Deep ultraviolet Raman spectroscopy for eyesafe standoff chemical threat detection

Abstract

Chemical threat detection has long been of interest to military, law enforcement, environmental agencies, and forensic investigators. Recently, as the propensity for both foreign and homegrown terrorism, illegal drug manufacture, and concern for environmental regulation continues to grow, the demand for rapid, portable chemical threat detection capabilities has increased dramatically. In particular, the ability to identify chemical threats (explosives, narcotics, toxic industrial chemicals, etc.) at a distance (standoff) is of special interest as it increases the safety of the end user during interrogation. Traditional analytical laboratory techniques such as high-performance liquid chromatography or gas chromatography coupled with mass spectrometry offer excellent sensitivity for detection and identification of trace amounts of threatening material. However, these techniques often lack the portability necessary for remote on-site interrogation as samples must be physically collected and brought to a laboratory for analysis. Vibrational spectroscopic techniques offer both the chemical identification and miniaturization capabilities required for portable, on-site chemical threat detection. Most importantly, spectroscopic techniques are inherently and uniquely standoff, where emitted or scattered photons are collected at some distance from the sample. The challenge then becomes miniaturizing the instrumentation while maximizing the distance at which accurate chemical detection can be made. Here we report on portable chemical threat detection instrumentation developed by Alakai Defense Systems, which employs deep ultra-violet Raman spectroscopy. We discuss the general system aspects such as basic optical design and ambient light rejection techniques. We also present data on the performance capabilities using several substances including actual narcotics and other compounds commonly used as cutting agents. Lastly, we discuss possible future directions including the ability for rapid spectroscopy while maintaining high photon detection sensitivity by employing an intensified scientific CMOS (sCMOS) and the propensity for NIR standoff Raman detection using deep-depletion CCD technology.