Reflectance probe

A diffuse reflection probe is used to measure the light reflected from a solid surface or powder. This probe is immune to specular reflections from the sample, and detects only diffusely reflected light. 

As differentiated from diffuse reflection, we use the term backscatter to mean a probe that detects both specular and diffuse light. The most common designs of these probes incorporate a number of illumination fibers (usually six) in a ring around a central detection fiber. This probe is useful in a slurry or solution high in particulates (e.g. crystallization). 

Sometimes, a probe must be shielded by a protecting glass (or quartz) tube because it is not catalytically inert with respect to the reaction mixture, or does not withstand its corrosive influence. In that case, it must be ascertained that the shielding does not falsify the signal, e.g. by specular reflection on the glass. 

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Reflectometry is a useful probe with which to investigate the structure of multilayers both in self-supporting films and adsorbed on surfaces [51]. Specular X-ray reflectivity probes the electron density contrast perpendicular to the film. The X-rays irradiate the substrate at a small angle (<5 °) to the plane of the sample, are reflected, and are detected at an equal angle. If a thin film is present on the surface... 

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. 

Fig. 2.6. Schematic illustration of the experimental setup for pump-probe anisotropic reflectivity measurements with fast scan method. PBS denotes polarizing beam splitter, PD1 and PD2, a pair of matched photodiodes to detect p- and s-polarized components of the reflected probe beam, PD3 another photodiode to detect the interference pattern of He-Ne laser in a Michelson interferometer to calibrate the scanning of the pump path length...

Design and selection of the sample interface is vital to provide the best-quahty data for an analysis. The sample interface may be located in the sample cavity of a spectrophotometer, as in the cases of laboratory cuvettes, vials, and flow cells. The sample interface may also be fiber-coupled and located closer to the process. Fiber-optic sample interfaces include flow cells, insertion probes, and reflectance probes. 

In the worse case, where either sample temperature, pressure or reactor integrity issues make it impossible to do otherwise, it may be necessary to consider a direct in situ fiber-optic transmission or diffuse reflectance probe. However, this should be considered the position of last resort. Probe retraction devices are expensive, and an in situ probe is both vulnerable to fouling and allows for no effective sample temperature control. Having said that, the process chemical applications that normally require this configuration often have rather simple chemometric modeling development requirements, and the configuration has been used with success. 

J.H. Cho, P.J. Gemperline, P.K. Aldridge, S.S. Sekulic, Effective mass sampled by NIR fiber-optic reflectance probes in blending processes. Anal Chim. Acta, 348, 303-310 (1997). 

Internal reflection spectroscopy is widely applied for on-line process control. In this type of application, the chemical reactor is equipped with an internal reflection probe or an IRE. The goal of this type of application is the quantification of reactant and/or product concentrations to provide real-time information about the conversion within the reactor. In comparison with other analytical methods such as gas chromatography, high-pressure liquid chromatography, mass spectrometry, and NMR spectroscopy, ATR spectroscopy is considerably faster and does not require withdrawal of sample, which can be detrimental for monitoring of labile compounds and for some other applications. 

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

D. Optical Reflectance Probe Bis(cyclopentadienyl)iron(II) Size Exclusion and Intrazeolite Chemistry... 

Sample Transparent, bubble and particulate-free liquids Powder samples require diffuse reflectance probe Transparent liquid, thin solid pellets and ideal for gases Special ATR probes needed Same probe is used for all samples liquids, slurries, emulsions, powders, solids, samples with particulates, and bubbles... 

Applied to the use of reactions to locate the position of a probe in a middle, the Curtin-Hammett principle suggests that not only the site of the probe but its reactivity at various sites must be considered. For example, if the reactivity of a probe at various sites along a detergent chain were independent of site, reactivity would reflect probe position, i.e., the extent of product formation would related to the concentration of probe at each site. If on the other hand, if the reactivity of the probe were much greater near the head of the detergent chain or near the tail... 

With optically dense dispersions such as occur in plate columns, the transmission method may fail because of intense multiple scattering. In such cases a reflectivity probe may be used (C5), where the optical reflectivity or backward scattered light from the dispersion is measured. The specific interfacial area is calculated by... 

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