CORE-BASED INTRINSIC FIBEROPTIC ABSORPTION SENSOR FOR THE DETECTION OF VOLATILE ORGANIC-COMPOUNDS

A core-based intrinsic fiber-optic absorption sensor has been develope d and tested for the detection of volatile organic compounds. The dist al ends of transmitting and receiving fibers are connected by a small cylindrical section of an optically clear silicone rubber. The silicon e rubber acts both as a light pipe and as a selective membrane into wh ich the analyte molecules can diffuse. The sensor has been used to det ect volatile organics (trichloroethylene, 1,1-dichloroethylene, and be nzene) in both aqueous solutions and in the vapor phase or headspace. Absorption spectra obtained in the near-infrared (near-IR) provide qua litative and quantitative information about the analyte. Water, which has strong broad-band absorption in the near-IR, is excluded from the spectra because of the hydrophobic properties of the silicone rubber. The rate-limiting step is shown to be the diffusion through the Nernst ian boundary layer surrounding the sensor and not the diffusion throug h the silicone polymer. The rate of analyte diffusion into the sensor, as measured by the t(90) values (the time required for the sensor to reach 90% of the equilibrium value), is 30 min for measurements in aqu eous solutions and approximately 3 min for measurements made in the he adspace. The limit of detection obtained with this sensor is approxima tely 1.1 ppm for trichloroethylene in an aqueous solution.