We demonstrate how molecular spectroscopy methods using NIR and MIR lasers can provide rapid detection and
identification of many threat materials. It is increasingly recognised that one spectroscopic method will not be suited to
every target in every scenario, both in terms of spectroscopic selectivity and the context e.g. vapour phase or within a
sealed container. The orthogonal selection rules and capabilities of IR and Raman in combination allow the identification
of a very broad range of targets, both in liquid and vapour phase. Therefore, we introduce the benefits of the combining
infra-red absorbance based on Quantum Cascade lasers (QC-IR) and NIR Raman spectroscopy for nitrogenous and
peroxide based materials. Rapid scan rates up to 10Hz for QC-IR and Raman and are demonstrated using current
technology. However, understanding of the chemistry and spectroscopic signatures behind such materials is necessary
for accurate fast fitting algorithms to benefit of the full advantage with advances in hardware. This is especially true as
future users requirements move towards rapid multiplexed analysis and data fusion from a variety of sensors.
A low cost technique is reported for the rapid screening of containers for materials that potentially could be used for terrorist activities. For peroxide based samples it is demonstrated that full characterisation can be achieved in a continuous curve fitting monitoring mode acquiring up to 10 spectra per second. This clearly demonstrates the potential for a Raman based method to be incorporated into a check-point whilst retaining fast throughput. A number of precursor compounds to nerve agents and peroxide and nitrate based improvised explosive materials have been studied. The potential strengths and weaknesses of using Raman for multiple target identification are discussed with regard to the common vibrations associated with each group of agents. Within this context we also introduce the use of fast Raman line mapping into the trace analysis of multiple component targets. The method presented is suited to volatile or light sensitive samples (such as derived peroxides) and can be employed on a variety of surfaces. As speed and throughput are traded against spectral bandwidth categorising threat compounds into groups based on common functionalities allows the full potential for multiplexed targeting to be realised.
Raman spectroscopy provides a very effective method of identifying an illicit substance in situ without separation or contact other than with a laser beam. The equipment required is steadily improving and is now reliable and simple to operate. Costs are also coming down and hand held portable spectrometers are proving very effective. The main limitations on the use of the technique are that it is insensitive in terms of the number of incident photons converted into Raman scattered photons and fluorescence produced in the sample by the incident radiation interferes. Newer methods, still largely in the development phase, will increase the potential for selected applications. The use of picosecond pulsed lasers can discriminate between fluorescence and Raman scattering and this has been used in the laboratory to examine street samples of illicit drugs. Surface-enhanced Raman scattering, in which the analyte requires to be adsorbed onto a roughened metal surface, creates a sensitivity to compete with fluorescence and quenches fluorescence for molecules on a surface. This provides the ability to detect trace amounts of substances in some cases. The improving optics, detection capability and the reliability of the new methods indicate that the potential for the use of Raman spectroscopy for security purposes will increase with time.
Surface enhanced resonance Raman scattering (SERRS) provides an increase in sensitivity over Raman scattering of
about 1014 and rivals fluorescence in terms of its quantum efficiency. With the use of modern edge and notch filters and
CCD cameras, the price and complexity of Raman spectroscopy equipment has decreased rapidly. This means that the
potential advantages of SERRS are now much easier to release for use for practical purposes. The technique has
specific advantages in terms of sensitivity and coding for use for tagging.
Surface-enhanced resonance Raman scattering (SERRS) is a very sensitive and selective detection method that can be used for the analysis of both DNA and P-450s. A number of factors have limited the broader application of the technique. These limitations are described and addressed. An approach to reduce the problems associated with variation of the silver colloids used to provide surface enhancement and chemical methodologies that ensure surface adsorption are presented. A practical approach was used to investigate the nature of the effect. This approach has highlighted the importance of resonance enhancement for ultimate sensitivity. Two approaches to achieve successful detection of DNA using SERRS are described, and, using these two approaches, the possibility of multiplexing is also demonstrated. The analysis of proteins by SERRS is discussed and P-450 is presented as a specific example of the information that may be gained from SERRS of proteins.
During the past five years work in our laboratory has been concentrated on developing SERRS spectroscopy and making it a simple and robust technique for the analyses of colorants. It has proved to be highly discriminative, extremely sensitive and possible to identify dyes in mixtures without their prior separation. Additionally, by using concentrated silver colloid solutions, in-situ analyses have now been accomplished with minimal or in some cases no visual destruction of the item being examined and with virtually no background interference from the surfaces on which the stains or smears have been deposited. To illustrate the methodology and the potential of SERRS various applications including the in-situ analyses of the dyes on cotton fibers and stains from cosmetics, shoe polishes, inks and drinks on various surfaces are presented.
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