In 2000, Andrzej Miziolek of the U.S. Army Research Laboratory (Aberdeen Proving Ground, MD) was using laser-induced breakdown spectroscopy (LIBS) to detect hazardous materials when he challenged us to create an alternative to the commercially available LIBS systems of the day. Our system would need to provide real-time analysis from 200 to 980 nm, provide optical resolution of 0.1 nm (FWHM) or better, and meet size, portability, and budgetary requirements.
LIBS is a relatively simple spark spectrochemical method for determining the elemental composition of a material. In LIBS, a researcher fires a high-power, short-pulse laser at a sample to create an intense, 15,000K plasma. As it decays (cools), the information-rich plasma emits light energy that can be spectrally analyzed to determine the elemental composition of the target sample. All elements can be identified using this spectroscopic technique, as they all emit over spectral lines between 200 to 980 nm (see figure).
Spectra of anthrax surrogates shows the ability of our LIBS technique to discriminate between closely related Bacillus species. Performance
We designed the LIBS system to detect all of the elemental emission lines (200 to 980 nm) in the plasma created by a single 10-ns laser pulse. A user can use any Q-switched pulsed laser as long as its energy is 30 mJ or greater. The system incorporates seven high-resolution spectrometers, each one with a grating engineered to effectively disperse light over a different segment of the target spectral range. The high resolution and broadband range of the system enables detection of parts-per-billion and picogram quantities of elements in samples at a resolution of 0.1 nm (FWHM).
Each spectrometer in the chain contains a 2048-element CCD-array detector with sensitivity that eliminates the need for the expensive intensifiers often required by traditional LIBS systems. In low-light level applications such as LIBS, the ability to collect enough light energy to attain a measurable signal is crucial. The Sony ILX511, a shallow-well detector, needs fewer photons to provide adequate signal-to-noise than does a deep-well detector common to conventional LIBS systems. Deep-well detectors require an image intensifier to produce a sufficient number of photons to provide ample signal-to-noise.
A master spectrometer controls the triggering function of all channels. We gate the simultaneous data acquisition of the spectrometers relative to the laser pulse. By controlling the pulse, we can start the integration time of the spectrometers prior to firing the laser. This allows us to see the plasma continuum, important ionization shifts, and temporal intensities. Additionally, since the spectrometer controls the firing of the laser, the time jitter between the start of the spectrometer integration and the firing of the laser is less than 250 ns. Portability and Cost
The miniature size of the spectrometers enabled us to enclose all seven channels in a standard 19-in. rack module only 5 in. high. The spectrometers connect to a compact (17.8 cm X 26.7 cm X 22.8 cm) sample chamber via a fiber-optic bundle.
We designed the sample chamber to have an eye-protective polymer that provides a clear view of the sample and attenuates the laser light by six orders of magnitude. A magnetic safety interlock in the sample chamber prevents the laser from firing when the chamber door is ajar. We included a blower/evacuation system that removes particles from the chamber and maintains an inert gas environment to eliminate spectral lines caused by the ionization of air. We incorporated a software-controlled x-y sample stage/probe holder, powered by a piezoelectric-based drive system.
A traditional LIBS system may cost up to $100,000. Using off-the-shelf components we fielded a system costing less than half that. We're leveraging continued advances in miniaturization to develop a lightweight, broadband, high-resolution modular spectrometer system that will be one-third the size of the current LIBS system. The proposed system will include a hand-carried probe, a backpack-mounted spectrometer/computer, a power supply, and an eye-level display. oe
Jay Thomason, Yvette Mattley, Roy Walters, Jeremy Rose
Jay Thomason is marketing communications manager, Yvette Mattley is senior scientist, Roy Walters is director of R&D, and Jeremy Rose is senior R&D software engineer at Ocean Optics Inc., Dunedin, FL.