Fibre-optic reflectance probes

Measurement of surface scattering from opaque surfaces (44) has been made with a fibre-optic reflectance probe, for determination of demineralization of tooth enamel, milk-fat content and the whiteness of the eyeball. A reference fibre optic probe on a white BaS04 calibrated surface provides a differential measurement. 

Figure 5. Single-point IR sensor head layouts a transmission probe with fibre coupling b transflectance probe with variable pathlength and single fibre coupling c (diffuse) reflection probe with single illumination fibre and collection fibre bundle d two-reflection ATR probe with fibre-optic coupling e multi-reflection ATR probe (DiComp -type) f ATR fibre...

In practice, very few applications of FEWS sensors can be found outside laboratory applications and demonstration systems. In the near-IR, suitable fibres are readily available but usually there is no real necessity to use them. Possible transmission pathlengths are sufficiently large to allow using standard transmission probes, while turbid samples can be measured using transflection or diffuse reflection probes. In the mid-IR, high intrinsic losses, difficulties in fibres handling and limited chemical and mechanical stability limit the applicability of optical fibres as sensor elements. 

M. Blanco, J. Coello, H. Iturriaga, S. Maspoch, C. Pezuela and E. Russo, Control analysis of a pharmaceutical preparation by near-infrared reflectance spectroscopy a comparative study of a spinning module and fibre optic probe, Anal. Chim. Acta, 298, 183-191 (1994). 

Reeves, J.B. Ill and McCarty, G.W. (2001) Quantitative analysis of agricultural soils using near infrared reflectance spectroscopy and a fibre-optic probe. Journal of Near Infrared Spectroscopy 9, 25-34. 

Gonzalez-Martin, I., Hemandez-Hierro, J. M., Vivar-Quintana, A., Revilla, I., and Gonzalez-Perez, C. (2009). The application of near infrared spectroscopy technology and a remote reflectance fibre-optic probe for the determination of peptides in cheeses (cow s, ewe s, and goa t s) with different ripening times. Food Chem. 114,1564—1569. 

Figure 3.31 Fibre-optic-coupled insertion transmittance and diffuse reflectance probes. (Courtesy of Axiom Analytical Inc., www.goaxiom.com)...

As discussed later, in Section 3.4, this technique is particularly well adapted to coupling to optical fibres, so a probe that can be immersed in a polymer reaction stream may be fabricated to enable spectra to be collected in real time. The principles of total internal reflection and its application in fibre-optics are discussed in more detail in Section 3.4.1. 

Double beam spectrophotometers allow differential measurements to be made between the sample and the analytical blank. They are preferable to the single beam instruments for cloudy solutions. The bandwidth of high performance instruments can be as small as 0.01 nm. For routine measurements such as monitoring a compound on a production line, an immersion probe is employed. Placed in the sample compartment of the apparatus this accessory contains two fibre-optics, one to conduct the light to the sample and another to recover it after absorption in the media studied. Two types exist by transmission for clear solutions and by attenuated total reflection (ATR) for very absorbent solutions (Figure 9.17). 

Figure 9.17 The principle of a spectrophotometer fitted with an immersion probe. Monochromatic light issuing from a spectrophotometer is guided towards an immersion cell and then returned to the detector. The route is confined by a fibre optic. Left transmission probe. Right ATR probe the sapphire prism has a refraction index greater than that of the solution. The schematic shows three reflections of the beam and its penetration into the solution (see explanation in Chapter 10, Section 10.9.3).

Hypoxia can now be detected and quantified by microelectrodes and the binding of reactive, reduced metabolites of nitroimidazoles, which reflect oxygen levels (because the radical-anion reduction intermediate is oxygen reactive) [30]. A new fibre-optic probe for tissue oxygenation, now available commercially, relies on measurements of emission from a ruthenium probe, which has an oxygen-dependent excited-state lifetime [31]. Acute or fluctuating hypoxia, which has features in common with ischaemia/reperfusion and therefore with myocardial infarction and stroke, can be detected by vascular stains such as bisbenzimidazole intercalators with different fluorescent characteristics. These can be administered intravenously a few minutes apart ... 

IR-transmitting optical fibres are evanescent wave sensors using a mathematical deconvolution technique to extract the absorbances and follow the concentrations of the components as they occur in both laboratory scale and process production. The fibre-optic probe used can be placed at specific locations within the samples or at the surface. The specificity of the technique, the speed of data acquisition and the portability of equipment make this method ideal as a tool to fundamentally probe polymer reactions and processes. Chalcogenide optical fibres are used to direct IR radiation from an FUR spectrometer through an attenuated total reflection (ATR) probe immersed in a reactor and back to the spectrometer. 

One of the first attempts to model NIR data and texture properties in beef was presented by Mitsumoto et al. (33). This report presented fitted correlations between four wavelengths and shear values from beef to 0.83 for NIR reflectance, 0.80 for NIR transmittance and 0.80 for NIR with a reflectance fibre-optic probe. 

I. Gonzalez Martin, C. Gonzalez Perez, J. Hernandez Mendez, N. Alvarez Garcia, J. L. Hernandez Andaluz. On-line non-destrnctive determination of proteins and infiltrated fat in Iberian pork loin by near infrared spectrometry with a remote reflectance fibre optic probe. Anal Chimica Acta 453(2) 281-288, 2002. 

Manufacturers rely on the ability of MRS for offline identification and classification purposes [297, 298] and on-line for process analysis [299]. In particular, the use of MR fibre optic probes for diffuse reflectance measurements and the minimal requirement for sample preparation minimises the likelihood of additional polymorphic forms being introduced during measurements. 

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. 

Specular reflectance is a much less common technique in IR analysis. It is not presently used for process analysis. It is more likely to find use in QC and product identification. It is also of interest to notice that a non-contact surface analyser uses an IR spectroscopic reflectance probe to obtain information without touching the surface. The probe is connected through mid-IR fibre-optic cables to an IR... 

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