Diffuse reflectance probe

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. 

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. 

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. 

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

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... 

These detection schemes are necessarily invasive. Non-invasive methods have been developed based on measuring the spatial distribution of light, diffusely reflected from the tissue surface, as illustrated in Figure 15. This can be analyzed to determine jj, and and thereby to calculate the light distributions e.g. the fluence-depth/radius curve using Equations (3) and (4) or (5) and (4), or computational models for any given source configuration. Specialized diffuse reflectance probes have been developed, both for accessible body surfaces and for endoscopic application. At present these techniques are not in common use for clinical PDT. 

We investigated the PVC compounds with an NIR in-line diffuse reflectance probe on the end of a twin screw extruder (Viskosystem, Reifenhaeuser). The probe was connected via optical fiber cables to the NIR spectrometer. On the end of the extruder we adapted a melt-at-die interface for different probes. We used the diffuse reflectance fiber optic probe FDR 650 (Axiom Analytical Inc., USA) with a sapphire-window. The pressure in the melt-at-die interface varied in the 100 to 270 bar (1500 to 4000 psi) range and the processing temperature was 190 °C. 

A process diffuse reflectance probe that works in contact with the sample, is available from Hellma GmbH Co KG. The Model 661.761.NIR probe has seven 200 /rm diameter illumination fibers illuminating the sample at a 30" angle, and one 600 / m diameter core pickup fiber at the center of the illumination fibers. The DN 25/PN40 standard flange mounted probe withstands a temperature ranging from 5 to 140°C and 10 bar pressure. Chemical resistance can be enhanced with the Hastelloy C-22 body (Figure 37.40). 

FIGURE 37.40 Process NIR diffuse reflectance probe. (Courtesy of Hellma GmbH Co KG.)... 

A natural application for DRIFTS is particulate minerals and fillers because the nature of the surfaces can easily be determined. Chalk-filled PP was analysed using the diffuse reflectance probe because this material is not transparent. A calibration model with 18 samples using three relevant spectral regions (5307-6275, 6838-7505, 7987-8894 cm-i) was developed for quantification of the filler content [139]. The interfaces of various organic coatings (PAA, PMMA, oleic or stearic acid) with ceramic or silica glass surfaces were studied by means... 

Fischer et al. [151] investigated the simultaneous quantification of several additives in PVC with an in-line diffuse reflectance probe. In cases where... 

FT-MRS using transflectance and diffuse reflectance probes was particularly successful in morphology identification and differentiation (using... 

Fischer et al. [101] investigated the simultaneous quantification of the content of several additives in PVC with an in-line diffuse reflectance probe. The signal from diffuse reflectance can be affected by a number of physical properties of the sample, rather than just its chemical make up. This makes obtaining quantitative data very difficult. Chemometric analysis showed the possibility of detecting even small amounts of additives (3%) with an absolute prediction error of 0.3%. Step-scan PA-FTIR spectroscopic studies were used to study surfactant exudation and film formation in PS-nBA latex films [102]. 

According to Jakisch et al. [79], FTIR spectroscopy is the preferred method for in-line investigation polymer melts and polymer melt reac-tions/kinetics, allowing quantitative determination of all components. FTIR analysis of compound melts enables additive level stability and effectiveness to be observed over multiple extrusion passes. The use of the ATR principle is suitable for in-line analysis of polymer melts in the extruder. The exit of the extruder was equipped with an on-line IR transmission process control system consisting of a 150 /um thick ZnSe melt flow cell. Characteristics of such systems have been described [71,74]. Another process spectrometer with an in situ ZnSe-ATR dipper probe was mounted at different positions in the extruder. For in-line ATR the residence time plays no role. Only the first 5 /xm (corresponding to the penetration depth of the IR radiation) are examined. Minor components are thus detected with difficulty. Jakisch et al. [79] monitored the conversion of styrene-maleic anhydride copolymers (SMA) with fatty amines into styrene-maleimide copolymer (SMI) during reactive extrusion by means of FTIR. In principle, both mid-IR and near-IR spectroscopy with ATR, transmission and diffuse reflectance probes are suitable for quantitative on- and in-line process analysis of multicomponent polymer... 

Commercial package (available from Process Analysis and Automation Ltd., Farnborough, UK). Diffuse reflectance probe. 

Atline/in-line probes ATR probes Transmission transflection diffuse reflection probes... 

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