Plasma decay in high-voltage nanosecond discharges in oxygen-containing mixtures

Plasma decay in high-voltage nanosecond discharges in CO₂: O₂ and Ar: O₂ mixtures at room gas temperature and a pressure of 10 Torr is studied experimentally and theoretically. The time dependence of the electron density during plasma decay is measured using microwave interferometry. The time evolution of the charged particle density, ion composition, and electron temperature is simulated numerically. It is shown that, under the given conditions, the discharge plasma is dominated for the most time by O ₂ ⁺ ions and plasma decay is determined by dissociative and three-body electron?ion recombination. As in the previous studies performed for air and oxygen plasmas, agreement between measurements and calculations is achieved only under the assumption that the rate of three-body recombination of molecular ions is much greater than that for atomic ions. The values of the rate constant of three-body recombination of electrons with О₂ ⁺ ions in a wide range of electron temperatures (500–5500 K), as well as for thermal (300 K) electrons, are obtained by processing the experimental results.     

Production of singlet oxygen in a non-self-sustained discharge

The production of O₂(a¹Δ_g) singlet oxygen in non-self-sustained discharges in pure oxygen and mixtures of oxygen with noble gases (Ar or He) was studied experimentally. It is shown that the energy efficiency of O₂(a¹Δ_g production can be optimized with respect to the reduced electric field E/N. It is shown that the optimal E/N values correspond to electron temperatures of 1.2–1.4 eV. At these E/N values, a decrease in the oxygen percentage in the mixture leads to an increase in the excitation rate of singlet oxygen because of the increase in the specific energy deposition per O₂ molecule. The onset of discharge instabilities not only greatly reduces the energy efficiency of singlet oxygen production but also makes it impossible to achieve high energy deposition in a non-self-sustained discharge. A model of a non-self-sustained discharge in pure oxygen is developed. It is shown that good agreement between the experimental and computed results for a discharge in oxygen over a wide range of reduced electric fields can be achieved only by taking into account the ion component of the discharge current. The cross section for the electron-impact excitation of O₂(a¹Δ_g and the kinetic scheme of the discharge processes with the participation of singlet oxygen are verified by comparing the experimental and computed data on the energy efficiency of the production of O₂(a¹Δ_g and the dynamics of its concentration. It is shown that, in the dynamics of O₂(a¹Δ_g molecules in the discharge afterglow, an important role is played by their deexcitation in a three-body reaction with the participation of O(³P) atoms. At high energy depositions in a non-self-sustained discharge, this reaction can reduce the maximal attainable concentration of singlet oxygen. The effect of a hydrogen additive to an Ar: O₂ mixture is analyzed based on the results obtained using the model developed. It is shown that, for actual electron beam current densities, a significant energy deposition in a non-self-sustained discharge in the mixtures under study can be achieved due to the high rate of electron detachment from negative ions. In this case, however, significant heating of the mixture can lead to a rapid quenching of O₂(a¹Δ_g molecules by atomic hydrogen.     

Glow discharge in singlet oxygen

Currently, there is no experimental data on the plasma balance in gas mixtures with a high content of singlet delta oxygen O₂(¹Δ_g). These data can be obtained by studying the parameters of an electric discharge in singlet oxygen produced by a chemical generator. The O₂(¹Δ_g) molecules significantly change the kinetics of electrons and negative ions in plasma. Hence, the discharge conditions at low and high. O₂(¹Δ_g) concentrations are very different. Here, the parameters of the positive column of a glow discharge in a gas flow from a chemical singlet-oxygen generator are studied. It is experimentally shown that, at an O₂(¹Δ_g) concentration of 50% and at pressures of 1.5 and 2 torr, the electric field required to sustain the discharge is considerably lower than in the case when all of the oxygen molecules are in the ground state. A theoretical model of the glow discharge is proposed whose predictions are in good agreement with the experimental data.     

Oxygen uptake,the circulatory system,and haemoglobin function in the intertidal polychaete Terebella haplochaeta (Ehlers)

The intertidal polychaete Terebella haplochaeta (Ehlers) shows a high degree of oxyregulation in declining pO₂ when confined to its burrow at low tide. This response is achieved by a number of adaptations to the respiratory system. The worm ventilates its burrow in a headward direction by rhythmical conractions of the body. The rate of these pulsations increases at low pO₂ and assists the circulation of the coelomic and vascular fluids. Haemoglobin in the vessels has a high affinity for oxygen and a sigmoidal equilibrium curve. Both the shape and position of the oxygen-binding curve are sensitive to changes in pH, pCO₂, and temperature in a way that suggests augmentation of oxygen delivery at low tide. The concentration of haemoglobin in the vessels is high and is further raised following warm acclimation, presumably to meet an increase in oxygen demand. The ultrastructure of the gills and blood vessels indicates a design for function at low oxygen tensions where diffusion distances must be short and surface areas large in order to enhance the rate of diffusion of oxygen from the near environment.     

Kinetics of Energetic O<Superscript>−</Superscript> Ions in the Discharge Plasmas of Water Vapor and H<Subscript>2</Subscript>O-Containing Mixtures

The process of relaxation of energetic O^– ions formed via dissociative attachment of electrons to molecules in the discharge plasmas of water vapor and H₂O: O₂ mixtures in a strong electric field is studied by the Monte Carlo method. The probability of energetic ions being involved in threshold ion–molecular processes is calculated. It is shown that several percent of energetic O^– ions formed via electron attachment to H₂O molecules in the course of plasma thermalization transform into OH^– ions via charge exchange or are destroyed with the formation of free electrons. The probabilities of charge exchange of O^– ions and electron detachment from them increase significantly (up to 90%) when O^– ions are formed via electron attachment to O₂ molecules in water vapor with an oxygen additive. This effect decreases with increasing oxygen fraction in the mixture but remains appreciable even when the fraction of H₂O molecules in the H₂O: O₂ mixture does not exceed several percent.     

Characterization of laser produced plasma using laser induced breakdown spectroscopy

The plasma parameter studies of the Nd:YAG (neodymium-doped yttrium aluminum garnet, Nd:Y₃Al₁₅O₁₂) crystal by using the fundamental (1064 nm) and second (532 nm) harmonics of Nd:YAG laser are reported. The electron temperature (T _e ) and electron number density (N _e) were determined using the Boltzmann plot method and the Stark-broadened line profile, respectively. An increase in the plasma parameters have been observed with an increase in the laser irradiance for both laser modes. The electron temperatures were calculated in the range of 0.53–0.66 eV for 1064 nm and 0.47–0.60 eV for 532 nm, and the electron number densities were determined in the range of 7.43 × 10¹⁵–3.27 × 10¹⁶ cm^?3 for 1064 nm and 1.35 × 10¹⁶–3.97 × 10¹⁶ cm^?3 for 532 nm in the studied irradiance range of 1.19–12.5 GW/cm². However, the spatial evolution of the plasma parameters investigated up to 2.75 mm away from the target surface at a fixed laser irradiance of 6.51 GW/cm2 showed a decreasing trend. In addition, the estimated values of the inverse bremsstrahlung (IB) absorption coefficients at both laser wavelengths showed that the IB process is dominant for the 1064-nm laser.     

Stability trends and fragmentation patterns of gaseous yttrium oxide clusters studied by fast atom bombardment tandem mass spectrometry

The fragmentation patterns of yttrium oxide cluster species YO⁺, Y₂O₂⁺, Y₂O₃⁺, Y₃O₄⁺, Y₄O₆⁺, Y₅O₇⁺, Y₆O₈⁺ and Y₇O₁₀⁺ were investigated at collision energies 30–110 and 170 eV by fast atom bombardment tandem mass spectrometry. The collision activated dissociation (CAD) spectra obtained revealed higher thermodynamic stability for the clusters of general formula Y_αO_(3α−1)/2⁺, where a is an odd number (e.g. YO⁺, Y₃O₄⁺, Y₅O₇⁺, Y₇O₁₀⁺) which are also the preferred CAD products for all oxide clusters studied. These most stable oxides are constituted by trivalent yttrium only whereas those containing formally tetravalent yttrium Y_aO_3a/2⁺, (where a is even) e.g. Y₂O₃⁺ and Y₄O₆⁺, are extremely unstable. The clusters Y_aO_(3a−2)/2⁺, (where a is even) containing divalent yttrium, e.g. Y₂O₂⁺ and Y₆O₈⁺, have considerable stability but their CAD products are again the thermodynamic products Y_aO_(3a−1)/2⁺. Electronic structures appear to have overriding significance in determining the thermo- dynamic stabilities of the oxide cluster species.     

Laserspray Ionization, a New Atmospheric Pressure MALDI Method for Producing Highly Charged Gas-phase Ions of Peptides and Proteins Directly from Solid Solutions

The first example of a matrix-assisted laser desorption/ionization (MALDI) process producing multiply charged mass spectra nearly identical to those observed with electrospray ionization (ESI) is presented. MALDI is noted for its ability to produce singly charged ions, but in the experiments described here multiply charged ions are produced by laser ablation of analyte incorporated into a common MALDI matrix, 2,5-dihydroxybenzoic acid, using standard solvent-based sample preparation protocols. Laser ablation is known to produce matrix clusters in MALDI provided a threshold energy is achieved. We propose that these clusters (liquid droplets) are highly charged, and under conditions that produce sufficient matrix evaporation, ions are field-evaporated from the droplets similarly to ESI. Because of the multiple charging, advanced mass spectrometers with limited mass-to-charge range can be used for protein characterization. Thus, using an Orbitrap mass spectrometer, low femtomole quantities of proteins produce full-range mass spectra at 100,000 mass resolution with <5-ppm mass accuracy and with 1-s acquisition. Furthermore, the first example of protein fragmentation using electron transfer dissociation with MALDI is presented.Two primary differences between ESI and MALDI methods are the sample environment (solution versus solid) and the observable charge state(s) (multiply versus singly charged). The multiply charged ions observed in ESI mass spectrometry (MS) enhance the yields of fragment ions, a key benefit in structure characterization, and allow analysis of high molecular weight compounds on mass spectrometers with a limited mass-to-charge (m/z) range. In contrast, MALDI MS is ideal for the analysis of heterogeneous samples because it often requires less sample, and spectra of singly charged ions are easier to interpret. We report here the astonishing observation of highly charged molecular ions by laser ablation of a solid matrix/analyte mixture typically used in MALDI MS analyses. The distribution and abundances of the observed ions are similar to those obtained by ESI. Importantly, the MALDI mechanism that produces singly charged ions can be “turned on” at the operator''s will by changing only the matrix or matrix preparation conditions; this capability is not available with any other ionization method. These findings show for the first time that singly charged ions as well as multiply charged ions are available in MALDI. Besides having important mechanistic implications relating to MALDI and ESI, our findings have enormous practical analytical utility.ESI and MALDI combined with MS revolutionized the study of biological materials and earned the Nobel Prize in Chemistry for their ability to ionize proteins for analysis using MS. However, after two decades of extensive studies, the mechanism for ion formation in MALDI remains controversial (1–8). At the heart of these debates lies the predominance of singly charged ions in MALDI mass spectra; the exception being very high mass compounds. A mechanism for the formation of multiply charged ions in MALDI has previously been proposed (1) based on molecular modeling studies (9, 10) and glimpses of multiply charged ions have been observed in lower molecular weight compounds (11–14). The formation of these multiply charged ions has been attributed to sample preparation, high laser fluence, a metal-free sample stage, use of an IR laser, and atmospheric pressure (AP)1 conditions. Multiply charged ions were also recently observed by laser ablation of a liquid surface in the presence of a high electric field (15). The inability in that experiment to observe ions from a solid MALDI matrix/analyte sample or in the absence of an electric field suggests an ionization process involving liquid droplets in a high field similar to ESI (16) or other liquid based, field-induced ionization methods (17, 18).Here, we show analytically useful ESI-like MALDI mass spectra obtained using standard MALDI conditions but using a nontraditional source (19) mounted in place of the standard atmospheric pressure ionization source on a mass spectrometer most commonly used with ESI. The utility of this MALDI MS method for extending the mass range of mass spectrometers as well as the capability of peptide/protein sequencing using electron transfer dissociation (ETD) (20) is demonstrated. Because highly charged ions have not previously been observed with any MALDI ion source configuration, we briefly discuss the fundamental concepts that lead to their production. Key aspects of laserspray ionization (LSI) are laser ablation using a UV laser aligned in transmission geometry (TG) (21–23), field-free (FF) at AP (24), using a heated AP to vacuum ion transfer capillary. In order to emphasize the MALDI sample preparation but distinguish laserspray from conventional AP-MALDI, the new ionization method will hereafter be referred to as FF-TG AP-MALDI.     

Inactivation of Escherichia coli by superoxide radicals and their dismutation products.

Escherichia coli cells are inactivated by the products of the reaction between dialuric acid and oxygen, of which the primary product is Superoxide. The rate of inactivation is decreased by Superoxide dismutase, by catalase, and by EDTA, whereas it is increased by addition of cupric ions or hydrogen peroxide. It is concluded that a toxic product is formed in a reaction involving Superoxide, hydrogen peroxide, and metal ions, which might be the Haber-Weiss reaction, O₂^? + H₂O₂ → OH + OH^? + O₂. In radiation chemical experiments it is shown that this reaction does not occur in the absence of metal ions.     

Transport processes in plasma with an admixture of several heavy impurities

A two-temperature magnetohydrodynamic model of an ideal, fully ionized magnetized plasma consisting of electrons and several types of ions is developed for the case in which the mass of ions of the first type is much lower than that of jth-type ions, where j = 2,3,…, m ₁ ? m _j, while the densities of heavy ions are so low that collisions between them can be neglected. The ion component is assigned a common velocity, common temperature, and common density, while its composition can vary in time and space.     

Breathable Carbon‐Free Electrode: Black TiO2 with Hierarchically Ordered Porous Structure for Stable Li–O2 Battery

This paper introduces oxygen‐deficient black TiO₂ with hierarchically ordered porous structure fabricated by a simple hydrogen reduction as a carbon‐ and binder‐free cathode, demonstrating superior energy density and stability. With the high electrical conductivity derived from oxygen vacancies or Ti³⁺ ions, this unique electrode features micrometer‐sized voids with mesoporous walls for the effective accommodation of Li₂O₂ toroid and for the rapid transport of reaction molecules without the electrode being clogged. In the highly ordered architecture, toroidal Li₂O₂ particles are guided to form with a regular size and separation, which induces the most of Li₂O₂ external surface to be directly exposed to the electrolyte. Therefore, large Li₂O₂ toroids (≈300 nm) grown from solution can be effectively charged by incorporating a soluble catalyst, resulting in a very small polarization (≈0.37 V). Furthermore, disordered nanoshell in black TiO₂ is suggested to protect the oxygen‐deficient crystalline core, by which oxidation of Ti³⁺ is kinetically impeded during battery operation, leading to the enhanced electrode stability even in a highly oxidizing environment under high voltage (≈4 V).     

A topical metabolic inhibitor to improve transcutaneous estimation of arterial oxygen tension in adults

The use of Eugenol as a contact medium in the measurement of transcutaneous oxygen (tcPO₂) partially inhibits the metabolism in the underlying tissue, thereby reducing oxygen consumption and increasing tcPO₂. Oxygen consumption in the tissue can be estimated from the rate at which tcPO₂ falls when blood flow is occluded, and blood flow in the tissue can be estimated from the rate at which tcPO₂ increases when the subject changes from breathing air to pure oxygen. Both these measurements have been made with Eugenol and distilled water as contact media. From these measurements it has proved possible to estimate the arterial oxygen tension (aPO₂) of healthy adults at a relatively low sensor temperature (43°C).     

Membrane transport of singlet oxygen monitored by dipole potential measurements

The efficiency of photodynamic reactions depends on 1), the penetration depth of the photosensitizer into the membrane and 2), the sidedness of the target. Molecules which are susceptible to singlet oxygen (¹O₂) experience less damage when separated from the photosensitizer by the membrane. Since ¹O₂ lifetime in the membrane environment is orders of magnitude longer than the time required for nonexcited oxygen (O₂) to cross the membrane, this observation suggests that differences between the permeabilities or membrane partition of ¹O₂ and O₂ exist. We investigated this hypothesis by releasing ¹O₂ at one side of a planar membrane while monitoring the kinetics of target damage at the opposite side of the same membrane. Damage to the target, represented by dipole-modifying molecules (phloretin or phlorizin), was indicated by changes in the interleaflet dipole potential difference Δφ_b. A simple analytical model allowed estimation of the ¹O₂ interleaflet concentration difference from the rate at which Δφ_b changed. It confirmed that the lower limit of ¹O₂ permeability is ∼2 cm/s; i.e., it roughly matches O₂ permeability as predicted by Overton's rule. Consequently, the membrane cannot act as a barrier to ¹O₂ diffusion. Differences in the reaction rates at the cytoplasmic and extracellular membrane leaflets may be attributed only to ¹O₂ quenchers inside the membrane.     

Influence of the plasma density and heating power on the intensity of electron cyclotron emission in the L-2M stellarator

Results are presented from experiments on studying the plasma behavior in the L-2M stellarator in regimes with a high power deposition in electrons during electron cyclotron heating at the second harmonic of the electron gyrofrequency (X mode) at heating powers of P _in=120–400 kW and average plasma densities from n _e≤3×10¹⁹ to 0.3×10¹⁹ m^?3. It is shown that, as the plasma density decreases and the heating power increases, the electron cyclotron emission spectrum is modified; this may be attributed to a deviation of the electron energy distribution from a Maxwellian and the generation of suprathermal electrons. At low plasma densities, the emission intensity at the second harmonic of the electron gyrofrequency increases, whereas the plasma energy measured by diamagnetic diagnostics does not increase. This poses the question of the correctness of determining the plasma electron temperature by electron cyclotron emission diagnostics under these conditions.     

DNA-binding and Oxidative Properties of Cationic Phthalocyanines and Their Dimeric Complexes with Anionic Phthalocyanines Covalently Linked to Oligonucleotides

Abstract

Design of chemically modified oligonucleotides for regulation of gene expression has attracted considerable attention over the past decades. One actively pursued approach involves antisense or antigene oligonucleotide constructs carrying reactive groups, many of these based on transition metal complexes. The complexes of Fe(II) and Co(II) with phthalocyanines are extremely good catalysts of oxidation of organic compounds with molecular oxygen and hydrogen peroxide. The binding of positively charged Fe(II) and Co(II) phthalocyanines with single- and double-stranded DNA was investigated. It was shown that these phthalocyanines interact with nucleic acids through an outside binding mode. The site-directed modification of single-stranded DNA by O₂ and H₂O₂ in the presence of dimeric complexes of negatively and positively charged Fe(II) and Co(II) phthalocyanines was investigated. These complexes were formed directly on single-stranded DNA through interaction between negatively charged phthalocyanine in conjugate and positively charged phthalocyanine in solution. The resulting oppositely charged phthalocyanine complexes showed significant increase of catalytic activity compared with monomeric forms of phthalocyanines Fe(II) and Co(II). These complexes catalyzed the DNA oxidation with high efficacy and led to direct DNA strand cleavage. It was determined that oxidation of DNA by molecular oxygen catalyzed by complex of Fe(II)-phthalocyanines proceeds with higher rate than in the case of Co(II)-phthalocyanines but the latter led to a greater extent of target DNA modification.     

Identification of methylated flavonoid regioisomeric metabolites using enzymatic semisynthesis and liquid chromatography-tandem mass spectrometry

Discrimination of isomeric methylated metabolites is an important step toward identifying genes responsible for methylation, but presents substantial challenges because authentic standards are often unavailable and mass spectra of isomers have been considered indistinguishable. In this report, an approach is described for identifying methyl group positions in multiply methylated flavonoid metabolites using combinations of tandem mass spectrometry, liquid chromatography retention, and site-selective methylation by recombinant O-methyltransferases from Solanum habrochaites LA1777. The basis for observed fragment ions in tandem mass spectra of multiply methylated myricetin was further established using enzymatic incorporation of deuterium-labeled methyl groups using S-adenosylmethionine-d ₃ as precursor.     

Oxygen supply to bacterial suspensions of high cell densities by hydrogen peroxide

The supply of heterotrophically growing suspensions of Alcaligenes eutrophus PHB^?4 with oxygen formed by the continuous addition of H₂O₂ in the presence of bovine liver catalase was found to be restricted to well-defined conditions. The catalase-H₂O₂ system proved to be suitable during the growth at low cell densities equivalent to 2 g dry weight/liter. When under these conditions the oxygen concentration was held constant at 1.8 mg O₂/liter, the cells grew for 6–8 hr at a rate almost identical to that observed with conventional aeration. However, aeration with H₂O₂ for longer durations (10–20 hr) and at higher cell densities (5?20 g dry weight/liter) led invariably to cell damage and retardation of growth. The impairment of growth observed during the oxygen supply by the catalase?H₂O₂ system was traced back to the formation of gradually increasing steady-state concentrations of H₂O₂ in the medium. Possible sites of cell damage by H₂O₂ such as membrane function, excretion and function of siderophores, and synthesis of cell polymers have been studied, and the cytotoxic mechanism of low concentrations of H₂O₂ was discussed.     

High-frequency underwater plasma discharge application in antibacterial activity

Plasma discharge is a novel disinfection and effectual inactivation approach to treat microorganisms in aqueous systems. Inactivation of Gram-negative Escherichia coli (E. coli) by generating high-frequency, high-voltage, oxygen (O₂) injected and hydrogen peroxide (H₂O₂) added discharge in water was achieved. The effect of H₂O₂ dose and oxygen injection rate on electrical characteristics of discharge and E. coli disinfection has been reported. Microbial log reduction dependent on H₂O₂ addition with O₂ injection was observed. The time variation of the inactivation efficiency quantified by the log reduction of the initial E. coli population on the basis of optical density measurement was reported. The analysis of emission spectrum recorded after discharge occurrence illustrated the formation of oxidant species (OH^?, H, and O). Interestingly, the results demonstrated that O₂ injected and H₂O₂ added, underwater plasma discharge had fabulous impact on the E. coli sterilization. The oxygen injection notably reduced the voltage needed for generating breakdown in flowing water and escalated the power of discharge pulses. No impact of hydrogen peroxide addition on breakdown voltage was observed. A significant role of oxidant species in bacterial inactivation also has been identified. Furthermore the E. coli survivability in plasma treated water with oxygen injection and hydrogen peroxide addition drastically reduced to zero. The time course study also showed that the retardant effect on E. coli colony multiplication in plasma treated water was favorable, observed after long time. High-frequency underwater plasma discharge based biological applications is technically relevant and would act as baseline data for the development of novel antibacterial processing strategies.     

Plasma decay in the afterglow of a high-voltage nanosecond discharge in air

The decay of air plasma produced by a high-voltage nanosecond discharge at room temperature and gas pressures in the range of 1–10 Torr was studied experimentally and theoretically. The time dependence of the electron density was measured with a microwave interferometer. The initial electron density was about 10¹² cm⁻³. The discharge homogeneity was monitored using optical methods. The dynamics of the charged particle densities in the discharge afterglow was simulated by numerically solving the balance equations for electron and ions and the equation for the electron temperature. It was shown that, under these experimental conditions, plasma electrons are mainly lost due to dissociative and three-body recombination with ions. Agreement between the measured and calculated electron densities was achieved only when the rate constant of the three-body electron-ion recombination was increased by one order of magnitude and the temperature dependence of this rate constant was modified. This indicates that the mechanism for three-body recombination of molecular ions differs from that of the well-studied mechanism of atomic ion recombination.     

Effects of nonextensivity and nonthermality on dust-acoustic Gardner solitons in dusty plasmas with distinct ion temperatures

The effects of nonextensivity and nonthermality of ions of two distinct temperatures on dustacoustic Gardner solitons (DAGSs) in an unmagnetized dusty plasma system are investigated theoretically. The constituents of the dusty plasma under consideration are negatively charged mobile dust fluid, Boltzmann-distributed electrons, and ions of two distinct temperatures following nonextensive (q) and nonthermal distributions, respectively. The Korteweg-de Vries (KdV), modified KdV, and Gardner equations are derived by using the reductive perturbation technique, and thereby their characteristic features are compared. It is observed that both the nonextensive and nonthermal ions significantly modify the basic properties and polarities of dust-acoustic solitary waves. The present investigation may be of relevance to space and laboratory dusty plasma systems.