Dielectric barrier discharges applied for soft ionization and their mechanism

Concentric Hybrid Nanoelectrospray Ionization-Atmospheric Pressure Chemical Ionization Source for High-Coverage Mass Spectrometry Analysis of Single-Cell Metabolomics

High-coverage mass spectrometry analysis of single-cell metabolomics remains challenging due to the extremely low abundance and wide polarity of metabolites and ultra-small volume in single cells. Herein, a novel concentric hybrid ionization source, nanoelectrospray ionization-atmospheric pressure chemical ionization (nanoESI-APCI), is ingeniously designed to detect polar and nonpolar metabolites simultaneously in single cells. The source is constructed by inserting a pulled glass capillary coaxially into a glass tube that acts as a dielectric barrier layer. Benefitting from the integrated advantages of nanoESI and APCI, its limit of detection is improved by one order of magnitude to 10 pg mL-1. After the operational parameter optimization, 254 metabolites detected in nanoESI-APCI are tentatively identified from a single cell, and 82 more than those in nanoESI. The developed nanoESI-APCI is successively applied to study the metabolic heterogeneity of human hepatocellular carcinoma tissue microenvironment united with laser capture microdissection (LCM), the discrimination of cancer cell types and subtypes, the metabolic perturbations to glucose starvation in MCF7 cells and the metabolic regulation of cancer stem cells. These results demonstrated that the nanoESI-APCI not only opens a new avenue for high-coverage and high-sensitivity metabolomics analysis of single cell, but also facilitates spatially resolved metabolomics study coupled with LCM.

Dielectric Barrier Discharge Ionization Mechanisms: Polycyclic Aromatic Hydrocarbons as a Case of Study

Dielectric barrier discharge ionization (DBDI) is a versatile tool for small-molecule mass spectrometry applications, helping cover from polar to low polar molecules. However, the plasma gas-phase interactions are highly complex and have been scarcely investigated. The ionization mechanisms of plasmas have long been assumed to be somewhat similar to atmospheric pressure chemical ionization (APCI). Here, we evaluated the ionization mechanisms of a two-ring DBDI ion source, using different discharge gases to analyze vaporized liquid samples. Polycyclic aromatic hydrocarbons (PAHs) were used as model analytes to assess the mechanisms' dominance: protonation, [M + H]⁺, or radical ion species formation, [M]^·+. In the present work, two different ionization trends were observed for APCI and DBDI during the PAH analysis; the compounds with proton affinities (PA) over 856 kJ/mol were detected as [M + H]⁺ when APCI was used as ionization source. Meanwhile, independently of the PA, DBDI showed the prevalence of charge exchange reactions. The addition of dopants in the gas-phase region shifted the ionization mechanisms toward charge exchange reactions, facilitating the formation of [M]^·+ ion species, showing anisole a significant boost of the PAH radical ion species signals, over nine times for Ar-Prop-DBDI analysis. The presence of high-energy metastable atoms (e.g., He^M) with high ionization potentials (IE = 19.80 eV) did not show boosted PAH abundances or extensive molecule fragmentation. Moreover, other species in the plasma jet region with closer and more appropriate IE, such as N₂ B³Π_g excited molecules, are likely responsible for PAH Penning ionization.

Hybrid Ionization Source Combining Nanoelectrospray and Dielectric Barrier Discharge Ionization for the Simultaneous Detection of Polar and Nonpolar Compounds in Single Cells

Single-cell metabolomics is expected to deliver fast and dynamic information on cell function; therefore, it requires rapid analysis of a wide variety of very small quantities of metabolites in living cells. In this work, a hybrid ionization source that combines nanoelectrospray ionization (nanoESI) and dielectric barrier discharge ionization (DBDI) is proposed for single-cell analysis. A capillary with a 1 μm i.d. tip was inserted into cells for sampling and then directly used as the nanoESI source for ionization of polar metabolites. In addition, a DBDI source was employed as a post-ionization source to improve the ionization of apolar metabolites in cells that are not easily ionized by ESI. By increasing the voltage of the DBDI source from 0 to 3.2 kV, the classes of detected metabolites can be shifted from mostly polar to both polar and apolar to mainly apolar. Plant cells (onion) and human cells (PANC-1) were investigated in this study. After optimization, 50 compounds in onion cells and 40 compounds in PANC-1 cells were observed in ESI mode (3.5 kV) and an additional 49 compounds in onion cells and 73 compounds in PANC-1 cells were detected in ESI (3.5 kV)-DBDI (2.6 kV) hybrid mode. This hybrid ionization source improves the coverage, ionization efficiency, and limit of detection of metabolites with different polarities and could potentially contribute to the fast-growing field of single-cell metabolomics.

Thermal desorption bridged the gap between dielectric barrier discharge ionization and dried plasma spot samples for sensitive and rapid detection of fentanyl analogs in mass spectrometry

Guided by finite element simulations and 3D-printing, we constructed a semi-covered flat-TD surface for sufficient thermal desorption and ionization of fentanyl analogs from dried plasma/blood spot samples.

Direct Ultratrace Detection of Lead in a Single Hair Using Portable Electromagnetic Heating Vaporization-Atmospheric Pressure Glow Discharge-Atomic Emission Spectrometry

In this paper, the first demonstration of direct ultratrace determination of lead in a single human hair by direct current-atmospheric pressure glow discharge-atomic emission spectrometry (DC-APGD-AES) coupled with electromagnetic heating vaporization (EMV) was described. Only the ultramicro mass of a human hair sample (about 0.15 mg, often a single human hair) was required during the analysis, and fast detection was implemented without tedious pretreatment processes, such as grinding and digestion. A limit of detection (LOD) of 30.8 μg kg^-1 (4.8 pg) for Pb was obtained under optimized conditions, which was even equivalent to that of conventional LA-ICP-MS/ETV-ICP-MS/GFAAS. EMV-APGD-AES, meanwhile, can facilitate miniaturization and portability with low power and small size. The accuracy and practicality of the method were verified by the analysis of certified reference materials (CRMs) GBW09101b (human hair) and human hair samples from three volunteers. A simple, efficient, and low-cost method for detecting Pb in human hair has been developed.

Evaluation of a novel controlled-atmosphere flexible microtube plasma soft ionization source for the determination of BTEX in olive oil by headspace-gas chromatography/mass spectrometry

Although electron impact ionization (EI) remains the standard ionization source for GC-MS, it presents extensive fragmentation as its main limitation. The potential of a novel plasma-based soft ionization source named controlled-atmosphere flexible microtube plasma (CA-FμTP) has been evaluated in this work for the determination of monoaromatic volatile BTEX group (namely benzene, toluene, ethylbenzene, and o-, m- and p-xylenes) in olive oil, based on headspace technique. The obtained results show an attractive advantage over EI due to no fragmentation was observed. A nitrosated ion [M + NO]⁺ is obtained as the most abundant species. Thus, the BTEX mass spectrum identification can be carried out without major effort. In general, the sensitivity for CA-FμTP was comparable to those obtained by EI, achieving LODs ranged from 0.6 to 1.0 μg kg^-1. The potential usefulness of GC-CA-FμTP-MS for the detection of BTEX was demonstrated by analyzing olive oil samples and identifying traces of these compounds in one sample. Therefore, the proposed plasma-based soft ionization is suitable for BTEX analysis in fatty complex matrixes as olive oil.

Solution anode glow discharge optical emission spectrometry: Volatile hydride introduction from the gas jet nozzle cathode for ultrasensitive determination of lead

An ultrasensitive method for the determination of Pb was developed by coupling solution anode glow discharge-optical emission spectrometry (SAGD-OES) with hydride generation (HG). Compared to solution cathode glow discharge, the introduction of analytes yielded via HG from the discharge cathode into the microplasma was demonstrated to be easily performed by SAGD in which the gas jet nozzle served as cathode and further enhanced sensitivity for Pb determination was achieved. The susceptibility of SAGD-OES to the matrix-induced interferences in the analysis of real samples was significantly improved owing to the coupling of HG. After a thorough optimization of the HG-SAGD-OES system parameters, the developed system achieved Pb detection limit of 0.061 ng mL^-1, with the corresponding relative standard deviation being <2.2% at analyte concentrations of 50 ng mL^-1. The potential application of this method was validated by successfully analyzing three certified reference materials (CRMs: GBW07311, GBW07312, and GBW07601a (GSH-1)) and human blood samples.

Ambient (desorption/ionization) mass spectrometry methods for pesticide testing in food: a review

Ambient mass spectrometry refers to the family of techniques that allows ions to be generated from condensed phase samples under ambient conditions and then, collected and analysed by mass spectrometry. One of their key advantages relies on their ability to allow the analysis of samples with minimal to no sample workup. This feature maps well to the requirements of food safety testing, in particular, those related to the fast determination of pesticide residues in foods. This review discusses the application of different ambient ionization methods for the qualitative and (semi)quantitative determination of pesticides in foods, with the focus on different specific methods used and their ionization mechanisms. More popular techniques used are those commercially available including desorption electrospray ionization (DESI-MS), direct analysis on real time (DART-MS), paper spray (PS-MS) and low-temperature plasma (LTP-MS). Several applications described with ambient MS have reported limits of quantitation approaching those of reference methods, typically based on LC-MS and generic sample extraction procedures. Some of them have been combined with portable mass spectrometers thus allowing "in situ" analysis. In addition, these techniques have the ability to map surfaces (ambient MS imaging) to unravel the distribution of agrochemicals on crops.

Stepwise optimization of a Flexible Microtube Plasma (FµTP) as an ionization source for Ion Mobility Spectrometry

The ionization source is the central system of analytical devices such as mass spectrometers or ion mobility spectrometers. In this study, a recently developed flexible microtube plasma (FμTP) is applied as an ionization source for a custom-made drift tube ion mobility spectrometer (IMS) for the first time. The FµTP is based on a highly miniaturized, robust and a small-footprint dielectric barrier discharge design with an outstanding ionization efficiency. In this study, the experimental setup of the FµTP was further improved upon to achieve optimal coupling conditions in terms of the ion mobility spectrometry sensitivity and the plasma gas consumption. One major focus of this study was the adjustment of the electrical operation parameters, in particular, the high voltage amplitude, frequency and duty cycle, in order to minimize the electric field disturbances and yield higher signals. Additionally, the consumption of helium plasma gas was reduced by refining the FµTP. It was found that the ionization efficiency could be significantly enhanced by increasing the plasma high voltage and through application of a duty cycle up to 90:10. Plasma gas flows could be reduced down to 3 mL min^-1 by increasing the plasma high voltage amplitude. Furthermore, a smaller wire electrode design enables the operation of the FµTP with nitrogen and clean air. Moreover, detection limits of a homologous series of ketones in the range of 330 ppt_v (N₂-FµTP, 2-decanone) down to 20 ppt_v (He-FµTP, 2-octanone) could be reached in the optimized setup. To sum up, this feasibility study demonstrates the potential of the optimized FµTP as a powerful ionization source for ion mobility spectrometry especially with regard to ionization efficiency.

Can low-temperature point discharge Be used as atomic emission source for sensitive determination of cyclic volatile methylsiloxanes?

Despite of increased interest in the application of miniature microplasma atomic spectrometry for environmental analytical chemistry, the amenable element detection range is limited to some metal elements and carbon due to it low power consumption. In this work, the generation of silicon atomic emission (251.6 nm and 288.2 nm) from the organosiloxanes was found possible in a low-temperature, low-power, and compact point discharge. Consequence, a tiny point discharge silicon optical emission spectrometer (μPD-OES) was exploited, and used as a novel GC detector for the determination of various cyclic volatile methyl siloxanes (cVMSs). Under the optimized conditions, the developed system provided limits of detection (LODs) of 0.2 mg L^-1, 0.04 mg L^-1, 0.03 mg L^-1 and 0.02 mg L^-1 of Si for hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane and dodecamethylcyclohexasiloxane, respectively. Meanwhile, relative standard deviations (RSDs) of better than 2.3% were obtained. In contrast to gas chromatography mass spectrometer, GC-μPD-OES significantly simplifies the experimental setup with low power consumption and a miniature configuration. As far as we know, this work reports for the first time that silicon atomic emission can be generated in such low temperature microplasma. The accuracy of this system was validated by determining cVMSs in five daily-used shampoo samples collected from retail store, providing satisfactory recoveries (84%-114%) and excellent agreement with values determined by GC-MS at the 95% confidence level.

Spectroscopic Measurements of Dissolved O3, H2O2 and OH Radicals in Double Cylindrical Dielectric Barrier Discharge Technology: Treatment of Methylene Blue Dye Simulated Wastewater

Advanced oxidation technologies (AOTs) focusing on nonthermal plasma induced by dielectric barrier discharge are adequate sources of diverse reactive oxygen species (ROS) beneficial for water and wastewater treatment. In this study, indigo, peroxytitanyl sulphate and terephthalic acid methods were used to approximate the concentrations of O3, H2O2 and OH produced in a double cylindrical dielectric barrier discharge (DCDBD) plasma configuration. The effect of pH and scavengers as well as the amount of chemical probes on the generation of oxidants was investigated. The efficiency of the DCDBD reactor was further evaluated using methylene blue (MB) as model pollutant. The results demonstrated that the formation of oxidants O3, H2O2 and OH in the DCDBD reactor was pH-dependent. Furthermore, the presence of scavengers such as phosphates, bicarbonates and carbonates in the solution diminished the amount of OH in the system and hence could impact upon the degree of detoxification of targeted pollutants during water and wastewater treatment. The MB simulated dye was totally decomposed into H2O, dissolved CO2 and simpler aqueous entities. Herein the DCDBD design is an adequate AOT that can be used worldwide for effective decontamination of water and wastewater.

Optical and spectroscopic study of a supersonic flowing helium plasma: energy transport in the afterglow

Flowing plasma jets are increasingly investigated and used for surface treatments, including biological matter, and as soft ionization sources for mass spectrometry. They have the characteristic capability to transport energy from the plasma excitation region to the flowing afterglow, and therefore to a distant application surface, in a controlled manner. The ability to transport and deposit energy into a specimen is related to the actual energy transport mechanism. In case of a flowing helium plasma, the energy in the flowing afterglow may be carried by metastable helium atoms and long-lived helium dimer ions. In this work a systematic investigation of the optical and spectroscopic characteristics of a supersonic flowing helium plasma in vacuum and its afterglow as function of the helium gas density is presented. The experimental data are compared with numerical modeling of the plasma excitation and helium dimer ion formation supported by a Computational Fluid Dynamic simulation of the helium jet. The results indicate that the plasma afterglow is effectively due to helium dimer ions recombination via a three-body reaction.

A Critical Review of Recent Progress and Perspective in Practical Denitration Application

Nitrogen oxides (NOx) represent one of the main sources of haze and pollution of the atmosphere as well as the causes of photochemical smog and acid rain. Furthermore, it poses a serious threat to human health. With the increasing emission of NOx, it is urgent to control NOx. According to the different mechanisms of NOx removal methods, this paper elaborated on the adsorption method represented by activated carbon adsorption, analyzed the oxidation method represented by Fenton oxidation, discussed the reduction method represented by selective catalytic reduction, and summarized the plasma method represented by plasma-modified catalyst to remove NOx. At the same time, the current research status and existing problems of different NOx removal technologies were revealed and the future development prospects were forecasted.

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.

Flexible Microtube Plasma (FμTP) as an Embedded Ionization Source for a Microchip Mass Spectrometer Interface

Dielectric barrier discharges are used as soft ionization sources for mass spectrometers or ion mobility spectrometers, enabling excellent possibilities for analytical applications. A new robust and small-footprint discharge design, flexible microtube plasma (FμTP), developed as a result of ongoing miniaturization and electrode design processes, is presented in this work. This design provides major safety benefits by fitting the electrode into an inert flexible fused silica capillary (tube). Notably, in this context, the small discharge dimensions enable very low gas flows in the range of <100 mL min^-1; portability; the use of hydrogen, nitrogen, and air in addition to noble gases such as helium and argon, including its mixtures with propane; and application in microchip environments. By coupling FμTP with gas chromatography/mass spectrometry, we show that the polarity principle of the new discharge design allows it to outperform established ionization sources such as dielectric barrier discharge for soft ionization (DBDI) and low-temperature plasma (LTP) at low concentrations of perfluoroalkanes in terms of sensitivity, ionization efficiency, chemical background, linear dynamic range, and limit of detection by a large margin. In negative ion mode, the limit of detection is improved by more than 3-fold compared with that of DBDI and by 8-fold compared with that of LTP. The protonation capability was evaluated by headspace measurements of diisopropyl methylphosphonate in positive ion mode, showing low fragmentation and high stability in comparison to DBDI and LTP.

Doped Argon Surface Desorption Dielectric-Barrier Discharge Ionization Mass Spectrometry for Fragile Compounds

Argon surface desorption dielectric-barrier discharge ionization (Ar-SDDBDI) in conjunction with dopants was reported for rapid and sensitive determining of fragile compounds. In dopant/Ar-SDDBDI, analytes are ionized primarily through proton transfer with dopant ions, which are formed in Ar plasma. Different from He, dopant/Ar-SDDBDI generates low energetic ions, and therefore, fragmentation is suppressed. It thus significantly simplifies the mass spectra and the assignment of one peak. Dopants ranging from organic solvents to gaseous materials were systematically studied. The application of dopant/Ar-SDDBDI was demonstrated by analysis of multiple compounds, including antibiotics, amino acids, fatty acids, hormones, pharmaceuticals, and peptides. Rapid profiling of chemicals in such complex matrixes including mixtures and drug tablets was also tested. Positive and negative mass spectra with little to no fragmentation for compounds in the pure state and as mixtures were readily achieved. Limits of detection (S/N = 3) were determined to be 0.60 and 0.36 pmol, respectively, for the analysis of l-alanine and metronidazole. Furthermore, the demonstration applications also included imaging of an "H" character under ambient conditions. These results indicate that the technique by combining of Ar-SDDBDI with dopants exhibits high sensitivity, high spatial resolution, and a very low degree of fragmentation, which render it a potential tool for fragile compound analysis in mass spectrometry imaging.

Mechanistic Understanding Leads to Increased Ionization Efficiency and Selectivity in Dielectric Barrier Discharge Ionization Mass Spectrometry: A Case Study with Perfluorinated Compounds

Perfluorinated compounds have unique properties and many practical applications, but are difficult to ionize efficiently with soft ionization methods. An active capillary plasma ionization source based on dielectric barrier discharge ionization (DBDI) coupled with mass spectrometry was used to study the ionization pathway of perfluorinated compounds (PFCs), with the aim of both increasing the ionization efficiency and influencing the selectivity for generating product ions in negative ion mode. Cyclic and linear perfluorinated alkanes were found to mainly form [M - F]^- and [M - F + O]^- ions, respectively; the [M]^-• ion was only obtained at low discharge voltage. Additionally, fluorine attachment [M + F]^- was observed mostly for perfluorinated alkenes. An isotope labeling experiment with ¹⁸O₂ showed that the primary source of oxygen in the substitution reaction is molecular oxygen, reacting with the analyte in the form of O^-• ions. The abundance of [M - F + O]^- ions can thus be enhanced by increasing the plasma voltage to produce a higher O^-• ion density. The loss of the fluorine (without substitution by oxygen) was mainly observed at high frequency, a fact which can be exploited for tuning the ionization toward specific product ions. Overall, the mechanistic understanding of the ionization of PFCs allowed to increase the selectivity of the product ions, resulting in increased ionization efficiency.

Probing the geometry reorganization from solution to gas-phase in putrescine derivatives by IRMPD, 1H-NMR and theoretical calculations

Geometry reorganization of ESI formed ions are demonstrated and explicit calculations of the solution phase are shown to be relevant.