Advanced stand-off Raman spectroscopy for remote chemical sensing

Fast, sensitive and straightforward remote detection and imaging of chemical substances, such as explosives, in today`s society is critical for the safety and well-being of any civilized society. An approach which can be used to meet these increasing demands is stand-off Raman spectroscopy. Raman spectroscopy is a powerful technique which provides molecular specific information. This information is obtained by inelastic scattering of a laser beam with the samples under investigation. Using pulsed laser systems, this spectral information can be obtained at remote distances in short periods of time (order of seconds). Raman spectroscopy though molecular specific, is a weak effect and is often hindered by fluorescence of the sample or the background on which the sample is detected. As a result new methods are required to improve the sensitivity of stand-off Raman spectroscopy. In this proposal, we intend to improve remote detection sensitivities by using UV excitation, shorter laser pulses (order of picoseconds) and Spatial Offset Raman Spectroscopy. In addition to this increased sensitivity, we intend to provide complementary imaging information of the substances at remote distances. Our close cooperation with Austrian Armed Forces will provide a field application where our technique can be robustly tested and implemented. In addition to explosive detection we propose to apply the method to on-line process monitoring in metallurgical and chemical reactor processes.

In light of increased risk of public attacks by a single perpetrator or small groups using improvised explosive devices or similar threats over the last years the need for mobile and robust detection instruments, which keep the user and the equipment safe, intensified. Hence, in this project the application of Raman spectroscopy for remote detection of potential hazardous or dangerous chemicals was employed. Here, a laser is used to illuminate a sample at a stand-off distance and the back-scattered photons are collected and spectrally analyzed. The acquired spectrum is equivalent to a fingerprint for a certain compound and can be directly linked to its chemical composition. The goal of the project included the improvement of the established stand-off Raman spectroscopy equipment, the downsizing and engineering into a mobile prototype capable of monitoring large areas and the implementation of classification algorithms for a fast and reliable identification of chemicals at stand-off distances. Using a spatial offset between excitation point and detection point, measurement of concealed substances inside various containers was achieved at a distance of 12 m, which proves especially useful in different scenarios where the substance of interest may be hidden. In parallel, the construction of a direct imaging system, employing an air- cooled, compact laser and a smaller detection unit, was undertaken. This new prototype is able to detect larger areas of interest with higher spatial resolution, whilst maintaining high optical throughput. This enabled us to build an automatic identification tool for several different explosives and precursors using chemometric tools and classification algorithms. Additionally, the new prototype was tested for the application of identification of pigments used in historic art works. Here, in cooperation with the Universidad de Jaén, pigment samples used in the décor of historic murals e.g. the Alhambra were prepared and tested. Concurrently, a cooperation with the Institute of Chemical Engineering led to the development of a new method for evaluation of fluid dynamics and chemical composition for process stream at the same local position. This was achieved by combining Laser-Doppler- Velocimetry and stand-off Raman spectroscopy. This novel technique was validated through a series of experiments involving miscible, as well as immiscible compounds.