A novel experimental design combining Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) in a unique integrated sensor is described. The sensor presented herein aims to demonstrate the applicability of a hybrid dual Raman-LIBS system as an analytical tool for the standoff analysis of energetic materials. Frequency-doubled 532 nm Nd:YAG nanosecond laser pulses, first expanded and then focused using a 10x beam expander on targets located at 20 m, allowed simultaneous acquisition of Raman-LIBS spectra for 4-mononitrotoluene (MNT), 2,6-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), C4 and H15 (plastic explosives containing 90% and 75% of RDX by weight, respectively), and Goma2-ECO (Spanish denominated dynamite class high explosive mainly composed of ammonium nitrate, nitroglycol, and dinitrotoluene among other compounds), sodium chlorate, and ammonium nitrate. With the use of a Cassegrain telescope, both Raman and LIBS signals from the same laser pulses were collected and conducted through a bifurcated optical fiber into two identical grating spectrographs coupled to intensified charge-coupled device (iCCD) detectors. With the use of the appropriate timing for each detection mode, adjustment of the laser power on the beam focal conditions is not required. The ability of the present single hybrid sensor to simultaneously acquire, in real time, both molecular and multielemental information from the same laser pulses on the same cross section of the sample at standoff distances greatly enhances the information power of this approach.
SummaryA Houbara survey of Lanzarote and the small island of Graciosa, during December 1993, resulted in a total count of 146 birds and an estimated total population of about 400 Houbaras. These numbers are higher than found on most previous surveys of Fuerteventura, considered as the main stronghold of this subspecies, and indicate that the Houbara population on Lanzarote is much more important than was formerly supposed. A study of habitat preferences showed that the species tended to select zones with greater height of shrubs.
In general, any standoff sensor for the effective detection of explosives must meet two basic requirements: first, a capacity to detect the response generated from only a small amount of material located at a distance of several meters (high sensitivity) and second, the ability to provide easily distinguishable responses for different materials (high specificity). Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) are two analytical techniques which share similar instrumentation and, at the same time, generate complementary data. These factors have been taken into account recently for the design of sensors used in the detection of explosives. Similarly, research on the proper integration of both techniques has been around for a while. A priori, the different operational conditions required by the two techniques oblige the acquisition of the response for each sensor through sequential analysis, previously necessary to define the proper hierarchy of actuation. However, such an approach does not guarantee that Raman and LIBS responses obtained may relate to each other. Nonetheless, the possible advantages arising from the integration of the molecular and elemental spectroscopic information come with an obvious underlying requirement, simultaneous data acquisition. In the present paper, strong and weak points of Raman spectroscopy and LIBS for solving explosives detection problems, in terms of selectivity, sensitivity, and throughput, are critically examined, discussed, and compared for assessing the ensuing options on the fusion of the responses of both sensing technologies.
The fact that a Raman spectrum may be considered the fingerprint of an interrogated target by providing specific information on the particular chemical structures of the molecules present, has boosted the use of Raman spectroscopy for explosives detection in homeland and security applications. Also, the possibility of direct and distant access to suspect targets by stand-off Raman measurements makes this analytical technique a valuable tool in operational scenarios for security forces. The modest detection power as a result of the well-known, inherently low efficiency of the Raman scattering requires a careful evaluation of the experimental parameters governing the analytical response of this technique, particularly in respect to the amenable distance to target, the data acquisition speed, and the sources of uncertainty in any given measurement. The present paper highlights the importance of adequate instrumental parameters of the sensor according to the operational scenario when analyzing unknown targets. Raman fingerprints collected from a wide range of high explosives and associated compounds under different analysis conditions and operational scenarios have been evaluated in terms of signal strength, signal-to-noise ratio, analytical sensitivity, and signal stability.
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