The Use Of Mid-Infrared Optical Fibers For Analytical Applications

Abstract

Fourier transform infrared (FT -M) spectrometry has general utility for chemical analysis.However, these analyses are limited by the requirement of the sample being accessible to the spectrometer and often undergoing some amount of preparation before analysis. This restriction has kept FT -IR from many potential applications, in particular, medical and industrial process control. The ability to make measurements at a remote site or as a reaction occurs offers a significant advance in infrared analysis. We have used a mid -IR fiber to transmit radiation outside of the spectrometer, to the sample, and then to the detector. A section of the fiber, with the protective cladding removed is used as the sampling device. In this declad region the fiber, acting as an internal reflectance element, contacts the sample and provides the chemical information for analysis. Two examples of the use of an optical fiber will be discussed. The first demonstrates the ease in which a liquid sample can be measured. The second demonstrates the use of a fiber to monitor the progress of a curing reaction in thermoset composite materials where the fiber was imbedded in the matrix. In addition to discussing the analytical utility; the instruments, fiber optic sampling, and optical characterization of the fibers will also be presented.

Theory of fiber-optic, evanescent-wave spectroscopy and sensors

“…This method has the advantages of fast, real-time, in situ, selective, nondestructive, and safe detection. It has been successfully employed in a variety of configurations for studies of solid-liquid interfaces, 3 chemical reaction rates, 4 complex material curing, 5 organometallic thin films, 6 liquid 7-10 and gas [11][12][13][14][15] detection and monitoring, concentration measurements, [16][17][18][19] and biological applications. 20,21 Usually the sensing elements are installed in a Fourier-transform IR 1FTIR2 spectrometer system, but recently tunable lasers were incorporated into FEWS systems 10,22 instead of conventional blackbody sources.…”

Sensor Systems Based on Mid‐Infrared Transparent Fibers

“…35,36 With a spectral window covering from about 2 to 11 µm and good resistance to moisture, several applications have been reported using chalcogenide fibers in direct sensing applications. Remote hydrocarbon gas sensing based on the FEWS principle has been demonstrated, 37,38 as has in-situ polymer curing monitoring, 22,39 remote monitoring of binary mixtures of water with e.g. acetone or ethyl alcohol, 40 and the determination of organic solutes in water.…”