Hansheng Zhang

Laser-induced breakdown spectrometry as a multimetal continuous-emission monitor

Laser-induced breakdown spectrometry (LIBS) has been used to detect atomic and molecular species in various environments. LIBS has the capability to be used as a continuous-emission monitor to monitor toxic-metal concentrations in stack emissions. Recently a mobile LIBS system was calibrated in our laboratory and tested as a multimetal continuous-emission monitor during a joint U.S. Department of Energy-Environmental Protection Agency (EPA) test. LIBS measurements were performed with three sets of metal concentrations at the EPA Rotary Kiln Incinerator Simulator. The LIBS system successfully measured concentrations of Cr, Pb, Cd, and Be in near real time in this test. Real-time LIBS data were averaged and compared with data obtained from an EPA reference method that was conducted concurrently with LIBS. The details of the LIBS calibration and results of these LIBS measurements are described.

Parametric study of a fiber-optic laser-induced breakdown spectroscopy probe for analysis of aluminum alloys

Abstract In the present work we demonstrate a fiber-optic laser-induced breakdown spectroscopy (FO LIBS) system for delivering laser energy to a sample surface to produce a spark as well as to collect the resulting radiation from the laser-induced spark. In order to improve the signal/background (S/B) ratio, various experimental parameters, such as laser energy, gate delay and width, detector gain, lenses of different focal lengths and sample surface, were tested. In order to provide high reliability and repeatability in the analysis, we also measured plasma parameters, such as electron density and plasma temperature, and determined their influence on the measurement results. The performance of FO LIBS was also compared with that of a LIBS system that does not use a fiber to transmit the laser beam. LIBS spectra with a good S/B were recorded at 2-μs gate delay and width. LIBS spectra of six different Al alloy samples were recorded to obtain calibration data. We were able to obtain linear calibration data for numerous elements (Cr, Zn, Fe, Ni, Mn, Mg and Cu). A linear calibration curve for LIBS intensity ratio vs. concentration ratio reduces the effect of physical variables (i.e. shot-to-shot power fluctuation, sample-to-surface distance, and physical properties of the samples). Our results reveal that this system may be useful in designing a high-temperature LIBS probe for measuring the elemental composition of Al melt.

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

Abstract 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.

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 ...

Laser-Induced Breakdown Spectra in a Coal-Fired MHD Facility

A study of laser-induced breakdown spectroscopy (LIBS) has been performed in a particle-loaded methane/air flame and in the Diagnostic Instrumentation and Analysis Laboratory/Mississippi State University (DIAL/MSU) test stand to evaluate its application for practical environments. The LIBS spectra collected from different observational directions and spectral regions are compared. The forward LIBS technique has been chosen to characterize the upstream region of a large magnetohydrodynamics (MHD) coal-fired flow facility (CFFF). The relative concentrations of several species are inferred by fitting the observed CFFF LIBS spectra with computer-simulated spectra. This paper reports the first LIBS experiments in a harsh, turbulent, and highly luminous coal-fired MHD combustion environment.

Investigation of the Effects of Atmospheric Conditions on the Quantification of Metal Hydrides Using Laser-Induced Breakdown Spectroscopy

A study was performed to evaluate the performance characteristics of a laser-induced plasma for real-time determination of various gas-phase metal hydrides, specifically Sn and As. The choice of carrier gas composition and the effect of the pressure on the temporal emission behavior of neutral atoms excited by the laser-induced plasma were investigated. Metal hydrides were generated by using a NaBH4-based hydride generation system. The hydrides were equilibrated into an evacuated cell and isolated from the generator prior to measurement. Laser-induced breakdown spectroscopy (LIBS) spectra of Sn and As were recorded in He and N2 atmospheres at 300 and 760 Torr. The temporal behavior of the LIBS signal was most affected by gas composition, gas pressure, and intensity of the laser beam. The Sn neutral atom emission (284.0 nm) in a N2 atmosphere decreased exponentially with time. In contrast, with a He atmosphere and identical experimental conditions, the Sn signal increased logarithmically with time over the first 100 s. Then the signal maintained a steady-state value until approximately 400 s, after which it decreased exponentially. The steady-state time depends on the concentration of metal hydride. The variation of the LIBS signal with time was mirrored for the As neutral atom emission in He and N2 atmospheres. Various experiments have been performed to find the possible reason for the signal variation with time. It was found that chemical reactions in the laser plasma that might deplete the metal from the gas volume were responsible for the decrease in the signal with time.

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.

Performance evaluation of laser-induced breakdown spectrometry as a multimetal continuous emission monitor.

ABSTRACT Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a multimetal continuous emissions monitor (CEM) at the U.S. Environmental Protection Agency (EPA) rotary kiln incinerator simulator (RKIS) facility in Raleigh, NC. Two detection systems with a bifurcated optical fiber bundle were used for simultaneously monitoring the concentrations of Be, Cd, Cr, and Hg in the test. Two calibration techniques were evaluated in the laboratory for the field measurements. On-line calibration of relative metal concentration was also performed in the simulated incinerator gas stream. Toxic metal concentrations measured with LIBS have been compared with the EPA reference method (RM) results.

Adaptive Signal Enhancement of Laser-Induced Breakdown Spectroscopy

A noise reduction technique to analyze laser-induced breakdown spectroscopy spectra is presented in this paper. This technique is an adaptive-type technique for noise minimization. It is based on an improved adaptive line enhancement to effectively increase the signal-to-noise ratio level in spectral data. It requires no a priori knowledge of the signal or of the noise, and it has the capability of on-line implementation. Medium and weak spectra are considered, and a comparison with traditional techniques is made to illustrate the applicability of the foregoing technique.

Laser-induced breakdown spectroscopy in a metal-seeded flame

Laser-induced breakdown spectroscopy (LIBS) has been used to detect atomic and molecular species in various environments. It has the capability to be used as a real time monitor in a harsh and turbulent environment. LIBS has been tested at the Diagnostic Instrumentation and Analysis Laboratorys (DIAL) combustion facility to optimize and characterize the LIBS operation in a flame. Three metals, Cu, Fe, and Ca, were injected into the combustor with a peristaltic pump. LIBS spectra were recorded at different delay times with a fixed gate width to determine the best detection gate. LIBS signal in the flame were also compared with the direct emission from the flame. It is found that the LIBS signal is stronger than the signal from the direct emission of the metal in the same flame and the LIBS method has a much better signal over background ratio. To test the calibration scheme, metal solutions were injected into the combustor with different rates. Linear calibration curves were obtained.