Detecting Explosives by Use of LIBS

June 1, 2008

Army Research Laboratory, Adelphi, Maryland

Laser-induced breakdown spectros - copy (LIBS) has been investigated for potential utility as a means of detecting trace amounts of chemical explosives and residues thereof in lawenforcement, forensic analysis, and military settings. In LIBS (see figure), a laser is used to rapidly generate a microplasma of a sample, and the light emitted by the microplasma is analyzed to identify (and determine the intensities of) spectral lines of elements and compounds in the sample. In previous applications for purposes other than detection of explosives, LIBS has been shown to enable remote, rapid, multielement micro-analysis of bulk samples (solid, liquid, gas, aerosol) of compounds having concentrations in the parts-per-million range.

LIBS has several features that make it especially attractive as a means of detecting explosives:

• Conceptually, it is simple and straightforward.
• Little or no preparation of samples is necessary.
• Only a very small sample is needed for production of a usable LIBS spectrum (typically, a sample mass in the approximate range of picograms to nanograms is sufficient).
• Response time is less than 1 s.
• LIBS sensors can be made rugged and field-portable: all components (including the laser, spectrometer, detectors, and computer) can be miniaturized.
• LIBS offers the flexibility of operation in a point detection (proximity) mode or a stand-off mode.

In the past, LIBS was used primarily to analyze one or a few elements — mostly metals — that can be identified via emission spectra lines within relatively narrow wavelength bands. More recently, the utility of LIBS for identifying compounds has been realized with the advent of high-resolution, broad-band spectrometers, which can capture the spectral lines emitted by all the elements in the laser-generated plasmas (provided that the elements are present in sufficient abundance). LIBS has already been shown to enable detection of a variety of toxic and otherwise hazardous compounds other than explosives.

Recent efforts at the Army Research Laboratory (ARL) have focused on optimizing the sensitivity and selectivity of LIBS for detecting explosive residues. The ability to detect trace amounts of materials with a single laser shot is especially important because the first shot can ablate all or most of a residue.

The basic idea of the ARL approach is to identify an explosive or a residue thereof on the basis of the relative abundances (and, thus, ratios between intensities of spectral lines) of elements in explosive compounds, as distinguished from relative abundances and corresponding spectral intensities of other materials likely to be present in samples that contain explosives and residues thereof. These relative abundances and corresponding relative spectral intensities have been compiled in survey of LIBS spectra of explosives and of other energetic materials, including petroleum fuels. In particular, a common characteristic of most military explosive compounds is that, in comparison with nonexplosive compounds, their nitrogen and oxygen contents are high, relative to their carbon and hydrogen contents.

In the case of residues, the difficulty of discriminating between explosive and non-explosive problems is increased by the entrainment of atmospheric oxygen and nitrogen into laser induced plasmas. In a laboratory setting, one can prevent the entrainment of atmospheric oxygen and nitrogen by using argon to displace air from the surface of a sample. In a stand-off application, it is not practical to use argon, but by use of double- pulse LIBS as described next, one can reduce the relative amount of air entrained in the laser-induced plasma and thereby improve the discrimination between explosives and non-explosives.

In double-pulse LIBS, two successive laser pulses are used to generate the microplasma. Typically, the laser pulses are separated by a few microseconds. The main advantage of double-pulse LIBS for most applications is increased signal strength. The extent of the increase depends on many factors, including pulse separation times, laser wavelength, pulse energies, the sample, and the experimental configuration. Another advantage of using two pulses is that separation of the analytical pulse from the ablation pulse increases the degree of reproducibility in most cases.

This work was done by Jennifer L. Gottfried, Frank C. De Lucia Jr., Russell S. Harmon, Chase A. Munson, Raymond J. Winkel, Jr., and Andrzej W. Miziolek of the Army Research Laboratory.

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Detecting Explosives by Use of LIBS

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