“Standoff Spatial Offset Raman Spectroscopy for the detection of concealed content in distant objects” - Abstract 

Authors: Zachhuber, B.; Gasser, C.; Lendl, B.

A pulsed (4.4 ns pulse length) frequency-doubled Nd:YAG laser operated at 10 Hz was used to generate Raman scattering of samples at a distance of 12 m. The scattered light was collected by a 6 in. telescope, and the Raman spectrum was recorded using an Acton SP-2750 spectrograph coupled to a gated intensified charge-coupled device (ICCD) detector. Applying a spatial offset between the point where the laser hit the sample and the focus of the telescope on the sample enabled collection of Raman photons that were predominantly generated inside the sample and not from its surface. This is especially effective when the content of concealed objects should be analyzed. High-quality Raman spectra could be recorded, within 10 s of data acquisition, from a solid (NaClO₃) as well as a liquid (isopropyl alcohol) placed inside a 1.5 mm thick opaque low-density polyethylene (LDPE) plastic bottle. The applied spatial offset was also advantageous in cases where the surface of the container was highly fluorescent. In such a situation, Raman spectra of the sample could not be recorded when the sampling volume (telescope observation field) coincided with the focus of the excitation laser. However, with the use of a spatial offset of some millimeters, a clear Raman spectrum of the content (isopropyl alcohol) in a strongly fluorescent plastic container was obtained.

“Stand-off spatial offset Raman spectroscopy – A distant look behind the scenes” – Abstract

Authors: Zachhuber, B.; Gasser, C.; Hobro, A.J.; Chrysostom, E.; Lendl, B. 

A pulsed (4.4 ns pulse length) frequency doubled Nd:YAG laser, operating at 10 Hz, was used to generate Raman scattering from samples at a distance of 12 m. The scattered light was collected by a 6 inch telescope and the Raman spectrum recorded using an Acton SP-2750 spectrograph coupled to a gated ICCD detector. To extend the potential applications further, employing a spatial offset between the point where the laser hit the sample and the focus of the telescope on the sample, enabled collection of Raman photons that were predominantly generated inside the sample and not from its surface. This is especially effective when the content of concealed objects should be analysed. Raman spectra of H₂O₂ in a 1.5 mm thick, fluorescent HDPE plastic bottle were recorded at a distance of 12 m. From the recorded spectra it was possible to determine the H₂O₂ concentration in the concentration range from 2-30%. Stand-off Raman spectra of eleven potentially dangerous chemicals (commercial and improvised explosives) were recorded at a distance of 100 m.

“Stand-off Raman spectroscopy: a powerful technique for qualitative and quantitative analysis of inorganic and organic compounds including explosives” - Abstract

Authors: Zachhuber, B.; Ramer, G.; Hobro, A.J.; Chrysostom, E.; Lendl, B.  

A pulsed stand-off Raman system has been built and optimised for the qualitative and quantitative analysis of inorganic and organic samples including explosives. The system consists of a frequency doubled Q-switched Nd: YAG laser (532 nm, 10 Hz, 4.4 ns pulse length), aligned coaxially with a 6″ Schmidt–Cassegrain telescope for the collection of Raman scattered light. The telescope was coupled via a fibre optic bundle to an Acton standard series SP-2750 spectrograph with a PI-MAX 1024RB intensified CCD camera equipped with a 500-ps gating option for detection. Gating proved to be essential for achieving high signal-to-noise ratios in the recorded stand-off Raman spectra. In some cases, gating also allowed suppression of disturbing fluorescence signals. For the first time, quantitative analysis of stand-off Raman spectra was performed using both univariate and multivariate methods of data analysis. To correct for possible variation in instrumental parameters, the nitrogen band of ambient air was used as an internal standard. For the univariate method, stand-off Raman spectra obtained at a distance of 9 m on sodium chloride pellets containing varying amounts of ammonium nitrate (0–100%) were used. For the multivariate quantifi- cation of ternary xylene mixtures (0–100%), stand-off spectra at a distance of 5 m were used. The univariate calibration of ammonium nitrate yielded R2 values of 0.992, and the multivariate quantitative analysis yielded root mean square errors of prediction of 2.26%, 1.97% and 1.07% for o-, m- and p-xylene, respectively. Stand-off Raman spectra obtained at a distance of 10 m yielded a detection limit of 174 μg for NaClO3. Furthermore, to assess the applicability of stand-off Raman spectroscopy for explosives detection in “real-world” scenarios, their detection on different back- ground materials (nylon, polyethylene and part of a car body) and in the presence of interferents (motor oil, fuel oil and soap) at a distance of 20 m was also investigated.

“Stand-off Raman spectroscopy of explosives” – Abstract

Authors: Zachhuber, B.; Ramer, G.; Hobro, A.J.; Lendl, B.  

We present our work on stand-off Raman detection of explosives and related compounds. Our system employs 532 or 355 nm laser excitation wavelengths, operating at 10 Hz with a 4.4 ns pulse length and variable pulse energy (maximum 180 mJ/pulse at 532 nm and 120 mJ/pulse at 355 nm). The Raman scattered light is collected by a co-axially aligned 6” telescope and then transferred via a fiber optic cable and spectrograph to a fast gating iCCD camera capable of gating at 500 ps. We present results including the effect of different excitation wavelengths, showing that 355 nm excitation gives rise to significantly stronger stand-off Raman signals compared to that of 532 nm. We also show the effect of appropriate detector gating widths for discrimination of ambient light and the reduction of high background signals in the obtained Raman spectra. Our system can be used to identify explosives and their precursors in both bulk and trace forms such as RDX and PETN in the low mg range and TNT in the 700 μg range at a distance of 20 m, as well as detection of a 1% or greater H₂O₂ solution at a distance of 6.3 m.