Diffuse reflectance spectroscopy fiber optics

A steady-state diffuse reflectance spectroscopy instrument typically includes a broadband fight source, intermediate optics, spatially separated delivery-collection optical fiber probes,77 and a CCD-based grating spectrometer. Frequency-based approaches have also been pursued.78 Correlations between the glucose concentration and the tissue transport scattering coefficient have been observed.77,78... 

Diffuse Reflectance Spectroscopy with Fiber Optics... 

Fig. 1 Accessories for diffuse reflectance spectroscopy (A) Integrating sphere with hemispherical radiation collection (B) Accessory based on ellipsoidal mirrors, used within the sample compartment of the spectrometer (C) Rotational ellipsoidal mirror device with dedicated detector and (D) Bifurcated fiber optic-based accessory (also shown is the random mixture of fibers for illumination and detection compared with devices based on reflection optics the acceptance cone for radiation delivery and collection is limited and depends on the refractive indices of the core and cladding material).

Blanco, M. Coello, J. Iturriaga, H. Maspoch, S. de la Pezuela, C. Quantitation of the active compound and major excipients in a pharmaceutical formulation by near infrared diffuse reflectance spectroscopy with fiber optical probe. Anal. Chim. Acta 1996, 333, 147-156. 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

With the ever-increasing need to improve quality and productivity in the analytical pharmaceutical laboratory, automation has become a key component. Automation for vibrational spectroscopy has been fairly limited. Although most software packages for vibrational spectrometers allow for the construction of macro routines for the grouping of repetitive software tasks, there is only a small number of automation routines in which sample introduction and subsequent spectral acquisition/data interpretation are available. For the routine analysis of alkali halide pellets, a number of commercially available sample wheels are used in which the wheel contains a selected number of pellets in specific locations. The wheel is then indexed to a sample disk, the IR spectrum obtained and archived, and then the wheel indexed to the next sample. This system requires that the pellets be manually pressed and placed into the wheel before automated spectral acquisition. A similar system is also available for automated liquid analysis in which samples in individual vials are pumped onto an ATR crystal and subsequently analyzed. Between samples, a cleaning solution is passed over the ATR crystal to reduce cross-contamination. Automated diffuse reflectance has also been introduced in which a tray of DR sample cups is indexed into the IR sample beam and subsequently scanned. In each of these cases, manual preparation of the sample is necessary (23). In the field of Raman spectroscopy, automation is being developed in conjunction with fiber-optic probes and accompanying... 

For process control, analytical techniques such as vibrational spectroscopies, which provide information about paint composition, are important based on (1) the popularization of FT instruments, with a better signal-to-noise ratio and fast data acquisition speed (2) reflectance techniques (total attenuated and diffuse), photoacoustic spectroscopy, and the development of optical fiber-based devices that provide easy spectrometric measurements on crude samples (3) vapor-phase generation coupled with FTIR for fast analysis of the volatile paint fractions ... 

A validation of this approach should be possible by using in situ DR (Diffuse Reflectance) Vis spectroscopy to monitor the color of the VPO under process conditions. Following Rodemerck et al., this should reflect the oxidation state of the catalyst. It is noteworthy that Golbig and Werther [9] report different colors of the VPO catalyst behind the riser (brown) and behind the regenerator (dark green) in their lab-scale riser-regenerator experiments. Preliminary experiments with high temperature (< 600°C) fiber optics show satisfactory spectral and temporal resolution. 

Earlier reports focused on the use of optical absorption properties of tissues for clinical applications (Wilson and Jacques, 1990). An example is blood oximetry, which is widely used clinically to monitor continuously blood oxygenation with the help of an optical fiber probe as described in the previous section. In this method the diffuse reflectance also collected by the fiber is analyzed based on the differences in the absorption bands of oxy and deoxyhemoglobins. Diffuse reflectance from the skin can be used to monitor changes induced, for example, by the UV radiation. Endoscopic reflectance spectroscopy from mucosa of the gastrointestinal tract has been used to determine blood content and oxygenation (Leung et al, 1989). 

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