Reactive Olfaction Ambient Mass Spectrometry

Charge inversion under plasma-nanodroplet reaction conditions excludes Fischer esterification for unsaturated fatty acids: a chemical approach for type II isobaric overlap

Direct infusion ionization methods provide the highest throughput strategy for mass spectrometry (MS) analysis of low-volume samples. But the trade-off includes matrix effects, which can significantly reduce analytical performance. Herein, we present a novel chemical approach to tackle a special type of matrix effect, namely type II isobaric overlap. We focus on detailed investigation of a nanodroplet-based esterification chemistry for differentiating isotopologue [M + 2] signal due to unsaturated fatty acid (FA) from the monoisotopic signal from a saturated FA. The method developed involves the online fusion of nonthermal plasma with charged nanodroplets, enabling selective esterification of saturated FAs. We discovered that unsaturated FAs undergo spontaneous intramolecular reaction via a novel mechanism based on a carbocation intermediate to afford a protonated lactone moiety (resonance stabilized cyclic carbonium ion), whose mass is the same as the original protonated unsaturated FA. Therefore, the monoisotopic signal from any saturated FA can be selectively shifted away from the mass-to-charge position where the isobaric interference occurs to enable effective characterization by MS. The mechanism governing the spontaneous intramolecular reactions for unsaturated FAs was validated with DFT calculations, experimentation with standards, and isotope labeling. This novel insight achieved via the ultrafast plasma-nanodroplet reaction environment provides a potentially useful synthetic pathway to achieve catalyst-free lactone preparation. Analytically, we believe the performance of direct infusion MS can be greatly enhanced by combining our approach with prior sample enrichment steps for applications in biomedicine and food safety. Also, combination with portable mass spectrometers can improve the efficiency of field studies since front-end separation is not possible under such conditions.

Plasma-Droplet Reaction Systems: A Direct Mass Spectrometry Approach for Enhanced Characterization of Lipids at Multiple Isomer Levels

Neutral triacylglyceride (TG) lipids are critical in cellular function, signaling, and energy storage. Multiple molecular pathways control TG structure via nonselective routes making them structurally complex and analytically challenging to characterize. The presence of C=C bond positional isomers exacerbates this challenge as complete structural elucidation is not possible by conventional tandem mass spectrometric methods such as collision-induced dissociation (CID), alone. Herein, we report a custom-made coaxial contained-electrospray ionization (ESI) emitter that allows the fusion of plasma discharge with charged microdroplets during electrospray (ES). Etched capillaries were incorporated into this contained-ES emitter, facilitating the generation of reactive oxygen species (ROS) at low (3 kV) ESI voltages and allowing stable ESI ion signal to be achieved at an unprecedented high (7 kV) spray voltage. The analytical utility of inducing plasma discharge during electrospray was investigated using online ionization of neutral TGs, in situ epoxidation of unsaturation sites, and C=C bond localization via conventional CID mass spectrometry. Collisional activation of the lipid epoxide generated during the online plasma-droplet fusion experiment resulted in a novel fragmentation pattern that showed a quadruplet of diagnostic ions for confident assignment of C=C bond positions and subsequent isomer differentiation. This phenomenon enabled the identification of a novel TG lipid, composed of conjugated linoleic acid, that is isomeric with two other TG lipids naturally found in extra virgin olive oil. To validate our findings, we analyzed various standards of TG lipids, including triolein, trilinolein, and trilinolenin, and isomeric mixtures in the positive-ion mode, each of which produced the expected quadruplet diagnostic fragment ions. Further validation was obtained by analyzing standards of free fatty acids expected from the hydrolysis of the TG lipids in the negative-ion mode, together with isomeric mixtures. The chemistry governing the gas-phase fragmentation of the lipid epoxides was carefully elucidated for each TG lipid analyzed. This comprehensive shotgun lipidomic approach has the potential to impact biomedical research since it can be accomplished on readily available mass spectrometers without the need for instrument modification.

Online Cross-Linking of Peptides and Proteins during Contained-Electrospray Ionization Mass Spectrometry

Recent advancements in mass spectrometry (MS) now enable all levels of protein structures to be characterized, including primary protein sequence, post-translational modifications, and three-dimensional protein conformations. However, protein conformational studies by MS require the use of many separate techniques that are performed independently of each other. Herein, we described a contained-electrospray (ES) experiment that has potential to integrate peptide/protein cross-linking with the general MS workflow. In our experiment, cross-linking of protein/peptide occurs simultaneously with ionization after analytes, and cross-linkers are sprayed from two separate ES emitters. The online cross-linking process occurring in the charged microdroplet environment was optimized using trilysine peptide and bis(sulfosuccinimidyl)suberate cross-linker. We detected the electrostatic complex between analyte and cross-linker, the mono-linked intermediate, and the fully cross-linked product, allowing us to correctly predict the sequence of reaction events in the cross-linking process. Importantly, we observed that the terminal fully cross-linked product is composed of two distinct conformations. In one form, the product involved cross-linking between two ε-NH₂ amines in lysine residues, while the other conformer was formed by a reaction between one ε-NH₂ amine and the N-terminus. The experimental conditions for selecting one cross-linked species over others during the online ES ionization-MS analysis have been detailed. Appropriate parameters enabled the reaction between α-lactalbumin proteins and cross-linkers using a non-denaturing spray condition. These results establish a framework for a future development in high-throughput structural MS method, where all levels of protein information can be gathered in a single experiment.

Hydrophobic and Rapid-Response Sensor Inks: Array-Based Fingerprinting of Perfumes

Counterfeited perfumes mixed with inexpensive additives for commercial purposes pose a great threat to cosmetic market competition and human health. Herein, a 24-element, solid-state colorimetric sensor array employing chemo-responsive dye inks for accurate discrimination of a variety of fragrance bases and "sniffing out" real perfumes from adulterated samples was first reported. The physiochemical robustness and gas response kinetics of the sensor array were optimized with the streamlined design of the channel geometry and hydrophobic modification of the sensor substrate. A unique and distinguishable color change profile was obtained within 2 min exposure of diluted vapor that enabled clear fingerprinting of chemically similar perfume samples. Four commercial perfume products were successfully distinguished and categorized according to their similarity to relevant perfume bases using chemometric methods including hierarchical clustering and principal component analysis. The sensor array also allows the discrimination of ethanol-diluted fragrance bases from the pristine sample, revealing its potential for quality assurance of perfumes and other cosmetics. Such easy-to-use, disposable, and miniaturized chemical sensing detectors therefore prove exceptionally valuable for fast analysis of luxuries such as perfumes and other industrial products with complex chemical compositions.

Portable Digital Linear Ion Trap Mass Spectrometer Based on Separate-Region Corona Discharge Ionization Source for On-Site Rapid Detection of Illegal Drugs

As narcotic control has become worse in the past decade and the death toll of drug abuse hits a record high, there is an increasing demand for on-site rapid detection of illegal drugs. This work developed a portable digital linear ion trap mass spectrometer based on separate-region corona discharge ionization source to meet this need. A separate design of discharge and reaction regions was adopted with filter air as both carrier gas for the analyte and protection of the corona discharge needle. The linear ion trap was driven by a digital waveform with a low voltage (±100 V) to cover a mass range of 50–500 Da with a unit resolution at a scan rate of 10,000 Da/s. Eighteen representative drugs were analyzed, demonstrating excellent qualitative analysis capability. Tandem mass spectrometry (MS/MS) was also performed by ion isolation and collision-induced dissociation (CID) with air as a buffer gas. With cocaine as an example, over two orders of magnitude dynamic range and 10 pg of detection limit were achieved. A single analysis time of less than 10 s was obtained by comparing the information of characteristic ions and product ions with the built-in database. Analysis of a real-world sample further validated the feasibility of the instrument, with the results benchmarked by GC-MS. The developed system has powerful analytical capability without using consumables including solvent and inert gas, meeting the requirements of on-site rapid detection applications.

Reactive Thread Spray Mass Spectrometry for Localization of C═C Bonds in Free Fatty Acids: Applications for Obesity Diagnosis

Cellulose thread substrates offer a platform for microsampling and reactive ionization of free fatty acid (FFA) isomers for direct differentiation by mass spectrometry. Ambient corona discharge forms when direct current high voltage is applied to the tiny subfibers on the thread substrate in the presence of a polar spray solvent (MeOH/H2O, 2:1, v/v), facilitating chemical reactions across a C═C bond of unsaturated fatty acids. The process was applied for diagnosis of obesity, which we observed to show better discriminatory power when compared to determinations based on body mass index. Overall, the integrated reactive thread-based platform is capable of (i) microsampling and dry-state, room-temperature storage (>30 days) of the biofluids, (ii) in-capillary liquid/liquid extraction, and (iii) in situ epoxidation reactions to locate the C═C bond position in unsaturated fatty acids via reactions with reactive oxygen species present in ambient corona discharge. The study showcased the ability to correctly characterize FFAs, including degree of unsaturation, and the determination of their relative concentrations in clinical biofluid samples.

Microsampling with a Solid-Phase Extraction Cartridge: Storage and Online Mass Spectrometry Analysis

This study aims to introduce the concept of utilizing a solid-phase extraction (SPE) cartridge for remote biofluid collection, followed by direct sample analysis at a later time. For this, a dried matrix spot was prepared in a syringe, in the form of SPE cartridge for the first time to enable small biofluid collection (microsampling), storage, shipment, and online electrospray ionization (ESI) mass spectrometry (MS) analysis of the stored dried samples. The SPE sorbents were packed into an ESI syringe and the resultant cartridge was used for sampling small volumes (<20 μL) of different complex biological fluids including blood, plasma, serum, and urine. The collected sample was stored in the dry state within the confinement of the SPE sorbent at room temperature, and analyte stability (e.g., diazepam) was maintained for more than a year. Direct coupling of the SPE cartridge to MS provides excellent accuracy, precision, and sensitivity for analyzing illicit drugs present in the biofluid. The corresponding mechanism of wrong-way positive ion generation from highly basic elution solvents was explored. Without chromatography, our direct SPE-ESI-MS analysis technique afforded detection limits as low as 26 and 140 pg/mL for raw urine and untreated plasma, respectively. These promising results proved that the new syringe-based SPE cartridge can serve as a good alternative to conventional microsampling techniques in terms of analyte stability, ease of operation and versatility, and analytical sensitivity and speed.

Enzyme-Catalyzed Hydrolysis of Lipids in Immiscible Microdroplets Studied by Contained-Electrospray Ionization

Enzyme-catalyzed hydrolysis of lipids was monitored directly in immiscible microdroplet environments using contained-electrospray mass spectrometry. Aqueous solution of the lipase enzyme from Pseudomonas cepacia and the chloroform solution of the lipids were sprayed from separate capillaries, and the resultant droplets were merged within a reaction cavity that is included at the outlet of the contained-electrospray ionization source. By varying the length of the reaction cavity, the interaction time between the enzyme and its substrate was altered, enabling the quantification of reaction product as a function of time. Consequently, enhancement factors were estimated by comparing rate constants derived from the droplet experiment to rate constants calculated from solution-phase conditions. These experiments showed enhancement factors greater than 100 in favor of the droplet experiment. By using various lipid types, two possible mechanisms were identified to account for lipase reactivity in aerosols: in-droplet reactions for relatively highly soluble lipids and a droplet coalescence mechanism that allows interfacial reactions for the two immiscible systems.

Uncatalyzed N-Alkylation of Amines in Ionic Wind from Ambient Corona Discharge

Ionic wind comprising of the drag of bulk air in the presence of electrical discharge enabled N-alkylation reactions under ambient conditions. By introducing reactant vapor as part of the discharge gas during the stages of electron acceleration, both neutral and charged species of the selected organic reactant gain energy through ion-neutral collisions, which is identified to facilitate chemical reactions. By performing this experiment in front of a mass spectrometer, chemical reactions occurring in the ionic wind were examined in real time. Reaction energetics were characterized via the use of benzylamine, which freely dissociates at a critical energy of 3.6 eV to give the resonance-stabilized benzyl cation as reaction intermediate. Benzylamine and many other primary amines were observed to undergo N-alkylation reactions by engaging in self-cross-coupling ion-molecule reactions. Because of the high energies of species involved and the fact that the ionic wind is generated at atmospheric pressure, it was straightforward to collect the ensuing reaction products without the use of complicated instrumentation. Water served as an effective collecting solvent allowing >0.1 mg of intact N-alkylated products to be collected under ambient conditions using a single plasma emitter. A novel N-alkylation reaction pathway involving the synthesis of N-benzyl-1-(methyleneamino)-1-phenylmethanaime was discovered through this offline product collection experiment, providing new insight into benzylamine dissociation in the ionic wind.

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.

Direct Analysis of Doping Agents in Raw Urine Using Hydrophobic Paper Spray Mass Spectrometry

In this study, the direct analysis of doping agents in urine samples with no sample preparation by a modified paper spray mass spectrometry (PS-MS) methodology has been demonstrated for the first time. We have described a paper surface treatment with trichloromethylsilane using a gas-phase reaction to increase the ionization of target compounds. This approach was applied for the analysis of two classes of banned substances in urine samples: anabolic agents (trenbolone and clenbuterol) and diuretics (furosemide and hydrochlorothiazide). Under optimized conditions, the developed methods presented satisfactory repeatability, and an analysis of variance showed linearity without lack-of-fit. Highly sensitive detections as low as sub-nanogram per milliliter levels, which is below the minimum required performance levels proposed by the World Anti-Doping Agency, have been reached using the hydrophobic PS-MS analysis without any preconcentration and cleanup step.

Droplet Imbibition Enables Nonequilibrium Interfacial Reactions in Charged Microdroplets

A droplet imbibition experiment is proposed to study interfacial effects, which appears to be the main factor influencing reaction acceleration in charged microdroplets produced by electrospray ionization (ESI). One reagent is deposited onto the surface of rapidly moving microdroplets containing the second reagent to be reacted. In this manner, reactions are hindered from reaching equilibrium and monitored in real time by mass spectrometry. We demonstrated this phenomenon using Katritzky chemistry, which is known to proceed either by the solvent-stabilized 2H-pyran intermediate or via the surface-active pseudobase intermediate. Comparisons with reactions performed using ESI show obvious surface effects in favor of the droplet imbibition experiment. By keeping reactant mole ratio constant, it was demonstrated that similar interfacial effects observed in the droplet imbibition experiment can be reached by allowing ESI microdroplets containing premixed reagents to traverse a distance >16 mm. At such spray distance, molecular diffusion and droplet lifetime become comparable allowing reactants to be enriched at droplet surface. Reactions were also conducted in rapid mixing, theta capillary-based droplets, which showed markedly reduced yields compared with the interfacial droplet imbibition experiment.

Direct Mass Spectrometry Analysis of Complex Mixtures by Nanoelectrospray with Simultaneous Atmospheric Pressure Chemical Ionization and Electrophoretic Separation Capabilities

Accurate and rapid analysis of complex microsamples are challenging tasks in translational research. Nanoelectrospray ionization (nESI) is the method of choice for analyzing small sample volumes by mass spectrometry (MS), but this technique works well only for polar analytes. Herein, we describe a versatile dual noncontact nESI/nAPCI (nanoatmospheric pressure chemical ionization) source that allows simultaneous detection of both polar and nonpolar analytes in microliter quantities of samples under ambient conditions and without pretreatment. The same device can be activated to enable electrophoretic separation. The noncontact nESI/nAPCI MS platform was applied to analyze different samples, including high sensitive direct analysis of biofluids and the efficient detection of proteins in buffers with high concentration of nonvolatile salts. Excellent linearity, accuracy and limits of detection were achieved for compounds with different chemical properties in different matrices. The high sensitivity, universality, simplicity, and ease of operation make this MS technique promising for use in clinical and forensic applications.