Fang Y. Yueh

2,4,6-Trinitrotoluene detection by laser-photofragmentation-laser-induced fluorescence.

Photofragmentation (PF) and subsequent nitric oxide (NO) laser-induced fluorescence (LIF) is being developed to measure the concentration of energetic materials (EMs) in soil and other media. Laser radiation near 226 nm photodissociates gas-phase EM to NO(2), which predissociates into NO that gives an intense luminescence. The EM concentration is inferred from the intensity of the NO fluorescence. We have studied the factors that affect the PF-LIF signal intensity, including the effect of buffer gas on the LIF spectrum of pure NO, the effect of 2,4,6-trinitrotoluene (TNT) pressure on the PF-LIF spectrum, the effect of buffer-gas pressure on the PF-LIF signal intensity of pure TNT, and the effect of temperature on the PF-LIF spectra of pure TNT and of TNT in simulated soil. Heating of the TNT sample above 343 K was found to increase the magnitude of the PF-LIF signal intensity significantly, but also was found to cause physical and chemical changes in the TNT sample. The effects of heating and evacuating on the TNT sample were investigated. TNT concentration calibration curves were obtained for TNT in simulated soil mixtures. The limit of detection of TNT in soil was estimated to be 40 parts in 10(9).

Double-pulse laser-induced breakdown spectroscopy with liquid jets of different thicknesses

Double-pulse laser-induced breakdown spectroscopy of magnesium in water has been performed with different jet thicknesses. A Meinhard nebulizer has been used to create a jet of 0.3-mm diameter, whereas a homemade liquid jet injector produced a thicker jet of 1.0-mm diameter. The relationship of line intensity to delay time between the two laser pulses for these two jets is compared and discussed. The limits of detection in these two jets are also determined and compared. The line intensity observed from the double-pulse measurement is correlated with the measured electron density calculated with the Halpha line. Also, the behavior of plasma density relative to time delay between the lasers is described.

High temperature fiber optic laser-induced breakdown spectroscopy sensor for analysis of molten alloy constituents

A fiber optic (FO) laser-induced breakdown spectroscopy (LIBS) sensor that measures the on-line, in situ elemental composition of a molten alloy inside the melt in a furnace is described. This sensor has applications as a process monitor and control tool for glass, aluminum, and steel melters. The sensor is based on the transmission of laser energy through a multimode optical fiber. The laser radiation from the fiber is collimated and finally focused inside the aluminum melt in the furnace by a specially designed stainless steel holder that holds the collimating and focusing lens. Atomic emission from sparks from the laser plasma is collected by the same stainless steel lens holder and transmitted back through the optical fiber and finally fed into the entrance slit of the spectrograph. The present design of the stainless steel holder is useful for obtaining a collimated LIBS signal over a long distance (the distance between the focusing and collimated lenses is more than 200 cm). Parametric studies such ...

Detection of trace elements in liquids by laser-induced breakdown spectroscopy with a Meinhard nebulizer

The laser-induced breakdown spectroscopy of magnesium, manganese, and chromium atoms by use of a commercial Meinhard nebulizer originally designed for inductively coupled plasma measurements is described. This is the first time, to our knowledge, that this nebulizer has been used for laser-induced breakdown spectroscopy measurements. The limit of detection is slightly lower when the nebulizer rather than a liquid jet is used in single-pulse laser excitation. In addition we present the response characteristics of the nebulizer, such as effects of variations in purge gas and liquid flow rate, that are different from normal operating specifications. The effects of gate delay, gate width, and laser power variations were also studied. The objective of the present research has been to consider a new operating mode and conditions in which a better limit of detection of trace elements in water can be obtained.

Effect of steady magnetic field on laser-induced breakdown spectroscopy

Effects of a steady magnetic field on the laser-induced breakdown spectroscopy of certain elements (Mn, Mg, Cr, and Ti) in aqueous solution were studied, in which the plasma plume expanded across an external steady magnetic field (approximately 6 kilogauss). Nearly 1.6 times enhancement in the line emission intensity was observed in the presence of the magnetic field. The temporal evolution of the line emission showed a significant enhancement in plasma emission between 2- and 7- micro(s) gate delays for Mg in the presence of the magnetic field (5-30 micro(s) for Mn). This enhancement in the emission is attributed to an increase in the rate of recombination because of an increase in plasma density due to a magnetic confinement after cooling the plasma. The increase in the optical line emission due to magnetic confinement was absent when the plasma was hot with a dominant background (continuum) emission. The limits of detection of Mg and Mn were reduced by a factor of two in the presence of a steady magnetic field of 5 kilogauss.

Laser-induced breakdown spectroscopy measurement in methane and biodiesel flames using an ungated detector

Laser-induced breakdown spectroscopy (LIBS) has been applied to measure the equivalence ratio of CH(4)/air flames using gated detection. In this work, we have developed an ungated, miniature LIBS-based sensor for studying CH(4)/air and biodiesel flames. We have used this sensor to characterize the biodiesel flame. LIBS spectra of biodiesel flames were recorded with different ethanol concentrations in the biodiesel and also at different axial locations within the flame. The sensor performance was evaluated with a CH(4)/air flame. LIBS signals of N, O, and H from a CH(4)/air flame were used to determine the equivalence ratio. A linear relationship between the intensity ratio of H and O lines and the calculated equivalence ratio were obtained with this sensor.

Study of optical emission from laser-produced plasma expanding across an external magnetic field

The laser-induced plasma obtained from the liquid target and expanding across a steady magnetic field has been studied using atomic emission spectroscopy. The line emission from the plasma was enhanced (> 1.5 times) in the prese nce of a magnetic field, whereas background emission decreases. Enhancement in line intensity was found to be mi.inly a function of plasma beta (F). An increased rate of three-body recombination in plasma particles due to the cooling of the plasma during its expansion and an increase in effective plasma density as a result of its confinement seems to be the reason behind this enhancement.

Fiber optic Raman sensor to monitor the concentration ratio of nitrogen and oxygen in a cryogenic mixture

A spontaneous Raman scattering optical fiber sensor was developed for a specific need of the National Aeronautics and Space Administration (NASA) for long-term detection and monitoring of the purity of liquid oxygen (LO(2)) in the oxidizer feed line during ground testing of rocket engines. The Raman peak intensity ratios for liquid nitrogen (LN(2)) and LO(2) with varied weight ratios (LN(2)/LO(2)) were analyzed for their applicability to impurity sensing. The study of the sensor performance with different excitation light sources has helped to design a miniaturized, cost-effective system for this application. The optimal system response time of this miniaturized sensor for LN(2)/LO(2) measurement was found to be in the range of a few seconds. It will need to be further reduced to the millisecond range for real-time, quantitative monitoring of the quality of cryogenic fluids in a harsh environment.

Study of atomic and molecular emission spectra of Sr by laser induced breakdown spectroscopy (LIBS)

Laser Induced Breakdown Spectroscopy (LIBS) is an ideal analytical technique for in situ analysis of elemental composition. We have performed a comparative study of the quantitative and qualitative analysis of atomic and molecular emission from LIBS spectra. In our experiments, a mixture of SrCl2 and Al2O3 in powder form was used as a sample. The atomic emission from Sr and molecular emission from SrCl and SrO observed in LIBS spectra were analyzed. The optimum laser energies, gate delays, and gate widths for selected atomic lines and molecular bands were determined from spectra recorded at various experimental parameters. These optimum experimental conditions were used to collect calibration data, and the calibration curves were used to predict the Sr concentration. Limits of detection (LODs) for selected atomic and molecular emission spectra were determined.

Comparative Study of Elemental Nutrients in Organic and Conventional Vegetables Using Laser-Induced Breakdown Spectroscopy (LIBS):

In this study, the laser-induced breakdown spectroscopy (LIBS) technique was used to identify and compare the presence of major nutrient elements in organic and conventional vegetables. Different parts of cauliflowers and broccolis were used as working samples. Laser-induced breakdown spectra from these samples were acquired at optimum values of laser energy, gate delay, and gate width. Both univariate and multivariate analyses were performed for the comparison of these organic and conventional vegetable flowers. Principal component analysis (PCA) was taken into account for multivariate analysis while for univariate analysis, the intensity of selected atomic lines of different elements and their intensity ratio with some reference lines of organic cauliflower and broccoli samples were compared with those of conventional ones. In addition, different parts of the cauliflower and broccoli were compared in terms of intensity and intensity ratio of elemental lines.