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Newsletter:  October 2003/ Issue 5
IR Spectral Database: A Library of Chemical Signatures

by Marina Skumanich, PNWCGS Staff

The international/national systems for nuclear nonproli-feration regime depends upon national systems for controlling and accounting for nuclear materials that are verified by the IAEA. While historically both political and technical attention has been paid primarily to nuclear materials control and accountability, increasing attention is now being given to accounting for non-nuclear materials such as toxic or other chemical agents as well as biological materials that might also be used in weapons of mass destruction. One priority in this area is the development of more accurate and portable chemical detection systems.

Researchers at the Pacific Northwest National Laboratory (PNNL) are conducting a series of projects for the U.S. Department of Energy in the area of chemical detection that include research and development (R&D) on both detection hardware tools and systems to support detection analysis. A key example of this research is the Infrared (IR) Spectral Database Project, managed by Steve Sharpe of PNNL’s Environmental Molecular Sciences Laboratory.

The IR Spectral Database includes quantitative, high-resolution IR (infrared) data on a wide range of vapor phase compounds, i.e., a reference library of “signatures” that can uniquely identify chemical compounds when they are airborne. The data are based on IR spectral analysis, which measures the amount of infrared energy absorbed by the compound.

IR spectral analysis—called IR spectroscopy—is a common way to identify chemicals. It relies on the fact that when low-energy (infrared) light shines on chemical compounds, the compounds begin to vibrate as the light is absorbed. However, just as a musical string only vibrates at certain frequencies, so different chemical compounds will only vibrate at certain characteristic frequencies. An IR spectrum (plural “spectra”) is therefore a chart of the amount of energy absorbed by the compound as different wavelengths of infrared light are shined on it.

A library of IR spectra is critical to IR-based chemical detection. That’s because having an excellent chemical detection device makes no difference if you are not able to identify the specific chemical being detected by comparing it to reference set. A good analogy is a fingerprint library. If a fingerprint is taken at a crime scene, it doesn’t really help to identify a suspect unless the print can be compared with a reference set. The IR Spectral Database fills that need for suspect chemicals.

The IR Spectral Database provides spectral data for a wide range of IR wavelengths. This is particularly important in the case of mixed or contaminated samples since it increases the likelihood the analyst will find a region of the spectra where a given chemical’s signature is clear from confounding signatures by other chemicals. Creating the Spectral Library is a true “labor of love,” said Steve Sharpe. “It requires attention to detail at every stage of development. For instance, acquiring the thousands of sets of “low level” laboratory data is essential yet time-consuming, boring, and sometimes frustrating. In order to keep the project on schedule and still maintain the sanity of those involved requires that we take turns working in the laboratory. The tedium and frustration is quickly offset by each new chemical we add to the library and watch the library grow into a truly unique national resource.”

The IR Spectral Database is also useful in the development of detection hardware. Knowing the signatures of certain important or critical chemical compounds, one can design technologies that are optimized to measure them in the field.

IR chemical detection and the IR Spectral Database provide support not only for nonproli-feration and security, but also for a wide range of chemical assessments, including ambient air quality assessment, greenhouse gas or pollution management, industrial process management, and occupational health assessments.

The Ultra-Selective Chemical “Sniffer” —a portable device being developed at PNNL that will use the IR Spectral Database to do in-field detection and identification—is intended for use with the IR Spectral Database. Together, these products should represent a real boon to efforts to identify chemical materials and to their control, when needed.