Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory

Jun 01, 2009
Volume 24, Issue 6

The United States Army Research Laboratory (ARL) has been applying standoff laser-induced breakdown spectroscopy (LIBS) to hazardous material detection and determination. We describe several standoff systems that have been developed by ARL and provide a brief overview of standoff LIBS progress at ARL. We also present some current standoff LIBS results from explosive residues on organic substrates and biomaterials from different growth media. These new preliminary results demonstrate that standoff LIBS has the potential to discriminate hazardous materials in more complex backgrounds.

The United States Army Research Laboratory (ARL) has been investigating the potential of standoff laser-induced breakdown spectroscopy (LIBS) for detection and discrimination of hazardous materials for several years. The detection of small amounts of hazardous materials at a standoff distance is of great interest to many government organizations and also to industry. LIBS has several attractive characteristics for future field use — real time detection, simple experimental components, and no sample preparation is required (1–7). LIBS is an atomic emission technique that involves focusing a pulsed laser beam to produce a microplasma on the target surface. A small amount of the target material is ablated and ionized by the plasma, leading to the generation of atomic–ionic emission in the plasma during cooling. A typical LIBS experiment involves a pulsed laser source, optics that focus the laser pulse to a sufficient energy density to generate the microplasma, optics to collect the emission from the plasma, and a spectrometer to resolve the emission into a spectrum that is analyzed by a computer. LIBS spectra comprise atomic emission peaks and some molecular emissions. Because LIBS is an optical technique, the focusing and collection optics can be configured for standoff operations up to tens of meters (8–15).

Early LIBS work at ARL involved using laboratory instruments to collect LIBS spectra from a variety of hazardous materials, including explosives and chemical and biological warfare agent surrogates (16–19). Other groups have also collected LIBS spectra of explosives (20,21) and biomaterials (22,23) with varying levels of subsequent data analysis. Methods to discriminate between hazardous residues and interferents needed to be developed. For example, the strategy for detection and discrimination of explosives is to track the elements of the oxidizer (usually an oxide of nitrogen component) relative to the fuel (usually a hydrocarbon component) (18,34). Most explosive material will have more oxygen and nitrogen relative to carbon and hydrogen compared with nonexplosive materials. In a similar manner, the major and minor elements of biological and chemical warfare agent surrogates are tracked and used to differentiate from potential interferent compounds (19,35–38). Air entrainment in the plasma can interfere with explosives detection (and biological and chemical agent detection to a lesser extent). Because we are tracking oxygen and nitrogen relative to carbon and hydrogen to determine if the sample is an explosive, the oxygen and nitrogen from the atmosphere will also contribute to the nitrogen and oxygen emission intensity. Thus, the true amounts of oxygen and nitrogen relative to carbon and hydrogen within a given sample will be obscured. As a solution, we began to use double-pulse LIBS, which involves spatially overlapping two collinear laser pulses and separating them temporally on the order of a few microseconds. One advantage of double-pulse LIBS is the enhancement of the LIBS signal (39–42). More importantly for explosives detection, the first pulse lowers the atmospheric oxygen and nitrogen density in the plasma, thus diminishing the effect of atmospheric oxygen and nitrogen atomic emission in the second analytical plasma (41). We demonstrated in the laboratory that double-pulse LIBS effectively reduced the air entrainment, thus giving a more accurate value of the nitrogen and oxygen to carbon and hydrogen atomic emission intensity ratios expected from an explosive sample (34).

Figure 1
The first standoff experiments (see time line in Figure 1) performed by ARL researchers were conducted at Yuma Proving Ground (Yuma, Arizona) in collaboration with Spanish researchers and industry. The initial proof of principal work demonstrated that standoff explosive residue detection appeared feasible (43). Subsequently, several standoff systems were developed by ARL and are described in the "Standoff Systems" section. We have collected standoff LIBS spectra from a wide variety of residues and bulk materials. Typically, a small amount of the residue material is spread on a substrate. In general, the coverage of the samples is estimated to be 10–100 mg/cm2, although this is not inclusive of all the sample coverage amounts we have investigated.

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