Novel developments in mid-IR fiber-optic spectroscopy for analytical applications

Mid-infrared spectroscopy using the attenuated total reflection (ATR) technique plays an important role in analytical chemistry. Among others, silver halide fibers are promising for the development of remote sensor probes. Studies on long term stability under various storage conditions were carried out, which demonstrate excellent stability for appropriately encased fibers. For the first time, fibers of square cross-section shape are presented and their advantages for infrared spectroscopic applications in comparison to fibers of circular cross-section are demonstrated. Various probes, employing such fibers, were developed for measurements of liquid and inhomogeneous solid samples. A short, u-shaped fiber was directly coupled to an MCT-detector, enabling ATR-measurements of solid samples with a minimum spot size of 20 mum X 60 mum. Using the same optical setup, lowest concentration detection limits were achieved for aqueous solutions. For routine analysis of liquid samples, two types of probes were developed with the use of silver halide fibers of square cross-section. One ATR-probe contained an exchangeable, u-shaped piece of fiber, whereas the second probe was made of a fiber coupled diamond micro-prism: the latter is especially advantageous for small sample volumes to be measured or when extreme chemical inertness of the ATR-crystal is required. Measurements of different beverages were performed with high spectral reproducibility. For routine analysis demanding ultimate sensitivity, exchangeable sensor elements with an Ag2S coating were tested successfully to give an ATR-absorption enhancement by more than a factor of 10 compared to uncoated fibers. (C) 2001 Elsevier Science B.V. All rights reserved.