Remote spectroscopy

Principles and Characteristics In on-line spectroscopic analysis, optical probes are often used for direct interrogation of chemical processes. The function of an optical probe is to 

Optical fibres can be used in the transmittance and ATR mode (a special ATR application is the remote sensor), and even in the reflectance mode. The development of special optical fibres for transmission, transflection or diffuse reflectance measurements favours on-line analysis of problematic product streams and reaction mixtures (solutions, suspensions, emulsions, melts, solids). Both quartz and fluoride (ZrF4-based) glass fibres are used, with the former having poor transmission characteristics above 2000 nm. 

Applications of the fibre optics transmittance or ATR probe are in quality control, reaction monitoring, skin analysis, goods-in checking, analysis at high and low temperature, radioactive or sterile conditions, and hazardous environments. Applications of the reflectance probe are for turbid liquids, powders, surface coatings, textiles, etc. By using an on-line remote spectrophotometer, real-time information is gathered about a chemical process stream (liquids, films, polymer melts, etc.), as often as necessary and without the need to collect samples. This determines more reliable process control. Remote spectroscopy costs less to maintain and operate than traditional techniques. Fernando et al. [48] have compared different types of optical fibre sensors to monitor the cure of an epoxy resin system. 

Schirmer et al. [47] have discussed the applications of chemical sensing using fibre optics and UV/VIS/NIR spectroscopy. 

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Advanced computerisation and sensorisation and developments in the field of multielement optical detectors (CCD and PDA) and fibre optic remote spectroscopy have added modularity and flexibility. Silica-silica fibres used for spectroscopy applications are multimode with core diameters from 50 to 1000 p,m. The application of new technologies to optical instrumentation (e.g. improved gratings in spectrographs, the use of... 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

Abstract Thin and flexible probes made with hollow-optical fibers may be useful for remote spectroscopy. Experimental results showed that these probes are useful for endoscopic measurements of infrared and Raman spectroscopy. A hollow-fiber probe has been used for remote FT-IR spectroscopy in the form of endoscopic measurement of infrared reflectometry spectra inside the body. This measurement was made possible by the hollow-fiber probe s flexibility, durability, nontoxicity, and low transmission loss. A hoUow-fiber probe with a ball lens at the end works as a confocal system for Raman spectroscopy. It can thus detect the molecular structure of biotissues with a high signal-to-noise ratio. Owing to their small diameter, the probes are useful for in vivo, noninvasive analysis using a flexible endoscope. 

A hollow-optical fiber is a prospective fiber-optic probe for infrared spectroscopy in medicine, owing to its nontoxicity and high mechanical and chemical stability [5]. However, it has been difficult to use it for remote spectroscopy because of its relatively high bending losses. Accordingly, the transmission efficiency should be improved by optimizing the fabrication conditions. 

The fundamental requirement of all radiation-transfer techniques in remote spectroscopy is that the radiation be transferred from the spectrometer to the sample, probe the reactions or transformations of interest and then return the modified beam of radiation to the spectrometer for the measurement of intensity at each wavelength in the spectral region of interest, all without any contributions from the transfer medium and with little loss in energy. While fibre-optics meets these requirements under favourable circumstances, there are potential artefacts and limitations, which may be understood from the principles of operation. 

The wavelength and angle dependences of the depth of penetration, d, were given earlier (Equation (3.22)), and this principle is used in the MIR for the sampling technique of ATR spectroscopy. Figure 3.13(c) illustrated this, and, for typical refractive indices of fibre core and sample of 2.5 and 1.5, respectively, and an angle of incidence of 45°, the depth of penetration of the evanescent wave is about 0.152. The effect of the evanescent wave on fibre-optic spectroscopy may be illustrated by the example of plastic-clad silica (PCS) fibre optics for remote spectroscopy in the NIR spectral region, as discussed below. 

Figure 3.40. Dead bands and negative bands arising in a transmission cell used for remote spectroscopy due to the core and cladding bands shown in Figure 3.39 (George et al, 1991).

Each of these questions can be answered provided that the data are collected with the appropriate level of precision. There are many specialized texts (Adams, 1995, Jolliffe, 1986, Malinowski and Howery, 1980, Pelikan et al, 1994) and review articles (Lavine, 2000) on chemometrics, and in the following section it is intended to address only those methods most commonly used in spectral analysis of chemorheological data using remote spectroscopy. 

Fiber-optic sensors are less expensive, more rugged, and smaller than electrodes in the future we may see the former replacing the latter in various areas of analytical and clinical chemistry. Fields of application include environmental monitoring, process control, remote spectroscopy in high-risk areas with radioactive, explosive, biological, or other hazards, titrimetry, and in-vivo bioanalysis. 

J. R. Berard, R. J. Burger, P. J. Melling, and W. R. Moser, Opitcal Fiber coupled Devices for Remote Spectroscopy in the Infrared, US patent 5,170,056. 

Le Coq D., Michel K., Fonteneau G., Hocde S., Boussard-Pledel C., and Lucas J., Infrared chalcogen glasses Chemical polishing and fibre remote spectroscopy, Int.J. Inorg. Mater., 3, 233-239 (2001). 

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