Spectroscopy Poised to Seek Terror Agents

Richard Gaughan

In the post-9/11 world, an envelope may hide disease and a shoe may conceal explosives. Detecting these threats is time-consuming and labor-intensive. According to scientists at the US Army Research Laboratory at Aberdeen Proving Ground in Maryland, laser-induced breakdown spectroscopy (LIBS) promises to ease the burden.

In laser-induced breakdown spectroscopy, a solid-state, pulsed laser is focused onto a sample to form a high-temperature plasma. The elemental composition of the sample can be identified by the characteristics of the light emissions from the atoms, ions and molecular fragments generated in the plasma. Courtesy of US Army Research Laboratory.

In the technique, an intense laser pulse is focused on the surface under examination, creating a plasma. The excited plasma decays, releasing a broadband emission that quickly diminishes. However, excited atoms within the plasma also emit electromagnetic radiation as they return to the ground state. Each unique atomic species has its own spectral signature in the 200- to 980-nm range.

Andrzej W. Miziolek is the research leader of the laser-induced breakdown spectroscopy development program at the Army lab. He noted that there are various laser-induced spectroscopy instruments in the field, some built by companies and some by universities. The laboratory has worked with Ocean Optics Inc. of Orlando, Fla., to develop the LIBS 2000+ broadband spectrometer.

The baseline configuration of the LIBS 2000+ employs as an excitation source the Ultra CFR from Big Sky Laser Technologies Inc. of Bozeman, Mont. The 1.06-µm Nd:YAG laser delivers 50 mJ per 10-ns pulse, with a repetition rate of 1 to 20 Hz. A high-temperature plasma is created at the target surface at a distance of a few centimeters to 100 m or farther (with the use of fiber optic delivery).

After approximately 1 µs, the continuum emission diminishes, and atomic and ionic spectra dominate. Seven spectrometers sample the response, each using a 2048-pixel linear CCD as the sensing element. Within moments after excitation, the system collects the data and displays the spectra.

Preliminary research demonstrates that the system can distinguish several strains of anthrax-surrogate bacteria. But although the technique is promising, the development work has not received significant help to date from the usual funding agencies. Because of this, Miziolek said, the researchers have not fully explored the limitations of the technology. Nevertheless, he said that the variety of applications in which laser-induced breakdown spectroscopy is finding a place would surprise nonexperts.

Further development of both the instrumentation and the software is necessary to satisfy the requirements of field use. This development will happen at an accelerated pace, he predicted, to help meet the needs of homeland security.