Optical reflectance probes

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). 

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

Herbert, P.M., Gauthier, T.A., Briens, C.L., and Bergougnou, M.A., "Application of Fiber Optic Reflection Probes to the Measurement of Local Particle Velocity and Concentration in Gas—Solid Flow", Powder Tech., 80, 243 (1994). 

Having established that the multi-wavelength transmission techniques offer a number of advantages, the Uv-vis reflectance spectra from concentrated polystyrene lattices have been recorded using fiber optics reflection probes. Although some averaging of the individual particle properties can be expected, reliable calibration and estimation of the particle properties is possible, since discrimination to the desired particle properties is found as it will be shown below. 

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. 

Fiber optic spectrometers with reflectance probes build an additional degree of flexibility into reflectance spectroelectrochemical measurements. Fiber optic reflectance probes are typically made of multiple fiber optics which direct the light from the source to the sample surrounding one or more fiber optics which collect the reflected light and direct it to the detector. These fibers are usually combined within a single cable which is bifurcated... 

Herbert PM, Gauthier TA, Briens CL, Bergougnou MA. Application of fiber optic reflection probes to the measurement of local particle velocity and concentration in gas solid flow. Powder Technol 80 243-252, 1994. 

Cho, J., et al.. Effective mass sampled hy NIR fiher-optic reflectance probes in blending processes. 

Reflective probes may be used over the entire rang of particle concentrations, from extremely dilute flows to the fixed bed state for several powder types in gas or liquid media, once they have been suitably calibrated [63, 85, 166, 238]. These probes are also nearly free of interference by temperature, humidity, electrostatics and electromagnetic fields. Lischer and Louge [116] developed a reflective F-OPT specifically for measuring solids concentration. In the fiber optical reflection probe developed by Hartge et al. [82], the emitted light from a laser diode reaches the... 

The charge transport and optical properties of the [Si(Pc)0]-(tos)y)n materials as y=0 -+ 0.67 are reminiscent of the [Si(Pc)0]-(BF4)y)n system, but with some noteworthy differences. Again there is an insulator-to-metal transition in the thermoelectric power near y 0.15-0.20. Beyond this doping stoichiometry, the tosylates also show a continuous evolution through a metallic phase with decreasing band-filling. However, the transition seems somewhat smoother than in the BF4 system for y)>0.40, possibly a consequence of a more disordered tosylate crystal structure. Both [Si(Pc)0]-(tos)y)n optical reflectance spectra and four-probe conductivities are also consistent with a transition to a metal at y 0.15-0.20. Repeated electrochemical cycling leads to considerably more decomposition than in the tetrafluoroborate system. 

Si(Pc)0] (S04)o.09)n> i-s limited by the oxidative stability of the sulfate anion. Thermoelectric power, optical reflectivity, magnetic susceptibility, and four-probe electrical conductivity measurements evidence behavior typical of an [Si(PcP+)0]n compound where p 0.20. That is, there is no evidence that the more concentrated counterion charge has induced significant localization of the band structure. 

As the potential of optical fiber probes for pH measurements was rapidly recognized, several other articles appeared within a few years75 83. Most were reflectance-based, and Seitz reported the first fluorescent pH sensors84, 78. The article by Janata85 on whether pH optical sensors can really measure pH is another "must" in the early literature since it points to aspects hardly addressed in pH sensor work. 

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. 

When metals have Raman active phonons, optical pump-probe techniques can be applied to study their coherent dynamics. Hase and coworkers observed a periodic oscillation in the reflectivity of Zn and Cd due to the coherent E2g phonons (Fig. 2.17) [56]. The amplitude of the coherent phonons of Zn decreased with raising temperature, in accordance with the photo-induced quasi-particle density n.p, which is proportional to the difference in the electronic temperature before and after the photoexcitation (Fig. 2.17). The result indicated the resonant nature of the ISRS generation of coherent phonons. Under intense (mJ/cm2) photoexcitation, the coherent Eg phonons of Zn exhibited a transient frequency shift similar to that of Bi (Fig. 2.9), which can be understood as the Fano interference [57], A transient frequency shift was aslo observed for the coherent transverse optical (TO) phonon in polycrystalline Zr film, in spite of much weaker photoexcitation [58],... 

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. 

Figure 9.11 Spectroelectrochemical cells using a bifurcated optical fiber probe (a) mercury pool working electrode (b) reflective metal working electrode. [From Ref. 67.)...

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

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