Evaluation of Optical Depths and Self-Absorption of Strontium and Aluminum Emission Lines in Laser-Induced Breakdown Spectroscopy (LIBS).

Laser-induced breakdown spectroscopy (LIBS) is a widely used laser spectroscopic technique in various fields, such as material science, forensic science, biological science, and the chemical and pharmaceutical industries. In most LIBS work, the analysis is performed using radiative transitions from atomic emissions. In this study, the plasma temperature and the product [Formula: see text] (the number density N and the absorption path length [Formula: see text]) were determined to evaluate the optical depths and the self-absorption of Sr and Al lines. A binary mixture of strontium nitrate and aluminum oxide was used as a sample, consisting of variety of different concentrations in powder form. Laser-induced breakdown spectroscopy spectra were collected by varying various parameters, such as laser energy, gate delay time, and gate width time to optimize the LIBS signals. Atomic emission from Sr and Al lines, as observed in the LIBS spectra of different sample compositions, was used to characterize the laser induced plasma and evaluate the optical depths and self-absorption of LIBS.

Helium detection in gas mixtures by laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a tool for monitoring trace levels of helium in gas mixtures consisting mostly of hydrogen. Calibration data for helium in hydrogen was investigated at different helium concentration levels. At high concentrations of helium (>7.25%), the LIBS signal is quenched due to Penning ionization. The hydrogen alpha line (656.28 nm) was observed to broaden as the concentration of helium impurities in the hydrogen gas mixture increased. The helium line at 587.56 nm was selected as the analyte line for helium impurity detection. The effects of laser energy, the delay time between the laser pulse and data acquisition, and the gas pressure on the LIBS signal of helium were investigated to determine the optimum conditions for helium detection. The LIBS signal from the helium line at 587.56 nm shows good linear correlation with helium concentration for He concentrations below 1%. Thus, LIBS can be reliably used to detect the low levels of helium. The limit of detection for helium was found to be 78 ppm.

Quantitative analysis of slurry sample by laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) has been employed for the analysis of slurry samples. Quantitative analysis of slurry samples is crucial and challenging. The problems associated with slurry samples include splashing, surface turbulence, and the difficulties of obtaining reproducible samples due to sedimentation. The LIBS analysis has achieved limited success due to inherent disadvantages when applied to slurry samples. In order to achieve improved measurement precision and accuracy, a spin-on-glass sampling method was evaluated. Five elements (Al, Ca, Fe, Ni, and Si) were examined in five slurry simulants containing varying amounts of each ion. Three calibration models were developed by using univariate calibration, multiple linear regression, and partial least square regression. LIBS analysis results obtained from the partial least square regression model were determined to be the best fit to results obtained from inductively coupled plasma optical emission spectroscopy analysis.

Photo-fragmentation cross-section of gaseous 2,4,6-trinitrotoluene at different ultraviolet wavelengths.

The photo-fragmentation cross-section of 2,4,6-trinitrotoluene (TNT) vapor at room temperature was determined at different ultraviolet wavelengths (254, 300, 340, and 400 nm) by measuring the concentration of NO molecule with cavity ring down spectroscopy and correcting for the photo-fragmentation cross-section of NO(2). Nitric oxide (NO) molecules are produced by the TNT photo-fragmentation processes via an intermediate production of NO(2). Our results reveal that the photo-fragmentation cross-section of TNT changes appreciably with change in wavelength with xenon arc lamp illumination, increasing with decreasing excitation wavelength. The maximum value of cross-section was observed at the shortest photo-fragmentation wavelength studied (254 nm), which is closest to the wavelength of an absorption peak of TNT near 220 nm.

Time-dependent single and double pulse laser-induced breakdown spectroscopy of chromium in liquid.

A study of aqueous solutions of chromium using single and double pulse laser-induced breakdown spectroscopy (LIBS) is presented. Three atomic emission lines show enhancement in emission under dual pulse LIBS excitation. The temporal evolution of line emission indicates that a shock wave front produced by the first laser pulse plays an important role in determining the decay rate of intensity by excitation transfer in single pulse LIBS and by plasma confinement in double pulse LIBS. The ratio of emission in dual pulse LIBS to single pulse LIBS with time shows a linear increase followed by the onset of saturation. A theoretical calculation of the enhancement is found to be in qualitative agreement with the experimental results, suggesting that material ablation in dual pulse LIBS should be > or = 3.5 times that of single pulse LIBS. There is indication that the increase in ablation and subsequent enhancement in emission may be due to the rarefied gas density inside the region enclosed by the shock wave produced by the first laser pulse. The limit of detection of Cr in aqueous solution has been improved by an order of magnitude with double pulse LIBS.

Theoretical model for double pulse laser-induced breakdown spectroscopy.

We present a simple theoretical model for the emission from double pulse laser-induced plasmas that was developed to better understand the processes and factors involved in enhancement of plasma emission. In this model, the plasma emission is directly proportional to the square of plasma density, its volume, and the fraction of second laser pulse absorbed through inverse bremsstrahlung absorption by the plasma plume of the first laser pulse. The electron-ion collision frequency determines the profile and location of the peak of emission enhancement with respect to the delay between the two lasers, whereas the amplitude of the enhancement is mainly dependent on the increase in the mass ablation rate after the second laser pulse. The effects of increase in temperature and in plasma volume after the second laser pulse are also discussed in light of this model.

Comparative study of laser-induced breakdown spectroscopy measurement using two slurry circulation systems.

The experimental conditions associated with slurry measurements to achieve good precision by using laser-induced breakdown spectroscopy (LIBS) are examined. LIBS analysis was applied to a special waste slurry sample that contains 85.4% water, 2.5% ferric oxide Fe(2)O(3), 1.7% alumina Al(2)O(3), and small quantities of oxides of boron and chromium. While liquids add challenge to LIBS measurements, the analysis was successfully performed on iron and aluminum. Two slurry circulation systems were devised to overcome the major technical problems associated with LIBS measurements of slurry samples, namely, sedimentation and change in the lens-to-sample distance during measurement. LIBS slurry measurements using both circulation systems are compared. The results show that the experimental configuration plays a crucial role for online slurry analysis.

Glass-batch composition monitoring by laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy is an almost ideal technique for the in situ monitoring of the composition of a glass batch before it enters the glass-melting furnace, saving a significant amount of energy by the optimization of the furnace parameters for a particular composition of the glass batch. We investigate this application of laser-induced breakdown spectroscopy by determining the elemental composition of the glass batch used (i) as a surrogate for radioactive glass waste and (ii) to manufacture the most common type of flat glass. The surrogate glass-batch and flat-glass calibration curves for the major constituents have been prepared using both the line intensity and the line-intensity ratio. The analytical figure of merit of the glass-batch data obtained from the two different detection systems, namely, the Czerny-Turner spectrometer with an intensified diode-array detector and the echelle spectrometer fitted with an intensified CCD camera, are compared.

Characterization of malignant tissue cells by laser-induced breakdown spectroscopy.

Cancer diagnosis and classification is extremely complicated and, for the most part, relies on subjective interpretation of biopsy material. Such methods are laborious and in some cases might result in different results depending on the histopathologist doing the examination. Automated, real-time diagnostic procedures would greatly facilitate cancer diagnosis and classification. Laser-induced breakdown spectroscopy (LIBS) is used for the first time to our knowledge to distinguish normal and malignant tumor cells from histological sections. We found that the concentration of trace elements in normal and tumor cells was significantly different. For comparison, the tissue samples were also analyzed by an inductively coupled plasma emission spectroscopy (ICPES) system. The results from the LIBS measurement and ICPES analysis were in good agreement.

Parametric study of pellets for elemental analysis with laser-induced breakdown spectroscopy.

The effect of various parameters on the accuracy of the laser-induced breakdown spectroscopy (LIBS) data taken from pellet samples has been investigated. The dependence of the standard deviation of the LIBS data on the amount and nature of the binder used, pressure used to press the powder into a pellet, and the position of the focal spot on the pellet has been investigated. Pellets made from industrially important materials such as silica, alumina, and lime with polyvinyl alcohol, sucrose, and starch as binders have been studied. The results thus obtained are tested by preparation of the calibration curves for Si, Fe, and B in the pellets made from the powder glass batch used as a surrogate for the batch employed for the vitrification of radioactive waste.

Laser-induced breakdown spectroscopy of molten aluminum alloy.

We have demonstrated that a fiber-optic laser-induced breakdown spectroscopy (LIBS) probe is suitable for measuring the concentration of minor constituents of a molten Al alloy in a laboratory furnace. For the first time to our knowledge we are able to record the LIBS spectra in several spectral regions of seven different molten Al alloy samples by inserting the LIBS probe inside the molten alloys, allowing us to obtain a ratio calibration curve for minor constituents (Cr, Mg, Zn, Cu, Si, etc.), using Fe as a reference element. A ratio calibration curve for Fe with a major element (Al) can also be obtained with which the concentration of Fe in the alloy can be determined. The effects of the surrounding atmosphere on the LIBS spectra of the molten alloy were investigated. Effects of focal length of the lens on the LIBS signals were also studied. LIBS spectra of a solid Al alloy recorded with the same LIBS probe were compared with the LIBS spectra of the molten alloy. Our results suggest that the LIBS probe is useful for monitoring the elemental composition of an Al melt in an industrial furnace at different depths and different positions inside the melt.

Optical emission from laser-induced breakdown plasma of solid and liquid samples in the presence of a magnetic field.

The optical properties of laser-induced plasma generated firm solid (Al alloy) and liquid (Mn, Cr, Mg, or Ti solutions) samples expanded across an external, steady magnetic field have been studied by atomic-emission spectroscopy. Various line emissions obtained from the constituents of the Al alloy and of the aqueous solution show an enhancement in intensity in the presence of an approximately 5-kG magnetic field. The enhancement of the signal was nearly a factor of 2 for the minor constituents of the solid samples and a factor of 1.5 for the elements in liquid phase. Temporal evolution of the emission from the solid sample showed maximum enhancement in emission intensity at 3-10-micros time delay after plasma formation in the laser energy range 10-50 mJ. However, for the liquid sample the maximum signal was for a gate delay of 3-25 micros the energy range 50-200 mJ. This enhancement in the emission intensity was found to be due to an increase in effective density of the plasma as a result of magnetic confinement when the plasma cooled after expansion. This enhanced emission was due to an increase in the rate of radiative recombination in the plasma.

Study of laser-induced breakdown emission from liquid under double-pulse excitation.

The application of laser-induced breakdown spectroscopy to liquid samples, by use of a Nd:YAG laser in double-pulse excitation mode, is described. It is found that the line emission from a magnesium ion or atom is more than six times greater for double-pulse excitation than for single-pulse excitation. The effect of interpulse separation on the emission intensity of a magnesium ion and a neutral atom showed an optimum enhancement at a delay of 2.5-3 micros. The intensity of neutral atomic line emission dominates the ion emission from the plasma for higher interpulse (>10 micros) separation. A study of the temporal evolution of the line emission from the plasma shows that the background as well as line emission decays faster in double-pulse excitation than in single-pulse excitation. The enhancement in the emission seems to be dominated by an increase in the volume of the emitting gas. The limit of detection for a magnesium solution improved from 230 parts per billion (ppb) in single-pulse mode to 69 ppb in double-pulse mode.

Evaluation of the potential of laser-induced breakdown spectroscopy for detection of trace element in liquid.

The analytical figure of merit of the potential of laser-induced breakdown spectroscopy (LIBS) has been evaluated for detection of trace element in liquid. LIBS data of Mg, Cr, Mn, and Re were studied. Various optical geometries, which produce the laser spark in and at the liquid sample, were tested. The calibration curves for Mg, Cr, Mn, and Re were obtained at the optimized experimental conditions with bulk liquid and in liquid jet. It was found that measurements using a liquid jet provide better detection limits than bulk liquid measurements. The limits of detection (LOD) of Mg, Cr, Mn, and Re in the present liquid jet measurement are found to be 0.1, 0.4, 0.7, and 8 ppm, respectively. The LOD of Mg using Mg 279.55 nm was compared with the values found in other liquid work.