), ADP and ATP provided vital information that could be used to adjust activities of the 'buffering enzymes'. Using the RTMS results as a feedback, we also characterized a bienzymatic energy buffer that enables the recovery of ATP in the cases where it is directly hydrolysed to AMP in the main enzymatic reaction. The significance of careful selection of enzyme activities-guided by RTMS-is exemplified in the synthesis of glucose-6-phosphate by hexokinase in the presence of a buffering enzyme, pyruvate kinase. Relative activities of the two enzymes, present in the reaction mixture, influence biosynthetic reaction yields. This observation supports the conclusion that optimization of chemical energy recycling procedures is critical for the biosynthetic reaction economy.

Food quality and authenticity screening via easy ambient sonic-spray ionization mass spectrometry

This review is the first to summarize a decade of studies testing the use of easy ambient sonic-spray ionization mass spectrometry (EASI-MS) and its several sister techniques, Venturi (V-EASI), thermal imprinting (TI-EASI) and Spartan (S-EASI) mass spectrometry in food quality control and authentication. Since minimal or no sample preparation is required, such ambient desorption/ionization techniques have been shown to provide direct, fast and selective fingerprinting characterization at the molecular level based on the pools of the most typical components. They have also been found to be applicable on intact, undisturbed samples or on simple solvent extracts. Fundamentals of EASI-MS and its sister techniques, including mechanisms, devices, parameters and strategies, as well as the many applications reported for food analysis, are summarized and discussed.

Thermal bursting ionization for ambient mass spectrometry

Ambient ionization source, thermal bursting ionization (TBI), was characterized for complex liquid sample analysis with mass spectrometry.

A dopant for improved sensitivity in easy ambient sonic-spray ionization mass spectrometry

Recently, 3-nitrobenzonitrile (3-NBN) has been used to improve sensitivity of sonic-spray ionization mass spectrometry. Easy ambient sonic-spray ionization (EASI) is one of the simplest, gentlest and most used spray-based desorption/ionization ambient techniques, but limited sensitivity has been commonly taken as its major drawback. Herein we investigate the use of 3-NBN as a dopant in EASI-MS for improved sensitivity. Using a few typical EASI samples as test cases, the presence of 10 ppm (µg ml(-1) ) of 3-NBN in the spray solvent showed two to fourfold gains in EASI-MS sensitivity as measured both by total ion current and S/N ratios, accompanied with significant reductions in chemical noise. Sensitivity for DESI using 3-NBN as a dopant also improved and dopant DESI versus dopant EASI sensitivities were compared. The use of solvent dopants seems therefore to be a promising strategy to improve sensitivity for spray-based ambient MS techniques. Copyright © 2015 John Wiley & Sons, Ltd.

Microfluidic self-aspiration sonic-spray ionization chip with single and dual ionization channels for mass spectrometry

In consideration of the miniaturization, integration, and universal disadvantages of microfluidic chip-based ionization coupled with mass spectrometry, this study proposed a novel microfluidic self-aspiration sonic-spray ionization chip.

Direct and Convenient Mass Spectrometry Sampling with Ambient Flame Ionization

Recent innovations in ambient ionization technology for the direct analysis of various samples in their native environment facilitate the development and applications of mass spectrometry in natural science. Presented here is a novel, convenient and flame-based ambient ionization method for mass spectrometric analysis of organic compounds, termed as the ambient flame ionization (AFI) ion source. The key features of AFI ion source were no requirement of (high) voltages, laser beams and spray gases, but just using small size of n-butane flame (height approximately 1 cm, about 500 (o)C) to accomplish the rapid desorption and ionization for direct analysis of gaseous-, liquid- and solid-phase organic compounds, as well as real-world samples. This method has high sensitivity with a limit of detection of 1 picogram for propyphenazone, which allows consuming trace amount of samples. Compared to previous ionization methods, this ion source device is extremely simple, maintain-free, low-cost, user-friendly so that even an ordinary lighter (with n-butane as fuel) can achieve efficient ionization. A new orientation to mass spectrometry ion source exploitation might emerge from such a convenient, easy and inexpensive AFI ion source.

Ambient Ionization Mass Spectrometry for Cancer Diagnosis and Surgical Margin Evaluation

BACKGROUND

There is a clinical need for new technologies that would enable rapid disease diagnosis based on diagnostic molecular signatures. Ambient ionization mass spectrometry has revolutionized the means by which molecular information can be obtained from tissue samples in real time and with minimal sample pretreatment. New developments in ambient ionization techniques applied to clinical research suggest that ambient ionization mass spectrometry will soon become a routine medical tool for tissue diagnosis.

CONTENT

This review summarizes the main developments in ambient ionization techniques applied to tissue analysis, with focus on desorption electrospray ionization mass spectrometry, probe electrospray ionization, touch spray, and rapid evaporative ionization mass spectrometry. We describe their applications to human cancer research and surgical margin evaluation, highlighting integrated approaches tested for ex vivo and in vivo human cancer tissue analysis. We also discuss the challenges for clinical implementation of these tools and offer perspectives on the future of the field.

SUMMARY

A variety of studies have showcased the value of ambient ionization mass spectrometry for rapid and accurate cancer diagnosis. Small molecules have been identified as potential diagnostic biomarkers, including metabolites, fatty acids, and glycerophospholipids. Statistical analysis allows tissue discrimination with high accuracy rates (>95%) being common. This young field has challenges to overcome before it is ready to be broadly accepted as a medical tool for cancer diagnosis. Growing research in new, integrated ambient ionization mass spectrometry technologies and the ongoing improvements in the existing tools make this field very promising for future translation into the clinic.

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.

Ambient ionization MS for bioanalysis: recent developments and challenges

Ambient ionization MS has become very popular in analytical science and has now evolved as an effective analytical tool in metabolomics, biological tissue imaging, protein and small molecule drug analysis, where biological samples are probed in a rapid and direct fashion with minimal sample preparation at ambient conditions. However, certain inherent challenges continue to hinder the vibrant prospects of these methods for in situ analyses or to replace conventional methods in bioanalysis. This review provides an introduction to the field and its application in bioanalysis, with an emphasis on the most recent developments and applications. Furthermore, ongoing challenges or limitations related to quantitation, sensitivity, selectivity, instrumentation and mass range of these ambient methods will also be discussed.