Fast-responding, fibre-optic based sensing system for the volatile anaesthetic halothane, using an ultraviolet absorption technique and a fluorescent film

An improved version of a fast-responding sensing system for the widely used volatile anaesthetic halothane (2-bromo-2-iodo-1,1,1-trifluoroethane) is described. The concentration of halothane is determined using an ultraviolet radiation (UV) absorption technique. Ultraviolet radiation at 230 nm is conveyed by a silica optical fibre to a gas flow-through cell containing the halothane, and the intensity of the UV reaching the other side of the cell is measured using a fluorescent polymer film. This paper describes the development of an efficient fluorescent polymer film for the sensor based on poly(ethylene glycol) containing two fluorophores {2,5-diphenyloxazole and tris[4,4,4-trifluoro-1-(2-thienyl)butane-1,3-diono]europium(III)}. The film fluoresces strongly with a red line spectrum when excited in the range from about 200 to 380 nm. There is evidence of direct energy transfer between the two fluorophores. A similar effect is observed in films prepared from Carbowax 20M and the europium chelate. An advantage of this approach is that the fluorescent radiation can be transmitted back to a silicon photo-detector using an inexpensive polymer optical fibre bundle. Two experimental sensor systems for halothane are described and the results show that, although the response does not obey the Beer–Lambert law, a reliable system for determining halothane can be constructed, which operates over the medically important range 0–3%. This paper also describes how the signal-to-noise ratio of the system can be improved by using the long fluorescence lifetime of the film. A system for relaying the fluorescent radiation efficiently to polymer optical fibres is described, which is based on a clear film of poly(methyl methacrylate) containing the two fluorophores. The sensor head is compact and has a fast response time, thereby making it suitable for end-tidal respiratory gas measurements. The sensing system could also be used with other gases or liquids that absorb in the range 200–380 nm.