Diffuse reflectance accessories types

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT).6 When IR radiation is directed onto the surface of a solid sample, two types of energy reflectance can occur specular and diffuse. The specular component is the radiation that reflects directly off the sample surface (i.e., not absorbed by the sample). Diffuse reflectance is the radiation that penetrates into the sample and then emerges. Diffuse reflectance accessories are designed to optimize the diffuse reflected energy and suppress the specular component. The optics therefore selectively directs the scattered radiation to the IR detector. 

Electronic absorption and diffuse reflectance spectra (ESDR) were obtained with a "Specord M-40" spectrophotometer. IR spectra were recorded with a "Perkin Elmer FT-IR 1725X" spectrophotometer provided with diffuse reflectance accessory for solid samples. EPR spectra were recorded with a SE/X-2543 spectrometer at 77 K and 300 K. Primary treating and simulation of EPR spectra were carried out by special algorithms using IBM PC/XT type computers. 

Fig. 5. Type I illumination system with integrating sphere (from Cary Model 1711 diffuse reflectance accessory manual).

Am Assoc of Cereal Chemists, St. Paul, MN, 330pp Yang PW, Mantsch HH, Baudais F (1986) A critical evaluation of three types of diffuse reflectance infrared accessories. AppI Spectrosc 40 974-978 Yasuda T, Sakakibara A (1981) Hydrogenolysis of protolignin in compression wood. V. 

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... 

In a similar vein, Wilson et al. [61] recorded the FTIR spectra of fruit jams of different types using a diffuse reflectance (DRIFT) samphng accessory. Despite distortions in the spectra, associated with the use of the DRIFT technique, the different jams were found to exhibit characteristic and reproducible patterns, indicating that jams of different fruit content can be differentiated on the basis of their FTIR spectra. The authors suggested the potential utihty of this technique for the detection of adulteration of jams, for instance, by the substitution of fruit content by cheaper vegetable material. 

Coarse or hard powders are not well served by either the compressed pellet or mull technique, mainly because of difficulties associated with grinding. In such situations, the best approaches require the use of an accessory, such as a diffuse reflectance or photoacoustic detector. Both diffuse reflectance and photoacoustic methods [99,100] may be applied to most forms of powdered solids. As a rule, photoacoustic measurements, which are the only form of true absorption measurement, are not significantly influenced by sample morphology. An alternative procedure for powders is ATR, especially a horizontal accessory, preferably equipped with a pressure applicator. Note that the use of pressure is recommended to ensure intimate contact between the sample and the IRE (internal reflectance element) surface. Normally, the sample must conform to the surface of the IRE, and because the strength of the IRE is typically limited, the procedure is recommended only for soft powders. However, with the introduction of diamond-based ATR accessories [101-103], it is possible to handle most types of powdered material. 

Performance. Modern FTIR spectrometers, even the least expensive types, generally offer a level of performance that is more than adequate for simple sampling techniques— such as KBr discs or mulls—and will normally give excellent results with more sophisticated accessories such as ATR or diffuse reflectance. Only the most demanding accessories (microscopes or photoacoustic devices) or technically difficult experiments (step-scanning or high-speed and time-resolved work) require more expensive machines. The prospective customer who requires FTIR for raw material confirmation and occasional forensic analysis can be confident that whatever machine is chosen should be up to the job. 

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. 

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. 

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... 

Several accessories are available for the higher end spectrophotometers, which enable performing different modalities of measurement on different types of samples, as for example, solid films, liquid solutions, powders, etc. Some of the modalities commonly available are transmittance, diffuse transmittance, diffuse and specular reflectance, and variable angle specular reflectance. Instruments... 

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