Kevin L. McNesby

Analysis of environmental lead contamination: comparison of LIBS field and laboratory instruments

Abstract The Army Research Office of the Army Research Laboratory recently sponsored the development of a commercial laser-induced breakdown spectroscopy (LIBS) chemical sensor that is sufficiently compact and robust for use in the field. This portable unit was developed primarily for the rapid, non-destructive detection of lead (Pb) in soils and in paint. In order to better characterize the portable system, a comparative study was undertaken in which the performance of the portable system was compared with a laboratory LIBS system at the Army Research Laboratory that employs a much more sophisticated laser and detector. The particular focus of this study was to determine the effects on the performance of the field sensors lower spectral resolution, lack of detector gating, and the multiple laser pulsing that occurs when using a passively Q-switched laser. Surprisingly, both the laboratory and portable LIBS systems exhibited similar performance with regards to detection of Pb in both soils and in paint over the 0.05–1% concentration levels. This implies that for samples similar to those studied here, high-temporal resolution time gating of the detector is not necessary for quantitative analysis by LIBS. It was also observed that the multiple pulsing of the laser did not have a significant positive or negative effect on the measurement of Pb concentrations. The alternative of using other Pb lines besides the strong 406-nm line was also investigated. No other Pb line was superior in strength to the 406-nm line for the latex paint and the type of soils used in the study, although the emission line at 220 nm in the UV portion of the spectrum holds potential for avoiding elemental interferences. These results are very encouraging for the development of lightweight, portable LIBS sensors that use less expensive and less sophisticated laser and detector components. The portable LIBS system was also field tested successfully at sites of documented Pb contamination on military installations in California and Colorado.

Laser-induced breakdown spectroscopy (LIBS) – an emerging field-portable sensor technology for real-time, in-situ geochemical and environmental analysis

Laser-induced breakdown spectroscopy (LIBS) is a simple spark spectrochemical sensor technology in which a laser beam is directed at a sample to create a high-temperature microplasma. A spectrometer/array detector is used to disperse the light emission and detect its intensity at specific wavelengths. LIBS has many attributes that make it an attractive tool for chemical analysis. A recent breakthrough in component development, the commercial launching of a small, high-resolution spectrometer, has greatly expanded the utility of LIBS and resulted in a new potential for field-portable broadband LIBS because the technique is now sensitive simultaneously to all chemical elements due to detector response in the 200 to 980 nm range with 0.1 nm spectral resolution. Other attributes include: (a) small size and weight; (b) technologically mature, inherently rugged, and affordable components; (c) in-situ analysis with no sample preparation required; (d) inherent high sensitivity; (e) real-time response; and (f) point sensing or standoff detection. LIBS sensor systems can be used to detect and analyse target samples by identifying all constituent elements and by determining either their relative or absolute abundances.

Spectroscopic analysis of fire suppressants and refrigerants by laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy is evaluated as a means of detecting the fire suppressants CF(3)Br, C(3)F(7)H, and CF(4) and the refrigerant C(2)F(4)H(2). The feasibility of employing laser-induced breakdown spectroscopy for time- and space-resolved measurement of these agents during use, storage, and recharge is discussed. Data are presented that demonstrate the conditions necessary for optimal detection of these chemicals.

Temporal evolution of the laser-induced breakdown spectroscopy spectrum of aluminum metal in different bath gases

The spectral emission of gas-phase aluminum and aluminum oxide was measured during and immediately after exposure of a bulk-aluminum sample to a laser-induced spark produced by a focused, pulsed laser beam (Nd:YAG, 10-ns pulse duration, 35 mJ/pulse, lambda = 1064 nm). The spectral emission was measured as a function of time after the onset of the laser pulse, and it was also measured in different bath gases (air, nitrogen, oxygen, and helium).

High-speed digital color imaging pyrometry

Temperature measurements of high-explosive and combustion processes are difficult to obtain due to the speed and environment of the events. To overcome these challenges, we have characterized and calibrated a digital high-speed color camera that may be used to measure the temperature of such events. A two-color ratio method is used to calculate a temperature using the color filter array raw image data and a graybody assumption. If the raw image data are not available, temperatures may be calculated from the processed images or movies, depending on proper analysis of the digital color imaging pipeline. We analyze three transformations within the pipeline (demosaicing, white balance, and gamma correction) to determine their effect on the calculated temperature. Using this technique with a Phantom color camera, we have measured the temperature of exploded C-4 charges. The surface temperature of the resulting fireball was found to rapidly increase after detonation, and subsequently decayed to a constant value of approximately 1980  K.

Density functional theory characterization of the structure and gas-phase, mid-infrared absorption spectrum of 2-azido-N,N-dimethylethanamine (DMAZ)

Abstract Non-local density functional theory utilizing the B3LYP exchange-correlation functionals with a 6-311++G(d,p) basis set was employed to characterize the geometric parameters and normal modes of 12 equilibrium conformations of 2-azido-N,N-dimethylethanamine. Also known as DMAZ, an experimentally acquired, mid-infrared absorption spectrum of this fuels vapor is analyzed based on the computational results. The analysis indicates that the relative populations of DMAZ conformers in a room temperature sample do not deviate significantly from expectations based on a Boltzmann distribution calculated from their theoretically determined zero-point corrected energies. The most abundant conformer is found to have the central nitrogen atom of the azido group aligned over the amine lone pair electrons. Since this configuration is likely to inhibit proton transfer to the amine site, it may play an influential role in DMAZs performance as a hypergol.

Application of tunable diode laser diagnostics for temperature and species concentration profiles of inhibited low-pressure flames.

We have employed tunable diode laser absorption spectroscopy (TDLAS) to characterize low-pressure premixed CH(4)/O(2)/Ar flames inhibited with Halon 1301 (CF(3)Br) and the candidate Halon alternative compounds FE-13 (CF(3)H) and HFC-125 (C(2)F(5)H). This work is part of a larger program designed to help identify replacement fire-suppression compounds for the currently used Halon 1301. We have used CO two-line thermometry to profile the temperature in low-pressure laminar flames and have determined concentration profiles for a large number of flame species, including reactive intermediates. To date, we have detected 12 flame species by using TDLAS in our laboratory and report on seven of them here: CH(4), H(2)O, CO, CF(2)O, CF(2)H(2), CF(3)H, and CF(4). To the best of our knowledge, this is the first time the last four species have been observed in flame by the use of TDLAS. Our data are important for validating the detailed kinetic mechanisms of chemical flame inhibition. Our results indicate that TDLAS is a versatile and powerful diagnostic technique for studying combustion processes.

Spectroscopic Studies of Low Pressure Opposed Flow Methane/Air Flames Inhibited by Fe(CO)5, CF3Br, or N2

Hydroxyl radical (OH) is measured using laser induced fluorescence in reduced pressure, non-premixed, opposed flow CH4/air flames inhibited by either N2, CF3Br, or Fe(CO)5. Emission spectroscopy is used to qualitatively monitor OH for each inhibited flame as well as to characterize the structure of a Fe(CO)5 inhibited flame. For the flames to which CF3Br and Fe(CO)5are added, the OH population decreases approximately proportional to the amount of inhibitor agent added. OH populations decrease faster for flames inhibited by Fe(CO)5 than for flames inhibited by CF3Br.

Tomographic analysis of CO absorption in a low-pressure flame.

Tomographic analysis is used to provide a correction to low-pressure stoichiometric premixed CH(4)/O(2) flame temperatures measured with tunable diode laser absorption spectroscopy employing CO two-line thermometry. It is shown that flame temperatures measured with line-of-sight-based two-line thermometry are always too low and that the correction to the observed temperature is a nonlinear function of the height above the burner surface. It is also shown that, at a given height in the flame, a constant temperature across the flame does not imply that vibrational populations are constant and that, at low pressures (<20 Torr), the flame spreads radially beyond the burner diameter and so may no longer be approximated by a one-dimensional model.

Laser-Based Detection Methods for Explosives

Publisher Summary Lasers offer multiple approaches for explosive detection that are not possible with other techniques. In general, these can be separated into two types: those based on the unique properties of lasers for long-distance propagation of intense energy and those that are based on the actual molecular and atomic spectroscopy, and as such utilize the high wavelength specificity that most lasers offer. Of course, the field of laser explosive detection is somewhat new, given the fact that lasers were invented fairly recently in 1958. As such, it is fair to say that laser explosive detection is still a work in progress, with much having been discovered in recent years and still more to be discovered in the near future, particularly as more exotic laser sources, for example, fem to second lasers, become more common, less expensive, more rugged, and generally more readily available. The expected dramatic improvements in probability of detection and reduction of false alarm rates suggest that laser-based explosive detection methods may evolve into a major new technology area in the next 1–3 years.