Applications of Fiber Optic Coupled-Grazing Angle Probe Reflection-Absorption FTIR Spectroscopy

Abstract

The recent increase in attention of detection of chemical threats, explosives and narcotics has led to the development of instruments and sensors that can be effective in a variety of operating environments. Various approaches can be used for in situ analysis of explosives, including the widely used technique, Ion Mobility Spectrometry (IMS). The major advantages of IMS are its sensitivity in the picogram range, its continuous real time monitoring capability, its reasonable price due to instrumental simplicity and the ease of automation (Salleras, 1995). A main disadvantage of IMS is its limited linear range and that it cannot be used for quantitative analysis (Salleras, 1995). It is relatively easy to overload an IMS and, therefore, sample size must be controlled with care (Brambilla, 1997). Another weakness is the response variation that occurs with different background gas compositions and with different sample compositions (Salleras, 1995). However, spectroscopic techniques have the potential to afford the best selectivity for explosives. The infrared spectra of molecules can provide an information-rich fingerprint that allows for near unambiguous identification. A few years ago, direct detection by infrared absorption spectroscopy was not possible because of the limited sensitivity of this method. Fourier Transform Infrared Reflection Absorption Spectroscopy (IRRAS), operating at the grazing-angle, is the most sensitive optical absorption technique available for measuring low concentrations of chemical compounds adhered to reflective surfaces such as metals (Griffiths, 1986). The disadvantage of conventional spectroscopic techniques for applications such as explosives detection is that the test materials must be placed physically within the spectrometer’s sample compartment for measurement. FT-IRRAS combined with grazing angle probe (GAP) can now be used outside the boundaries of the sample compartment. Fiber-optic cables (FOCs) that transmit in the mid-IR (MIR) range have made it possible to develop a range of spectroscopic probes for in situ analysis (Melling, 2001; Melling, 2002; Mehta, 2003; Bacci, 2001). Thus, FTIR spectroscopy can now be effectively used outside the confinement