Yixiang Duan

Development and investigation of microwave plasma techniques in analytical atomic spectrometry

Microwave plasma (MWP) sources have found extensive use in spectrochemical analysis during the past decades. As the MWP offers some attractive features, it has been widely used either as an excitation source for atomic emission spectrometry (MWP-AES) or as an ionization source for mass spectrometry (MWP-MS). The use of MWPs as an atomization source for atomic absorption spectrometry (MWP-AAS) and atomic fluorescence spectrometry (MWP-AFS) has also appeared. The historical development and recent improvements in these MWP techniques are presented in this review with emphasis on the analytical performance, characteristics, interferences and applications. Research on the fundamental properties of MWPs is also given. Both the advantages and limitations of MWPs in atomic spectrometry are discussed.

Cold plasma brush generated at atmospheric pressure

A cold plasma brush is generated at atmospheric pressure with low power consumption in the level of several watts (as low as 4 W) up to tens of watts (up to 45 W). The plasma can be ignited and sustained in both continuous and pulsed modes with different plasma gases such as argon or helium, but argon was selected as a primary gas for use in this work. The brush-shaped plasma is formed and extended outside of the discharge chamber with typical dimension of 10-15 mm in width and less than 1.0 mm in thickness, which are adjustable by changing the discharge chamber design and operating conditions. The brush-shaped plasma provides some unique features and distinct nonequilibrium plasma characteristics. Temperature measurements using a thermocouple thermometer showed that the gas phase temperatures of the plasma brush are close to room temperature (as low as 42 degrees C) when running with a relatively high gas flow rate of about 3500 ml/min. For an argon plasma brush, the operating voltage from less than 500 V to about 2500 V was tested, with an argon gas flow rate varied from less than 1000 to 3500 ml/min. The cold plasma brush can most efficiently use the discharge power as well as the plasma gas for material and surface treatment. The very low power consumption of such an atmospheric argon plasma brush provides many unique advantages in practical applications including battery-powered operation and use in large-scale applications. Several polymer film samples were tested for surface treatment with the newly developed device, and successful changes of the wettability property from hydrophobic to hydrophilic were achieved within a few seconds.

Optical diagnostics of a low power?low gas flow rates atmospheric-pressure argon plasma created by a microwave plasma torch

We employ a suite of optical techniques, namely, visual imaging, optical emission spectroscopy and cavity ringdown spectroscopy (CRDS), to characterize a low power, low gas flow rates, atmospheric-pressure argon microwave induced plasma. The plasma is created by a microwave plasma torch, which is excited by a 2.45 GHz microwave with powers ranging from 60 to 120 W. A series of plasma images captured in a time-resolution range of as fine as 10 µs shows that the converging point is actually a time-averaged visual effect and the converging point does not exist when the plasma is visualized under high time resolution, e.g. <2 ms. Simulations of the emission spectra of OH, N2 and in the range 200–450 nm enable the plasma electronic excitation temperature (Texc) to be determined at 8000–9000 K, while the vibrational temperature (Tv), the rotational temperature (Tr) and the gas temperature (Tg) at different locations along the axis of the plasma column are all determined to be in the range 1800–2200 K. Thermal equilibrium properties of the plasma are discussed. OH radical concentrations along the plasma column axis are measured by CRDS and the concentrations are in the range 1.6 × 1013–3.0 × 1014 cm−3 with the highest density at the tail of the plasma column. The upper limit of electron density ne is estimated to be 5.0 × 1014 cm−3 from the Lorentzian component of the broadened lineshape obtained by ringdown spectral scans of the rovibrational line S21 of the OH A–X (0–0) band.

Diode Laser Microwave Induced Plasma Cavity Ringdown Spectrometer: Performance and Perspective

Recent studies combining an atmospheric-pressure plasma source (inductively coupled plasma or microwave induced plasma) with cavity ringdown spectroscopy (plasma-CRDS) have indicated significant promise for ultra-sensitive elemental measurements. Initial plasma-CRDS efforts employed an inductively coupled plasma as the atomization source and a pulsed laser system as the light source. In an effort to improve the portability and reduce the cost of the system for application purposes, we have modified our approach to include a compact microwave induced plasma and a continuous wave diode laser. A technique for controlling the coupling of the continuous wave laser to the ringdown cavity has been implemented using a standard power combiner. No acouto-optic modulator or cavity modulation is required. To test the system performance, diluted standard solutions of strontium (Sr) were introduced into the plasma by an in-house fabricated sampling device combined with an ultrasonic nebulizer. SrOH radicals were genera...

Capillary-discharge-based portable detector for chemical vapor monitoring

Conventional portable instruments for sensing chemical vapors have certain limitations for on-site use. In this article, we develop a genuinely portable detector that is sensitive, powerful, rugged, of simple design, and with very low power needs. Such a detector is based on a dry-cell battery-powered, capillary-discharge-based, microplasma source with optical emission detection. The microscale plasma source has very special features such as low thermal temperature and very low power needs. These features make it possible for the plasma source to be powered with a small dry-cell battery. A specially designed discharge chamber with minielectrodes can be configured to enhance the plasma stability and the system performance. A very small amount of inert gas can be used as sample carrier and plasma supporting gas. Inert gases possess high excitation potentials and produce high-energy metastable particles in the plasma. These particles provide sufficient energy to excite chemical species through Penning ionization and/or energy transfer from metastable species. A molecular emission spectrum can be collected with a palm-sized spectrometer through a collimated optical fiber. The spectrum can be displayed on a notebook computer. With this design and arrangement, the new detector provides high sensitivity for organic chemical species. The advantages and features of the newly developed detector include high sensitivity, simple structure, low cost, universal response, very low power consumption, compact volume with field portable capability, and ease of operation.

Development of a new high-efficiency thermal ionization source for mass spectrometry

Abstract A thermal ionization source for mass spectrometry has been designed and tested. The ion source is based on a tungsten crucible with a deep cavity into which the sample is loaded. The crucible is heated by high energy electron bombardment from a tantalum filament surrounding the crucible. As the sample evaporates inside the crucible, gaseous analyte atoms are produced which interact with the inner surface of the crucible walls to produce positive ions through surface ionization. The ions are extracted from the cavity through a small opening at the end of the crucible. Regulation of the electron emission current makes it possible to control the energy and power applied to the crucible and, therefore, the crucible temperature. A number of elements have been tested in this source. The ionization efficiencies measured using a high transmission isotope separator spectrometer show 10 to 100 times higher efficiency than in conventional surface thermal ionization sources.

Design and development of a highly sensitive, field portable plasma source instrument for on-line liquid stream monitoring and real-time sample analysis

The development of a highly sensitive, field portable, low-powered instrument for on-site, real-time liquid waste stream monitoring is described in this article. A series of factors such as system sensitivity and portability, plasma source, sample introduction, desolvation system, power supply, and the instrument configuration, were carefully considered in the design of the portable instrument. A newly designed, miniature, modified microwave plasma source was selected as the emission source for spectroscopy measurement, and an integrated small spectrometer with a charge-coupled device detector was installed for signal processing and detection. An innovative beam collection system with optical fibers was designed and used for emission signal collection. Microwave plasma can be sustained with various gases at relatively low power, and it possesses high detection capabilities for both metal and nonmetal pollutants, making it desirable to use for on-site, real-time, liquid waste stream monitoring. An effectiv...

Cavity ringdown measurements of mercury and its hyperfine structures at 254 nm in an atmospheric microwave plasma: spectral interference and analytical performance

The plasma-cavity ringdown spectroscopic (Plasma-CRDS) technique has been demonstrated as a powerful tool for elemental and isotopic measurements in recent studies. This work reports the first application of plasma-CRDS to measurements of elemental mercury and its stable isotopes at the 254 nm transition under atmospheric conditions. A microwave-induced plasma (MIP) operating at 80–100 W is used to generate Hg atoms from standard HgCl2 solutions diluted by 2% nitric acid solvent. It is found that a background absorption, attributed to the overlap of two broadened rovibrational transitions R21(21) and P1(15) of the OH A-X (3-0) band located at 253.65 nm, generates significant spectral interference with the absorption peak of Hg at 254 nm. With an optimized operating condition, including plasma powers, gas flow rates, and laser beam positions in the plasma, the detection sensitivity of Hg is determined to be 9.1 ng ml−1 in aqueous solution, equivalently 221 pptv in the gas phase; this detection limit is approximately 2-fold higher than the theoretical detection limit, 126 pptv, which was estimated by using the parameters of the instrument system and the calculated absorption cross-section, 2.64 × 10−14 cm2 atom−1, of the transition under atmospheric plasma conditions. High-resolution spectral scans show a clear contour of the stable isotopes of the 254 nm transition. The technical challenges encountered and the potential for further development of the Hg analyzer using the MIP-CRDS technique are discussed.

Simple, sensitive nitrogen analyzer based on pulsed miniplasma source emission spectrometry

The development of pulsed miniplasma source emission spectrometry for trace nitrogen determination in inert gases is described in this article. The instrument consists of a pulsed miniplasma source generated by an in-house fabricated portable high-voltage supply, an optical beam collection system, an integrated small spectrometer with a charge-coupled-device detector, an interface card, and a notebook computer for controlling spectrometer parameters and signal processing. Trace nitrogen in the inert gases, such as helium and argon, was determined by monitoring the emission intensities from nitrogen molecules at 357 and 337 nm. The analytical performance was examined under various experimental conditions. The system has a detection limit of about 15 ppb (v/v) for nitrogen in helium with a relative standard deviation of 1.5%. The newly developed instrument offers a simple, low-cost, and sensitive method for continuously monitoring trace nitrogen in high-purity inert gases.

Characterization of an improved thermal ionization cavity source for mass spectrometry

A new thermal ionization source for use with a quadrupole mass spectrometer has been designed and characterized. The new source provides significant advantages over the previously reported prototype source and traditional filament-type thermal ionization sources. The operational interface between the source and the quadrupole mass spectrometer has been redesigned. A vacuum interlock, a translational stage, and an adjustable insertion probe are added to improve the source performance. With these modifications, the source is easier to operate while maximizing sample throughput. In this work, the performance of the newly developed source is examined. The ionization efficiencies are measured with a quadrupole mass spectrometer. The efficiency values obtained with this system are comparable to those obtained from a large scale isotope separator. The relationships among the ionization potential, vapor pressure, and measured ionization efficiency results are discussed. The crucible lifetime has been quantitatively estimated by measuring the crucible sputtering rate. Diagnostic studies of the new source show that the crucible position is a crucial parameter for sensitivity and performance. Stability tests demonstrate that the source can be run several weeks at a fixed emission current without significant degradation.