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

Ortho C-H Bond Activations in an Atmospheric Microwave Plasma Ion Source

C-H bond ortho-substitution reaction has always been a significant and challenging topic in organic chemistry. We proposed a synthesis method based on microwave plasma torches. High-resolution mass spectrometry was used to monitor rapid reaction products. 2-Alkylbenzimidazole can be formed through the reaction of phenylnitrenium ion and nitriles on a millisecond scale. This reaction can achieve the one-step formation of benzimidazoles from benzene ring single-substituted compounds without the addition of external oxidants or catalysts. A similar C-H bond activation reaction can be accomplished with ketones. Meanwhile, the microwave plasma reactor was modified, and the resulting 2-methylbenzimidazole was successfully collected, indicating the device has good application potential in organic reactions such as C-H bond activation reaction.

Plasma-Droplet Fusion-Mass Spectrometry Reveals Sub-Millisecond Protein Unfolding Dynamics Induced by Reactive Oxygen Species

Investigating the connection between reactive oxygen species (ROS) and oxidative protein unfolding is critical to reveal the mechanisms underlying disease involving elevated ROS and protein misfolding. This could inform the development of therapeutics targeting cells based on their redox status. In this study, we developed a plasma-droplet fusion-mass spectrometry platform to rapidly assess protein resilience to ROS. This home-built system fuses ROS generated from the microplasma source with protein microdroplets from a tunable nanospray source. At the droplet-plasma intersection, ROS interact with proteins before entering the mass spectrometer for mass identification and structural characterization. Benefiting from the small-sized microdroplet with adjustable traveling velocity, the platform enables the first sub-millisecond kinetic study of ROS-induced protein unfolding, with a rate constant of approximately 1.81 ms-1. Capturing ROS-induced protein unfolding intermediates and the resultant ligand release dynamics can be extended to many more protein systems. We foresee broad applications for establishing previously undetected protein unfolding events when biologically impactful ROS are enriched in time and space with functional proteins and complexes.

Dual Tunability for Uncatalyzed N-Alkylation of Primary Amines Enabled by Plasma-Microdroplet Fusion

The fusion of non-thermal plasma with charged microdroplets facilitates catalyst-free N-alkylation for a variety of primary amines, without halide salt biproduct generation. Significant reaction enhancement (up to >200×) is observed over microdroplet reactions generated from electrospray. This enhancement for the plasma-microdroplet system is attributed to the combined effects of energetic collisions and the presence of reactive oxygen species (ROS). The ROS (e.g., O2 ⋅- ) act as a proton sink to increase abundance of free neutral amines in the charged microdroplet environment. The effect of ROS on N-alkylation is confirmed through three unique experiments: (i) utilization of radical scavenging reagent, (ii) characterization of internal energy distribution, and (iii) controls performed without plasma, which lacked reaction acceleration. Establishing plasma discharge in the wake of charged microdroplets as a green synthetic methodology overcomes two major challenges within conventional gas-phase plasma chemistry, including the lack of selectivity and product scale-up. Both limitations are overcome here, where dual tunability is achieved by controlling reagent concentration and residence time in the microdroplet environment, affording single or double N-alkylated products. Products are readily collected yielding milligram quantities in eight hours. These results showcase a novel synthetic strategy that represents a straightforward and sustainable C-N bond-forming process.

Catalyst-Free Oxidation Reactions in a Microwave Plasma Torch-Based Ion/Molecular Reactor: An Approach for Predicting the Atmospheric Oxidation of Pollutants

The atmospheric oxidation of chemicals has produced many new unpredicted pollutants. A microwave plasma torch-based ion/molecular reactor (MPTIR) interfacing an online mass spectrometer has been developed for creating and monitoring rapid oxidation reactions. Oxygen in the air is activated by the plasma into highly reactive oxygen radicals, thereby achieving oxidation of thioethers, alcohols, and various environmental pollutants on a millisecond scale without the addition of external oxidants or catalysts (6 orders of magnitude faster than bulk). The direct and real-time oxidation products of polycyclic aromatic hydrocarbons and p-phenylenediamines from the MPTIR match those of the long-term multistep environmental oxidative process. Meanwhile, two unreported environmental compounds were identified with an MPTIR and measured in the actual water samples, which demonstrates the considerable significance of the proposed device for both predicting the environmental pollutants (non-target screening) and studying the mechanism of atmospheric oxidative processes.

Applications of ambient ionization mass spectrometry in 2021: An annual review

Ambient ionization mass spectrometry (AIMS) has revolutionized the field of analytical chemistry, enabling the rapid, direct analysis of samples in their native state. Since the inception of AIMS almost 20 years ago, the analytical community has driven the further development of this suite of techniques, motivated by the plentiful advantages offered in addition to traditional mass spectrometry. Workflows can be simplified through the elimination of sample preparation, analysis times can be significantly reduced and analysis remote from the traditional laboratory space has become a real possibility. As such, the interest in AIMS has rapidly spread through analytical communities worldwide, and AIMS techniques are increasingly being integrated with standard laboratory operations. This annual review covers applications of AIMS techniques throughout 2021, with a specific focus on AIMS applications in a number of key fields of research including disease diagnostics, forensics and security, food safety testing and environmental sciences. While some new techniques are introduced, the focus in AIMS research is increasingly shifting from the development of novel techniques toward efforts to improve existing AIMS techniques, particularly in terms of reproducibility, quantification and ease-of-use.

Excited-State N Atoms Transform Aromatic Hydrocarbons into N-Heterocycles in Low-Temperature Plasmas

The direct formation of N-heterocycles from aromatic hydrocarbons has been observed in nitrogen-based low-temperature plasmas; the mechanism of this unusual nitrogen-fixation reaction is the topic of this paper. We used homologous aromatic compounds to study their reaction with reactive nitrogen species (RNS) in a dielectric barrier discharge ionization (DBDI) source. Toluene (C₇H₈) served as a model compound to study the reaction in detail, which leads to the formation of two major products at "high" plasma voltage: a nitrogen-replacement product yielding protonated methylpyridine (C₆H₈N⁺) and a protonated nitrogen-addition (C₇H₈N⁺) product. We complemented those studies by a series of experiments probing the potential mechanism. Using a series of selected-ion flow tube experiments, we found that N⁺, N₂⁺, and N₄⁺ react with toluene to form a small abundance of the N-addition product, while N(⁴S) reacted with toluene cations to form a fragment ion. We created a model for the RNS in the plasma using variable electron and neutral density attachment mass spectrometry in a flowing afterglow Langmuir probe apparatus. These experiments suggested that excited-state nitrogen atoms could be responsible for the N-replacement product. Density functional theory calculations confirmed that the reaction of excited-state nitrogen N(²P) and N(²D) with toluene ions can directly form protonated methylpyridine, ejecting a carbon atom from the aromatic ring. N(²P) is responsible for this reaction in our DBDI source as it has a sufficient lifetime in the plasma and was detected by optical emission spectroscopy measurements, showing an increasing intensity of N(²P) with increasing voltage.

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.