Fiber Optic Detection of Ammonia Gas

Abstract

Bathochromic shifts accompanying the formation of several bivalent metallic complexes containing 5-(4’-dimethylaminophenylimino) quinolin-8-one (L1), and 7-chlore-5(4’-diethylamino-2-methylphenylimino) quinolin-8-one (L2) ligands in ethanol solutions were evaluated by VIS-NIR spectroscopy. The [L1-Cu-L1] sulphide complex was selected as a reagent for further tests on optical fibres. Samples of multimode siloxane-clad fused-silica fibre were sensitized by diffusing an ethanol/chloroform solution of the dye into the cladding polymer, and tested by VIS-NIR optical spectroscopy (12 cm long fibre sections), and optical time domain reflectometry (OTDR; 20 ns laser pulses, wavelength 850 nm, 120 m long fibre sensitized within the interval 104–110 m). A well-resolved absorption band of the reagent could be identified in the absorption spectra of the fibres. After exposure to dry ammonia/nitrogen gas with increasing ammonia concentration (0–4000 ppm), the short fibre samples showed subsequent decay of NIR optical absorption; saturation was observed for higher ammonia levels. The concentration resolution r ? 50 ppm and forward response time t90 ? 30 sec were obtained within the interval 0–1000 ppm. The OTDR courses showed an enhancement of the back-scattered light intensity coming from the sensitized region after diffusion of the initial reagent, and decay after exposure to concentrated ammonia/nitrogen gas (10000 ppm).