Sensing Suspicious Powders Using Noncontact Optical Methods

April 1, 2015

Army RDECOM, Durham, North Carolina

Suspicious powder incidents continue to be a disruptive and costly problem. In-situ assessment of suspicious powders within inorganic matrices, particularly with powders of biological origin, is currently limited to detection by biochemical methodologies that react with monomers such as amino acids, nucleic acids, lipids, or macromolecule compounds comprised of these basic subunits. Current optical methods such as Raman spectroscopy using excitation in the near infrared at 785 nm or visible at 532 nm, have not been able to detect or distinguish biological materials from background or other materials.

Figure 1. The laser/computer side (left) and spectrometer/detector side (right) of the BRANE sensor instrument.

A handheld technology was developed that employs a fusion of deep UV excited Raman and fluorescence spectroscopic methods. The proposed Biological Reconnaissance & Analysis using Non-Contact Emission-spectroscopy (BRANE) sensor enables detection and classification of a broad range of suspicious powders in real time at the site of a contamination incident without the need for reagents, labels, or other consumables. It also avoids contact with or disturbing the suspicious powder and the subsequent need for decontamination of the sensor, or spreading, growing, or dispersing biological threats in suspicious powders. It provides rapid analysis; for each 60-μs laser pulse, a full spectrum analysis is possible.

The BRANE sensor has the noncontact, no-sample-handling advantages of Raman-based sensors, with the advantages of deep UV excitation of samples at wavelengths that enable simultaneous detection of both Raman emissions without obscuration by fluorescence, and fluorescence emissions without alteration by Raman emissions. This can only be conducted in the deep UV below 250 nm. The BRANE sensor is capable of making millions of tests with no consumables cost in an instrument with indefinite field lifetime. It is also capable of detection and classification of a much broader range of contaminants beyond biological that includes chemical and explosive hazards. All this is done in a single sensor without the need for modification for different applications.

The BRANE sensor optically analyzes the nucleic, protein, and lipid components of biological threats to assess their presence and identity. This is enabled by three new technology elements: 1) a miniature deep UV laser with narrow linewidth that enables simultaneous Raman and fluorescence detection; 2) a high-speed linear CCD array detector that enables handheld miniaturization; and 3) chemometric algorithms that enable classification of biological material using a fusion of deep UV Raman and fluorescence spectroscopy. The BRANE instrument weighs about 7 pounds, including batteries, and measures about 4" wide by 9" high by 16" deep.

Figure 2. Photos of the BRANE sensor left and right sides, with most key elements physically in place.

Operating at excitation wavelengths below 250 nm, both Raman and fluorescence emissions can be collected simultaneously on a single array detector, with Raman emissions providing information about molecular bonds with the unknown sample, and fluorescence emissions providing information about overall electronic structure of the sample.

Figure 1 illustrates the BRANE sensor’s laser/computer and spectrometer/detector sides. The right side of the instrument houses the laser, laser drive, and control electronics; input/output control board with onboard battery charging electronics and embedded Microprocessor; memory for storing all suspicious powder library data and processing all spectroscopic data to compare with library data to determine identity of a suspicious powder; and optical element for processing the laser output. The I/O board will also include an onboard GPS to enable logging of position along with date and time for each data set collected. The left side has the objective lens, contextual imaging camera (not yet shown), laser injection optics, and spectrometer input optics and entrance slit with recollimation optics, grating, spectrometer imaging lens, linear CCD array detector, and detector control and processing electronics.

Controls include a simple mechanical on/off button or switch to power up the basic system, energize the display, and prepare the sensor for data acquisition. Under this standby condition, BRANE will draw battery power of about 5W. A trigger in the handle activates the laser and detection electronics, and the system begins taking data on suspicious powders at a rate of about 40 combined Raman and fluorescence spectra per second, proving real-time display of the results. For some weak materials, it may be necessary to dwell on a sample for up to about 10 seconds, although most data can be achieved in less than 1 second.

This work was done by William F. Hug, Ray D. Reid, and Rohit Bhartia of Photon Systems for the Army RDECOM. ARL-0174