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UK scientists have developed a new approach for enhancing fluorescence and Raman signals recorded from turbid media in a conventional backscattering geometry. The technique, which offers a "substantial enhancement in observed signal", was achieved by employing a custom-made dielectric band-pass filter, which sits directly in front of the sample in the path of the laser. The resulting enhancement in signal allows the use of shorter acquisition times. The new technology developed by Kevin Buckley, Neil Macleod, Anthony Parker, and Pavel Matousek of the STFC's Rutherford Appleton Laboratory, in Oxfordshire and Allen Goodship of the Royal National Orthopaedic Hospital in Stanmore, Middlesex has potential applications in pharmaceutical research, forensic science and security screening. Buckley and colleagues explain how signal intensity in fluorescence and Raman spectroscopy is important for reaching desirable sensitivity levels with diffusely scattering media. The analysis of pharmaceutical products, for instance, would benefit from improved signal-to-noise ratios stemming from the higher signal levels. One of the main regions in which photons, and ultimately sensitivity, are lost, is at the sample/air interface at a point of the laser illumination. "For opaque samples such as powders, over 90% of the incident laser radiation can be scattered backward after travelling through a sample only a few millimetres thick," the researchers point out. By coupling the laser radiation into the sample with much greater efficiency, the researchers hoped to overcome this obstacle as a means to significantly improve sensitivity and/or reduce data acquisition times. The "unidirectional" dielectric mirror, placed directly in front of the sample, "permits laser light that escapes from the sample surface to be reflected back into the sample where it can be more usefully employed in generating Raman and fluorescence signals," the researchers explain, "This leads to improved Raman signal, higher signal-to-noise ratio, and shorter acquisition times." Proof of principle tests were performed on standard pharmaceutical tablets and on sheets of polytetrafluoroethylene (PTFE, or Teflon) with a single enhancing element. They were able to demonstrate signal enhancement factors of six and three in fluorescence and Raman experiments, respectively. The choice of a suitable dielectric material means that photons shifted to longer wavelength (due to fluorescence or Raman scattering) can pass through it and on to the detection system of a conventional backscattering spectrometer. In addition, the team suggests that conventional broadband mirrors can be used to bounce back light at any of the other air-sample interfaces and so reduce photon loss still further at all wavelengths. The team adds that aside from boosting sensitivity and shortening acquisition times, the new approach will be especially useful in situations where safety or other constraints mean the laser cannot be concentrated on to a very small area, for example, when probing human skin, analysing putatively explosive powders or other hazardous materials. "The solution presented here is fully compatible with the defocused laser beams used in such conditions," the researchers say. Related links:
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Matousek, reflecting on Raman |
