OPTICAL SENSORS BASED ON INHOMOGENEOUS WAVE-GUIDING IN THE WALLS OF CAPILLARIES (CAPILLARY WAVE-GUIDE OPTRODES)

Glass capillaries with a chemically sensitive coating on the inner sur face are used for optical chemical sensing in gaseous and liquid sampl es. The light from an LED or a laser is coupled into and out of the ca pillary under a well-defined angle. The coated glass capillary acts as an inhomogeneous waveguide, in which light is guided in both the glas s and the coating. The portion of the light that is absorbed in the ch emically sensitive coating is a function of the concentration of a che mical analyte. Typical relative light-intensity signal changes with th is type of optical interrogation are as high as 98%. Such capillary wa veguide optrodes possess several attractive features: (a) they can ser ve as sample cavities or flow cells for gases and liquids; (b) they ar e suitable for direct sampling; (c) their relative signal change can b e optimized by adjusting the optical pathlength; (d) the optical pathl ength is much longer compared to other optrode designs based on absorp tion or reflection, therefore the coatings can be made thinner, and le ss loaded with chemically active components; (e) the sensors are hardl y cross-sensitive to color and turbidity of the sample; (f) cheap ligh t sources and detectors can be used in connection with these novel cap illary waveguide optrodes.