Hidekazu Miyahara

Investigation of reactive species using various gas plasmas

In this study, atmospheric nonequilibrium plasmas were generated with six gas species using a multi-gas plasma jet. Singlet oxygen, OH radicals, H radicals, and NO radicals, in reaction with a solution interface, were measured using electron spin resonance. Carbon dioxide plasma generated the largest amount (90 μM) of singlet oxygen at 30 s, and argon-containing vapor gas plasma generated the largest amount (210 μM) of OH radicals. Among the homo-atomic gas species, nitrogen plasma generated the largest amount (130 μM) of OH radicals. In addition, H radicals were generated with argon, helium, and nitrogen plasmas. NO radicals were generated with nitrogen–oxygen plasma, and the largest amount of NO radicals was generated at a 1 : 1 volume ratio. These measurement results of the reactive species generated by individual gas plasmas permit identification of the production processes of reactive species.

Surface Hydrophilization of Polyimide Films Using Atmospheric Damage-Free Multigas Plasma Jet Source

Atmospheric-pressure plasma jet sources have proven useful for surface treatments; however, conventional plasma and near-plasma sources have limitations in the number of different plasma gas species that they can handle. We previously developed a damage-free multigas plasma jet source that can generate stable atmospheric-pressure plasma using various gas species without thermal/electrical discharge damage. Herein, we investigate the fundamental characteristics of the generated plasma such as gas temperature (<;57°C) and emission properties. In addition, we evaluate the industrial potential of the jet source by using it with various gas species to induce surface hydrophilization in a polyimide film. The jet source proved useful, and carbon dioxide proved the most effective of the studied gas species for the purpose with the hydrophilized surface maintaining a contact angle of about 30° for over 32 days after plasma irradiation.

Fundamental properties of a non-destructive atmospheric-pressure plasma jet in argon or helium and its first application as an ambient desorption/ionization source for high-resolution mass spectrometry

Various plasma sources were used as ionization sources for ambient desorption/ionization mass spectrometry (ADI-MS) in the past. In the present study, a plasma jet, termed an atmospheric-pressure damage-free multi-gas plasma jet, which can generate a stable atmospheric low-temperature plasma with various gas species, was investigated and used as an ionization source for ambient MS. First, the OH rotational temperature and electron number density of the plasma were determined spectroscopically. It was found that a relatively low temperature (<350 K) and high-density (1014 cm−3) plasma can be generated in helium and argon. Second, the amount of reactive species in the glow-like discharge was indirectly compared to those typically found in a dielectric barrier discharge jet by means of differences in hydrophilization efficiencies of polyimide films. It was found that the plasma jet was more reactive when the plasma exit was positioned close to the sample (<3 mm). Third, the plasma source was coupled to a high-resolution molecular mass spectrometer (Exactive with Orbitrap mass analyzer) and used for direct solid sample analyses. Commercially available acetaminophen, loratadine, and aspirin tablets were successfully analyzed without any sample pre-treatment. The plasma source was also used for direct solution analysis of model compounds to demonstrate the analytical capacity. Calibration curves were obtained with correlation coefficients of ≧0.9975, and limits of detection were in the picogram to nanogram range for acetaminophen, loratadine, and aspirin.

Fundamental properties of a touchable high‐power pulsed microplasma jet and its application as a desorption/ionization source for ambient mass spectrometry

Plasma-based ambient desorption/ionization mass spectrometry (ADI-MS) has attracted considerable attention in many fields because of its capacity for direct sample analyses. In this study, a high-power pulsed microplasma jet (HPPMJ) was developed and investigated as a new plasma desorption/ionization source. In an HPPMJ, a microhollow cathode discharge is generated in a small hole (500 µm in diameter) using a pulsed high-power supply. This system can realize a maximum power density of 5 × 10(8) W/cm(3). The measured electron number density, excitation temperature and afterglow gas temperature of the HPPMJ were 3.7 × 10(15) cm(-3), 7000 K at maximum and less than 60 °C, respectively, which demonstrate that the HPPMJ is a high-energy, high-density plasma source that is comparable with an argon inductively coupled plasma while maintaining a low gas temperature. The HPPMJ causes no observable damage to the target because of its low gas temperature and electrode configuration; thus, we can apply it directly to human skin. To demonstrate the analytical capacity of ADI-MS using an HPPMJ, the plasma was applied to direct solid sample analysis of the active ingredients in pharmaceutical tablets. Caffeine, acetaminophen, ethenzamide, isopropylantipyrine and ibuprofen were successfully detected. Application to living tissue was also demonstrated, and isopropylantipyrine on a finger was successfully analyzed without damaging the skin. The limits of detection (LODs) for caffeine, isopropylantipyrine and ethenzamide were calculated, and LODs at the picogram level were achieved. These results indicate the applicability of the HPPMJ for high-sensitivity analysis of materials on a heat-sensitive surface.

Performances of Ne-like Ar Soft X-ray Laser using Capillary Z-Pinch Discharge

We have designed, fabricated and tested a soft X-ray device, which uses a capillary discharge to achieve neon-like argon lasing. The ceramic capillary has an inner diameter of 3 mm and a length of 150 mm. Lasing has been confirmed when operating the device with a current of 9 to 32 kA with a rise time of 55 ns, in an argon gas pressure range from 100 to 800 mTorr. The relation between the observation of the laser spike and occurrence of the moving striation has been studied by a series of side-view observations. The appropriate starting time of the main discharge with respect to that of the predischarge current has been studied. When the predischarge is properly applied, constant laser amplification of the gain-length product of gl=12 (g=0.8 cm-1) was obtained, with a current of 32 kA at a pressure of 400 mTorr.

Direct Decomposition of Anesthetic Gas Exhaust Using Atmospheric Pressure Multigas Inductively Coupled Plasma

In our research group, an atmospheric multigas inductively coupled plasma (ICP) source was developed for industrial plasma processing and elemental analysis. The plasma source can stably generate atmospheric thermal plasma not only by Ar but also by He, O2, N2, CO2, air, and their mixture gas. In this paper, the plasma source was applied for anesthetic gas decomposition including N2O, which is a kind of global warming gases. Atmospheric thermal plasma was generated by mixture gas of N2O, O2, and air. It corresponds to actual gas composition for up to three surgery exhaust. Spectroscopic characteristics of generated plasma and composition of treated gas were investigated. By using optimally shaped ICP torch, 99.9% of decomposition rate and 942 g/kWh of high energy efficiency were achieved. However, around 3% of NOx was generated as by-products due to high gas temperature.

Temperature Controllable Atmospheric Plasma Source

An atmospheric pressure plasma source in which the gas temperature can be accurately controlled from below freezing point up to a high temperature has been developed. In general plasma devices, an electrical discharge is passed through a gas at about room temperature to generate plasma, so the plasma is at a temperature higher than room temperature; moreover, the gas temperature is determined by the discharge condition such as discharge power and plasma gas flow rate, so accurate temperature control is difficult. In the plasma source that has been developed in this research, the gas that is to be supplied to the discharge unit is first cooled using a gas cooler and then heated by a heater. The gas temperature of the produced plasma is measured, and feedback is sent to the heater. Thus, plasma at a desired temperature can be generated. Gas temperature control of the plasma over a range from -54 °C to +160 °C with a standard deviation of 1 °C was realized. Spectroscopic characteristics of generated plasma were investigated. This plasma source/technique will realize that effective plasma is applicable for heat-sensitive materials such as paper, textile, polymer, and especially human tissue. Furthermore, it enables us to generate the plasma at optimal gas temperature for chemical reaction of each plasma treatment.

Development of a Gas-Cylinder-Free Plasma Desorption/Ionization System for On-Site Detection of Chemical Warfare Agents

A gas-cylinder-free plasma desorption/ionization system was developed to realize a mobile on-site analytical device for detection of chemical warfare agents (CWAs). In this system, the plasma source was directly connected to the inlet of a mass spectrometer. The plasma can be generated with ambient air, which is drawn into the discharge region by negative pressure in the mass spectrometer. High-power density pulsed plasma of 100 kW could be generated by using a microhollow cathode and a laboratory-built high-intensity pulsed power supply (pulse width: 10-20 μs; repetition frequency: 50 Hz). CWAs were desorbed and protonated in the enclosed space adjacent to the plasma source. Protonated sample molecules were introduced to the mass spectrometer by airflow through the discharge region. To evaluate the analytical performance of this device, helium and air plasma were directly irradiated to CWAs in the gas-cylinder-free plasma desorption/ionization system and the protonated molecules were analyzed by using an ion-trap mass spectrometer. A blister agent (nitrogen mustard 3) and nerve gases [cyclohexylsarin (GF), tabun (GA), and O-ethyl S-2-N,N-diisopropylaminoethyl methylphosphonothiolate (VX)] in solution in n-hexane were applied to the Teflon rod and used as test samples, after solvent evaporation. As a result, protonated molecules of CWAs were successfully observed as the characteristic ion peaks at m/z 204, 181, 163, and 268, respectively. In air plasma, the limits of detection were estimated to be 22, 20, 4.8, and 1.0 pmol, respectively, which were lower than those obtained with helium plasma. To achieve quantitative analysis, calibration curves were made by using CWA stimulant dipinacolyl methylphosphonate as an internal standard; straight correlation lines (R(2) = 0.9998) of the peak intensity ratios (target per internal standard) were obtained. Remarkably, GA and GF gave protonated dimer ions, and the ratios of the protonated dimer ions to the protonated monomers increased with the amount of GA and GF applied.

A pulse-synchronized microplasma atomic emission spectroscopy system for ultrasmall sample analysis

Regenerative medicine and environmental science require the analysis of ultrasmall samples such as cells and nanoparticles. Therefore, a microplasma atomic emission spectroscopy (AES) system was developed in this study. This system combined microdroplet injection, which can introduce a small droplet of a few tens of picoliters into a plasma excitation/ionization source, with a microhollow cathode plasma source exhibiting a volume of a few microliters. When a 14 pL droplet of 100 mg L−1 sodium solution was introduced into an 8 W DC microplasma, no emission was observed possibly because of an insufficient excitation of the sample at low plasma gas temperature. Therefore, a pulsed discharge, producing high-intensity electric input power, was implemented to give a pulse-synchronized microplasma AES system. A 15 μs pulsed power reaching a maximum value of 100 kW was obtained using a laboratory-built high-intensity pulsed power supply and was applied to generate the plasma. This system achieved an excitation temperature of 7000 K, exceeding that of the common inductively coupled plasma. Pulsed plasma generation and sample droplet introduction into the plasma were synchronized to provide a high-sensitivity AES analysis. To this end, the time interval before the end of the generation and the beginning of the pulsed plasma generation was adjusted using a delay circuit. The optimal delay amounted to 40 ms. A droplet comprising Na, Ca, Mg, and K at 100 mg L−1 was analyzed using this system and the limits of detection equaled 300, 50, 30, and 640 fg for these analytes, respectively.

Design and evaluation of dual inlet ICP torch for low gas consumptionPresented at the 2002 Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, USA, January 6???12, 2002.

In this study, we concentrated on improving the vortex flow uniformity, which was considered as a key factor for our new ICP torch design. To obtain a uniform vortex flow with lower plasma gas consumption, our new ICP torch had a dual gas inlet, and convertibility for helium and argon ICP. With this modification, helium ICP was able to operate with a 10–20% lower plasma gas flow rate than our previous torch. To confirm the effects of the dual inlet, the flows were compared using a high-speed video system, and the noise-power spectra of the emission line spectrum of helium and argon were measured. As the result of these measurements, the ICP with the dual inlet had higher frequency of plasma rotation than that of the single inlet torch. It is shown that the dual inlet gives less damping of the plasma gas flow and that adequate vortex flow can be generated with lower plasma gas consumption.