Takahiro Iwai

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.

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.

Development and fundamental investigation of He plasma ionization detector (HPID) for gas chromatography using DC glow discharge

In analytical chemistry, gas chromatography (GC) has been widely used because of the short measurement time and the low running costs. High-sensitivity detection is required for further improvement of the analytical performance. To date, several types of detectors have been developed. Thermal conductivity detectors (TCDs) are the most common GC detectors, and they are applicable for almost all samples. However, element-specific detection is impossible, and the limit of detection (LOD), which is on the order of 100 ppm, is not good. In this study, a new type of GC detector using atmospheric-pressure He plasma was developed. He has the highest ionization (24.58 eV) and metastable (19.82, 20.62 eV) energies among the elements. This means that He plasma can effectively ionize and excite all elements. In the helium plasma ionization detector (HPID), DC-powered He plasma and rod-like electrodes were utilized for ionization of the samples. For an ionization detector, the generation of very stable plasma is important. Therefore, we used DC He plasma. The sample was ionized when it was mixed into the He afterglow plasma. The concentration of each sample was then measured by the detection of the ion or electron current. When a 2 mL mixture of 20 ppm H2, N2, O2, CH4 and CO was introduced for GC, a LOD of 21–67 ppb was achieved. Thus, the detection ability of HPID was more than 1000 times better than that of TCD.

Evaluation of the analytical performances of a valve-based droplet direct injection system by inductively coupled plasma-atomic emission spectrometry

We have developed a sample introduction system using a magnetic valve type dispenser, named “droplet direct injection nebulizer (D-DIN)”. In the case of the D-DIN, sample solution is directly injected as a single droplet or a series of droplets into the plasma. The droplet volume can be controlled across a wide range of droplet size from 700 pL (110 μm) to 100 nL (580 μm) by changing the inner diameter of the nozzle tip, back pressure and valve open time. The D-DIN system additionally enables direct injection of cells contained in a droplet into the plasma. In this study, the droplet system was optimized, and droplet characterization and analytical performances by emission profiles were investigated. When the emission intensities of 15 nL-volume droplets were measured by side-on observation, the detection limits of Na, Mg and Sr were 20.3 pg (1.35 μg mL−1), 56.5 pg (3.77 μg mL−1) and 20.6 pg (1.37 μg mL−1), respectively. Finally, a single droplet containing yeast cells was directly introduced into ICP and the emission profile of Na was measured with a satisfactory signal to noise ratio.

Development of a dual plasma desorption/ionization system for the noncontact and highly sensitive analysis of surface adhesive compounds

We developed a dual plasma desorption/ionization system using two plasmas for the semi-invasive analysis of compounds on heat-sensitive substrates such as skin. The first plasma was used for the desorption of the surface compounds, whereas the second was used for the ionization of the desorbed compounds. Using the two plasmas, each process can be optimized individually. A successful analysis of phenyl salicylate and 2-isopropylpyridine was achieved using the developed system. Furthermore, we showed that it was possible to detect the mass signals derived from a sample even at a distance 50 times greater than the distance from the position at which the samples were detached. In addition, to increase the intensity of the mass signal, 0%–0.02% (v/v) of hydrogen gas was added to the base gas generated in the ionizing plasma. We found that by optimizing the gas flow rate through the addition of a small amount of hydrogen gas, it was possible to obtain the intensity of the mass signal that was 45–824 times greater than that obtained without the addition of hydrogen gas.