Whilst Raman is useful for measuring solids and liquids, the weak Raman scattering effect makes analysis of gases more challenging due to the diffuse nature of the target. Consequently, for gas analysis, alternative techniques are often employed, particularly Fourier Transform Infrared (FTIR) spectroscopy and Gas Chromatography (GC). However, whilst both are established, mature technologies, each has drawbacks: FTIR requires an additional analyser to identify homonuclear diatomic molecules. GCs, as well as being bulky and expensive, are complex and require a highly trained operator to run the measurements and interpret the results. Perhaps the biggest drawback with GC though, is the requirement for anticipatory column and detector selection based on the gases the operator is expecting to find.
ISI has been working on a Raman instrument capable of characterising substances in gaseous form. Previously, for Raman measurements of gases, it has been necessary to employ techniques to maximise the opportunity for collection of the Raman signal. These have included sample pressurisation; using a high-powered laser (or both); and increasing the path length through the gas sample to increase the laser-gas molecule interaction pathlength. Silver-coated capillary and mirrored chambers which allow the laser to be reflected multiple times through the gas sample are two examples of maximising the laser-gas interaction pathlength. Each approach has been demonstrated however sensitivity often remains a significant challenge, so ISI has been working on a hollow core micro-structured optical fibre based Raman instrument to overcome the sensitivity hurdle. The hollow fibre is filled with the target gas and the laser transmitted down the fibre. The unique guidance mechanism facilitated by the fibre results in the efficient transmission of the laser over extended lengths resulting in substantial laser-gas interaction path lengths and consequently increased sensitivity to gas species.
