Spatially offset Raman spectroscopy (SORS) allows for sub-surface and through barrier detection and has applications in drug analysis, cancer detection, forensic science, as well as defense and security. This paper reviews previous efforts in SORS and other through barrier Raman techniques and presents a discussion on current research in defense and security applications.
The detection of materials through containers is a vital capability for security screening applications at high risk
locations, such as airports and checkpoints. Current detection procedures require suspect containers to be opened and the
contents sampled, which is laborious and potentially hazardous to the operator. The capability to detect through-barrier
would overcome these issues.
Spatially Offset Raman Spectroscopy (SORS) is an innovative spectroscopic technique that avoids fluorescence and
Raman scatter from containers, which can mask the Raman signature from the sample. This novel approach enables noninvasive
detection of hazardous and benign materials through a wider range of container materials than is possible using
conventional Raman spectroscopy.
SORS spectra were acquired from explosive compounds and benign materials within a range of coloured glass and
plastic containers. The SORS spectra were compared to the reference Raman signatures of the materials studied. Two
data analysis methods were then applied to the resultant data to investigate the ability of SORS to detect the target
materials through the barriers tested. Furthermore, the potential for reduction of sample fluorescence was investigated by
using longer excitation wavelength (1064 nm) than is typically used in commercially available Raman instruments that
use silicon detector technology. For some fluorescent samples, Raman spectral features that were masked by
fluorescence at 785 nm were revealed at 1064 nm.
The capability to detect toxic chemicals and explosive materials through a wide range of container types has a variety of
applications, including liquid screening at airport entrance points. Conventional Raman spectroscopy is commonly used
for chemical detection, but can result in an intense spectral response due to scattering and/or fluorescence from the
container when used for through-barrier applications. Such a response can reduce the effectiveness of the technique for
analysis of the container contents by swamping the Raman signature of the target material.
By producing two spectra containing different contributions from the container and the contents, spatially offset Raman
spectroscopy (SORS) allows a spectrum of the contents to be obtained, even through fluorescing containers. This
innovative technique could provide a through-barrier detection capability for a wider range of containers than
conventional Raman spectroscopy, including containers made from coloured glass and opaque plastic. In this paper, the
use of SORS for through-barrier detection is introduced, and its ability to detect a range of analytes through a range of
container materials evaluated. The potential advantages of using a longer excitation wavelength (e.g. 1064 nm) to reduce
sample fluorescence are also explored, focussing on target analytes mixed with fluorescent materials.
An ultraviolet (UV) laser induced fluorescence (LIF) light detection and ranging (LIDAR) system has been constructed
and commissioned by Dstl and demonstrated to be an effective technique for discriminating between some common
fluorescent potentially interfering aerosols and biological warfare agent (BWA) simulants at a distance remote from the
release. The Mk 3 UV-LIF LIDAR employs the fundamental wavelength (1064 nm) of a Nd:YAG laser to spatially map
aerosol clouds, and the fourth harmonic (266 nm) to excite fluorescence. The fluorescence emission is spectrally
resolved into ten detection channels between 300-500 nm, permitting classification by a discrimination algorithm. The
UV-LIF LIDAR was trialled in 2007 in the Joint Ambient Breeze Tunnel (JABT) and on the open range, at the US Army
Dugway Proving Ground (DPG), Utah. In the JABT, calibration instruments were used to characterise the BWA
simulant and interferent aerosol releases, permitting calculation of the system's limits of detection (LoD) and
Light detection and ranging (LIDAR) has potential to be a successful technique for remote detection of airborne
biological warfare agents (BWA) that pose a health hazard. Potential techniques for detecting BWA often use
spectroscopy to probe molecular structure properties (e.g. UV-fluorescence, Raman and differential absorption
spectroscopy). An alternative approach is to differentiate BWA from background interferents by their differing
morphology; depolarisation offers one such method. Here, we investigate the feasibility of introducing depolarisation
into a short range (approximately 10 m) LIDAR designed to be a simple, inexpensive, low power consumption, portable
T-matrix calculations are presented for a randomly oriented, polydisperse size distribution of Bacillus atrophaeus
spheroids. The relationship between backscatter depolarisation and particle aspect ratio is investigated at several incident
wavelengths corresponding to those produced by low cost, commercially available laser sources. Through a series of
simulations, we determine the best combination of wavelengths for a multi-wavelength instrument design that exploits
the concept of normalised depolarisation to determine particle aspect ratio, with the possibility of facilitating BWA
Aerosol droplets are trapped and manipulated with a single-beam gradient-force optical trap for timescales of hours. By coupling the optical trap with cavity enhanced Raman scattering, the size of the trapped droplet can be determined with nanometre accuracy and high time resolution. This allows the evolution in droplet size and composition to be monitored during the growth or evaporation of a single trapped droplet, providing a method for characterising the factors that govern aerosol droplet size. The simultaneous trapping of two or more aerosol droplets in parallel optical traps can permit studies of aerosol coagulation.