Laser-induced breakdown spectroscopy (LIBS) is an emerging atomic emission spectroscopic technique that offers the prospect highly- selective, sensitive, and of real-time detection and analysis of both natural and man-made materials. Because LIBS is simultaneously sensitive to all chemical elements due to detector response in the 200-980nm range with 0.1 nm spectral resolution, the technique has many attributes that make it an attractive tool for a variety of military, security, and environmental applications.
Laser Induced Breakdown Spectroscopy (LIBS) is an atomic emission spectroscopic technique that utilizes a pulsed laser to create a microplasma on the target together with an array spectrometer to capture the transient light for elemental identification and quantification. LIBS has certain important characteristics that make it a very attractive sensor technology for military uses. Such attributes include that facts that LIBS (1) is relatively simple and straightforward, (2) requires no sample preparation, (3) generates a real-time response, and (4) only engages a very small sample (pg-ng) of matter in each laser shot and microplasma event, (5) has inherent high sensitivity, and (6) responds to all forms of unknowns, and, therefore, is particularly suited for the sensing of dangerous materials. Additionally, a LIBS sensor system can be inexpensive, configured to be man-portable, and designed for both in-situ point sensing and remote stand-off detection with distances of up to 20-25 meters. Broadband LIBS results covering the spectral region from 200-970 nm acquired at the Army Research Laboratory (ARL) under laboratory conditions for a variety of landmine casings and explosive materials. This data will illustrate the potential that LIBS has to be developed into a hand-deployable device that could be utilized as a confirmatory sensor in landmine detection. The concept envisioned is a backpack-size system in which an eyesafe micro-laser is contained in the handle of a deminer's probe and light is delivered and collected through an optical fiber in the tapered tip of the probe. In such a configuration, analyses can be made readily by touching the buried object that one is interested in identifying.
A first generation, microphotonic sensor for rapid (10 ms response time) measurement of vapors from the hydrocarbon-based fuels JP-8, DF-2, and gasoline has been developed at the U.S. Army Research Laboratory. This sensor is based upon a previously reported laser mixing technique that uses two tunable diode lasers emitting in the near-infrared spectral region to measure concentrations of gases having unstructured absorption spectra. The fiber-mixed laser beam consists of two wavelengths, one of which is absorbed by the fuel vapor, and one of which is not absorbed. By sinusoidally modulating the power of the two lasers at the same frequency but 180 degrees out of phase, a sinusoidal signal is generated at the detector (when the target gas is present in the line of sight). The signal amplitude, measured using standard phase sensitive detection techniques, is proportional to fuel vapor concentration. A second generation sensor, designed to measure the full envelope of the first overtone C-H vibrations in middle distillate fuels is currently being developed. Both sensors are described. Limits of detection using the first generation sensor are reported for vapors of the three fuels studied.
KEYWORDS: Signal attenuation, Semiconductor lasers, Hydrogen fluoride lasers, Sensors, Visibility, Signal detection, Systems modeling, Fiber coupled lasers, Time metrology, Signal processing
Tunable diode laser absorption spectroscopy is used to measure the time evolution of hydrogen fluoride concentrations produce from a series of enclosed heptane/air pan fires extinguished by FE-36 or FE-36 plus ammonium polyphosphate (APP). Recent advances in room temperature fiber coupled, near-IR diode lasers provide isolation of the laser and signal processing electronics from the hostile sampling site. For the fires studied, the change in HF gas concentration with time is dependent upon the fire fighting chemical used to extinguish the fire. The presence of APP is observed to accelerate the dissipation of HF from the fire enclosure. Visible attenuation spectroscopy is also used to measure the amount of light attenuation that occurs as a hand held fire extinguisher containing powder fire fighting agents released in the crew space of a M1-Abrams land combat vehicle. Obscuration test demonstrate that release of APP from extinguishers in an occupied space does not present a visibility challenge to the vehicle personnel.
Raman spectroscopy has been shown to be a useful tool for characterizing neat crystalline explosive samples and for identifying principle components in many propellant and explosive formulations. Recently, we have been investigating changes in Raman spectra of explosives and propellant formulations which occur as the temperature approaches the melting point of the sample. We report recent measurements of Raman spectra of explosives and propellant formulations during bulk heating, and recent measurements of laser heating of the samples during measurement of Raman spectra. The results of these measurements are important to investigators using Raman spectroscopy to measure vibrational spectra at the surface of burning propellant samples.
Fourier transform Raman (FTR) spectroscopy employing near-IR laser radiation at 1.06 microns as the scattering source is used to obtain Raman spectra of some pure energetic materials and some propellant formulations containing those energetic materials. It is shown that FTR spectroscopy is a useful tool in determining the principle crystalline ingredient in many non-colored propellant formulations.
Line of sight Fourier transform infrared absorption spectra through low pressure premixed methane/nitrous oxide and methane/oxygen flames have been analyzed using computed tomography. The reconstruction method used is Abel inversion, which takes advantage of the fact that when the object of interest is axisymmetric, only one set of parallel projection data is necessary. Relative absorbances for several species in these flames have been mapped for various `slices' of the flame at different heights in the flame.
The techniques of Fourier transform infrared spectroscopy (FT-IR) and Fourier transform Raman spectroscopy (FT-R) are compared for use in high pressure environments such as those encountered when using supercritical fluid solvents. FT-IR spectra in the 1000 to 3000 cm-1 region show severe pressure broadening of high-pressure molecular solvents, which eliminates much of the useful spectral range when using this technique. These solvent interferences are not a problem with FT-R at pressures as high as 35 MPa. Spectra of naphthalene in liquid carbon dioxide are used to compare these two spectroscopic techniques.
An attenuated total reflectance FTIR (ATR-FTIR) technique has been developed for measuring the diffusion coefficients of liquids in polymer films. Data is being obtained for diffusion of plasticizers in nitrocellulose (NC), cellulose acetate butyrate (CAB), and NC/CAB mixtures.
A low pressure premixed gas burner flame is probed using FTIR spectroscopy. Species profiles obtained from line of sight absorbance spectra are analyzed using tomographic reconstruction. Future plans for applying this technique to the study of flames are discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.