George P. Miller

Cavity Ringdown Spectroscopy for Diagnostic and Analytical Measurements in an Inductively Coupled Plasma

The use of cavity ringdown spectroscopy (CRDS) for atomic absorption measurements in a 27-MHz low-power argon inductively coupled plasma (ICP) is described. These results are used to demonstrate the utility of CRDS for both plasma diagnostic and analytical measurements. In these experiments, an aqueous solution of lead was introduced into a modified torch designed to enhance the ICP conditions for atomic absorption measurements. Absorption intensity characteristics of the lead 283.3-nm absorption line as a function of observation height and lateral position in the plasma were recorded for three different ICP powers (700, 500, and 200 W). The radial distribution of the ground-state lead atom density was derived from Abel inversion of the lateral measurements. At the novel 200 W operating condition, spectral line shapes vs. height and lateral position were fitted to Voigt profiles. Line-of-sight values of the gas kinetic temperature and electron density at different plasma locations were estimated from Gaussian and Lorentzian broadening components, respectively. The results are discussed and compared with those from other methods. The unique flexibility of CRDS for atomic and ionic absorption measurements in an ICP and the potential application of the ICP-CRDS technique for analytical measurements are demonstrated. Analytical results are compared with theoretical estimates of the lead detection limit.

Effect of thermal treatment of wood lumbers on their adhesive bond strength and durability

Wood used in outdoor applications needs to undergo either chemical or thermal treatment to improve its decay resistance. Thermal treatment permits to avoid the use of toxic chemicals, increases the dimensional stability and gives a dark color to the wood. However, this process deteriorates the mechanical properties of wood, i.e., the wood becomes more fragile and rigid. The chemical transformation of wood that takes place during the heat treatment changes the interaction between the wood surface and the adhesive. In this work, the interfacial bonding strength (the resistance to the shear stress by compression in parallel direction to the glued interface) and cyclic delamination (resistance to delamination during accelerated exposure) for different wood species and adhesives were tested in accordance with the ASTM D2559 standard. Four wood species: scott pine (Pinus sylvestris), aspen (Populus tremuloides), yellow poplar (Liriodendron tulipifera) and jack pine (Pinus banksiana) both treated and non-treated, and two structural adhesives, phenol resorcinol formaldehyde (PRF) and polyurethane (PUR), were used in the testing. Among the studied species, jack pine is found to be the easiest to bond, while aspen is found to be the most difficult. With the wood species and adhesives evaluated in this study, non-treated wood is found to provide a better bonding strength than the treated wood.

Measurements of OH Radicals in a Low-Power Atmospheric Inductively Coupled Plasma by Cavity Ringdown Spectroscopy

Cavity ringdown spectroscopy is applied to line-of-sight measurements of OH radicals in an atmospheric-pressure argon inductively coupled plasma, operating at low power (200 W) and low gas flows (∼18 liters/min). Density populations of the single S21(1) rotational line in the OH A2Σ+–X2Π (0–0) band are extracted from the measured line-of-sight absorbance. Plasma gas kinetic temperatures, derived from the recorded line shapes of the S21(1) line, ranged from 1858 to 2000 K with an average uncertainty of 10%. Assuming local thermodynamic equilibrium, an assumption supported by the comparison of the experimental and simulated spectra, the spatially averaged total OH number density at different observation heights was determined to be in the range of 1.7 × 1020–8.5 × 1020 (m−3) with the highest OH density in the plasma tail. This work demonstrates that ringdown spectra of the OH radical may be used both as a thermometer for high-temperature environments and as a diagnostic tool to probe the thermodynamic properties of plasmas.

Isotopic Measurements of Uranium Using Inductively Coupled Plasma Cavity Ringdown Spectroscopy

Inductively coupled plasma cavity ringdown spectroscopy (ICP-CRDS) is applied to isotopic measurements of uranium. We have successfully obtained the isotopic-resolved spectra of uranium at three different atomic/ionic transition lines, 286.57, 358.49, and 409.01 nm. Of the three lines, the largest isotope shift of approximately 9 pm was measured at the 286.57 ionic line. Isotopic-resolved spectra were recorded in ratio of 1:1 (235U/238U, 2.5 μg/mL) and at the natural abundance ratio of 0.714% (235U/238U, 1.25 μg/mL 235U). The smallest measurable isotope shift of approximately 3 pm was determined for the 409.01 nm ion spectral line. Detection limits (DL) were obtained under optimized ICP operating conditions to be in the range of 70∼150 ng/mL, except for the 238U component of the 286.57 nm line (300 ng/mL). This latter result was determined to be due to a strong, previously unreported, absorption interference from the argon plasma. The 235U isotope component (DL 70 ng/mL) was found to be unaffected. This work demonstrates the applicability of ICP-CRDS for uranium isotopic measurements. The potential of development of a field-deployable, on-line uranium isotope monitor using plasma-CRDS is discussed.

Determination of elemental mercury by cavity ringdown spectrometry

Cold vapor cavity ringdown spectroscopy has been successfully applied to the detection of elemental mercury. Using an absorption cell 0.18 m in length, detection limits of 0.027 and 0.12 ng were obtained using peak area and peak height measurements, respectively. For the peak area measurement, this corresponds to a gas phase concentration of less than 25 ng m−3. For comparison, using a similar absorption cell, standard AAS yielded a Hg detection limit (peak height) of 9 ng, (gas phase concentration of ≡ 8.3 μg m−3).

On-line mercury speciation in exhaust gas by using solid-phase chemical reduction

Speciation of mercury species in exhaust gas from combustion sources is important for both the design of equipment for mercury pollution control and incinerator operation control. A simple, portable atomic absorption spectrometer is described that can monitor, in real time, the mercury species present in stack gas. A SnCl2-loaded reduction column was used to convert molecular mercury to mercury atoms, which were then detected by atomic absorption spectrometry. The adoption of solid-phase reduction of the molecular species simplified the instrument design and made analysis easier. Results presented in this paper demonstrate the potential of the proposed method for field application.

AOTF-echelle spectrometer for air-ICP-AES continuous emission monitoring of heavy metals and actinides

A spectrometer system consisting of a quartz acousto-optic tunable filter (AOTF) and an echelle grating has been assembled and tested for ICP-AES continuous emission monitoring of heavy metal and actinide elements in stack exhaust offgases introduced into an air plasma. The AOTF is a rapidly tunable bandpass filter that is used to select a small wavelength range (0.1 to 0.6 nm) of optical emission from the air plasma; the echelle grating provides high dispersion, yielding a spectral resolution of approximately 0.004 to 0.008 nm from 200 to 425 nm. The AOTF-echelle spectrometer, equipped with a photodiode array or CCD, provides rapid sequential multielement analysis capabilities. It is much more compact and portable than commercial ICP-AES echelle spectrometers, allowing use of the system in field and on-line process monitoring applications. Data will be presented that detail the resolution, detection limits, capabilities, and performance of the AOTF-echelle spectrometer for continuous emission monitoring of heavy metals (As, Be, Cd, Cr, Hg, and Pb) and actinides (including U isotopes). The potential use of the AOTF-echelle spectrometer with other emission sources and for other monitoring applications will be discussed.

A continuous sampling air-ICP for metals emission monitoring

An air-inductively coupled plasma (air-ICP) system has been developed for continuous sampling and monitoring of metals as a continuous emission monitor (CEM). The plasma is contained in a metal enclosure to allow reduced-pressure operation. The enclosure and plasma are operated at a pressure slightly less than atmospheric using a Roots blower, so that sample gas is continuously drawn into the plasma. A Teflon sampling chamber, equipped with a sampling pump, is connected to the stack that is to be monitored to isokinetically sample gas from the exhaust line and introduce the sample into the air-ICP. Optical emission from metals in the sampled gas stream is detected and monitored using an acousto-optic tunable filter (AOTF)-echelle spectrometer system. A description of the continuous sampling air-ICP system is given, along with some preliminary laboratory data for continuous monitoring of metals.

Emission profiles of torch generated plasmas

Emission spectroscopic techniques are employed to investigate and characterize the spectral output from a 75-kW dc arc. Torch parameters such as stability, temperature, and electron density were determined. After initially characterizing and optimizing the system, we investigated the composition of plasma spectral emissions with the intent of designing and testing a novel electrode health monitor. A prototype device was built and tested. The results presented clearly demonstrate that such a device is capable of providing early warning of impending electrode failure and has the potential of playing a significant role in system control and maintenance. (Author)

Plasma Cavity Ringdown Spectrometer for Elemental and Isotopic Measurements: Past, Present, and Future

Recent studies using Plasma Cavity Ringdown Spectroscopy (plasma-CRDS) show much promise of this newly developed technique for ultra-sensitive elemental/isotopic measurements. Plasma-CRDS, since its introduction in 1997, has experienced three major stages: (i) the early stage demonstration of the technical feasibility, (ii) the recent advancement on its technical improvements and extensive applications for elemental/isotopic measurements as well as plasma diagnostics and (iii) the most recent progress on the improvement of the instrument configurations based on a diode laser-compact microwave plasma-CRDS. Research and development in many aspects of this technique is vigorously under processing in our laboratories. This paper reports a brief review on the plasma-CRDS technique, its applications and the most recent advancement. Discussions on future developments toward a new generation of plasma- CRDS-based spectrometers for ultra-sensitive elemental/isotopic measurements are also presented.