Accessories for Diffuse Reflection

Accessories of this type have one major drawback. The particles in the top layer of the sample are often aligned so that one of their planes is parallel to the macroscopic plane of the sample (especially when the top of the sample is flattened with a spatula or razor blade). In this case, much of the front-surface reflection is collected along with the diffusely reflected reflection that has penetrated into the sample before reemerging from its top surface. The result is that bands become distorted and plots of/(/ oo) versus concentration become nonlinear at low concentration. Thus, DR accessories with an on-axis geometry are best used for qualitative measurements. 

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Fig. 1 Accessories for diffuse reflectance spectroscopy (A) Integrating sphere with hemispherical radiation collection (B) Accessory based on ellipsoidal mirrors, used within the sample compartment of the spectrometer (C) Rotational ellipsoidal mirror device with dedicated detector and (D) Bifurcated fiber optic-based accessory (also shown is the random mixture of fibers for illumination and detection compared with devices based on reflection optics the acceptance cone for radiation delivery and collection is limited and depends on the refractive indices of the core and cladding material).

Three types of devices have been described for the measurement of mid-infrared spectra on-axis accessories, off-axis accessories that are mounted in the sample compartment of the spectrometer, and integrating spheres. On-axis accessories for diffuse reflection spectrometry are very much like highly efficient specular reflection accessories. They usually have a higher optical efficiency than either of the other two designs. A typical design of a DR accessory with on-axis geometry is shown in Figure 16.2. 

As diffuse reflection spreads out over a large solid angle from the surface of a powder sample, an optical system is required to collect reflection radiation as efficiently as possible over a large solid angle. An example of an accessory for diffuse-reflection measurements is schematically illustrated in Figure 12.5. Two types of cups for diffuse-reflection measurements are commonly commercially available the diameter of the larger (ordinary) one... 

Optical Accessory Design for Diffuse Reflectance UV-vis-NIR Spectroscopy... 

The designs of various commercially available mirror optics for diffuse reflectance in IR or UV-vis spectroscopy are, in principle, similar to each other. Each of these accessories is characterized by six mirrors, four flat and... 

In a typical UV-VIS diffuse/directional instrument, the source, generally a tungsten halogen lamp, is placed inside an integrating sphere. The diffuse illuminated sample is imaged onto a slit and then to a polychromator and diode array. By having two sets of sample holders, one with a wedge and one at 0°, both specular-included and specular-excluded measurements can be made. The 0° sample holder ensures that the sample is not illuminated at 0° incidence and is thus specular excluded. A typical accessory for diffuse/ directional measurement of reflectance is shown in Fig. 17. 

Fig. 10.7 Accessory diagram for diffuse reflectance, interaction of radiation with the sample, and specular reflection, respectively...

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

All of the usual sampling techniques used in infrared spectroscopy can be used with FT-IR instrumentation. The optics of the sampling chamber of commercial FT-IR instruments are the same as the traditional dispersive instruments so the accessories can be used without modification for the most part. To make full use of the larger aperature of the FT-IR instrument, some accessories should be modified to accomodate the larger beam. The instrumental advantages of FT-IR allow one to use a number of sampling techniques which are not effective using dispersive instrumentation. Transmission, diffuse reflectance and internal reflectance techniques are most often used in the study of epoxy resins. 

The introduction of Fourier Transform Infrared Spectroscopy (FTIR) brought along a number of typical solid sample techniques. DRIFTS (Diffuse Reflectance Fourier Transform Infrared Spectroscopy) is probably most commonly known. Another technique, developed specifically for measuring solid, opaque samples is PAS (Photo Acoustic Spectroscopy). This accessory is less known, probably due to its high cost and its rather difficult modus operandi. 

Figure 12 The Spectra-Tech Collector diffuse reflectance accessory. A typical accessory for the study of particles and powders by diffuse reflectance FTIR. 

Fig. 4.1.4. Diffuse reflectance accessory (Spectra-Tech Inc.). / Movable stage with Blocker device to diminish the specular component of reflectance 2 two movable parts of the hemiellipsoidal mirror. The mirror opens for sample positioning 3 mirror system to reflect the IR beam from the source to the detector A macro sample holder B micro sample holder...

Figure 6.4-19 Optical schemes of different diffuse-reflection accessories, (a) in-plane configuration, usable with blocker (b) out-of-plane configuration (c) attachment for large samples (Korte and Otto, 1988).

Hedelman, S., and W. N. Mitchell (1968). Some new diffuse and specular reflectance accessories for the Cary models 14 and 15 spectrophotometer. In ... 

Various accessories were designed for recording diffuse reflectance spectra. Apart from special devices developed by different groups and described in the literature, several commercially available types must be noted. A few accessories are shown schematically in Fig. 1, which are representative of the diversity of optics. For a long time, integrating spheres have been in use, in particular for UV/VIS and near-infrared spectroscopy, although a few applications with sensitive MCT detectors also can be found within the mid-infrared. Usually, a baffle is placed within the sphere... 

Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously 9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K-M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). All Raman and infrared spectra were taken at 2 cm resolution. 

R. Marbach and H. M. Heise, Optical Diffuse Reflectance Accessory for Measurements of Skin Tissue by Near-Infrared Spectroscopy, Appl. Optics, 34(4), 610 (1995). 

Figure 9.24 Optical diagram of a diffuse reflectance accessory for an FTIR instrument.

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