DRIFTS specular reflection

Diffuse reflection iavolves reflecting the iafrared beam off of a soHd sample, as ia specular reflectioa, but it is the aoaspecular portioa of the reflected radiatioa that is coUected. Whea an ftir spectrometer is used, diffuse reflection is caUed DRIFTS (diffuse reflectance iafrared Fourier-transform... 

Thus, for Knudsen cosine scattering, / = 1, and for specular reflection, / = 0. Equation (59) may be solved for the drift velocity of the scattered molecule to give Uf = (1 - f)ut. The viscous force transmitted to the wall during gas collisions is the product of the number of collisions per second and the momentum change per collision,... 

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is used to obtain spectra of powders and rough polymeric surfaces such as textiles and paper. IR radiation is focused onto the surface of the sample in a cup resulting in both specular reflectance (which directly reflects off the surface having equal angles of incidence and reflectance) and diffuse reflectance (which penetrates into the sample subsequently scattering in all angles). Special mirrors allow the specular reflectance to be minimized. 

For DRIFT studies, a wood wafer, paper sheet, or milled wood sample dispersed in KBr (or KC1) is placed in a cup at the focal point of the concave, ellipsoidal mirror so that the incident light is focused on the sample. The scattered light coming from the sample is collected from the concave mirror and directed by a suitable mirror system to the detector of the FTIR instrument. The pressure used for smoothing the sample has to be adjusted so that reproducible results can be obtained (Yeboah et al. 1984). The contribution of specular reflectance can be diminished by reducing the particle size and by increasing the sample dilution. For powder samples, as indicated above, the diluent is KBr or KCI. Good results are normally obtained with alkali halide powders that contain 1-2% of sample. In certain cases, the sample concentration may be increased up to 10%. 

One important practical problem in DR measurement is the need to cancel the specular reflectance from the front surface of the sample, which generates negative bands in the DRIFT spectrum, so causing apparent shifts in the true absorption bands. This is achieved by using appropriate cell designs in commercial... 

One of the major sample-handling problems in FTIR analysis of carbonaceous materials is that many of them are effective blackbody absorbers and thus are too opaque for direct transmission analysis in the midinfrared spectral region. Addition of KBr intensifies the signal to obtain transmission infrared spectra. It is time consuming, and grinding conditions and moisture are known to affect the spectrum of the sample [238]. Alternative techniques such as specular reflectance, diffuse reflectance (DRIFT), photoacustic spectroscopy (FTIR-PAS), and total... 

IR spectroscopy is one of the few analytical techniques that can be used for the characterization of solid, liquid, and gas samples. The choice of sampling technique depends upon the goal of the analysis, qualitative identification or quantitative measurement of specific analytes, upon the sample size available, and upon sample composition. Water content of the sample is a major concern, since the most common IR-transparent materials are soluble in water. Samples in different phases must be treated differently. Sampling techniques are available for transmission (absorption) measurements and, since the advent of FTIR, for several types of reflectance (reflection) measurements. The common reflectance measurements are attenuated total reflectance (ATR), diffuse reflectance or diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and specular reflectance. The term reflection may be used in place of reflectance and may be more accurate specular reflection is actually what occurs in that measurement, for example. However, the term reflectance is widely used in the literature and will be used here. 

Figure 4.25. Optical diagram of DRIFTS accessory (1) flat mirror (50 x 50 mm), (2) flat mirror (70 X 70 cm), (3) concave spherical reflector, and (4) sample cell D = diffuse reflection S = specular reflection. Dashed lines show optical path of diffuse reflection. Solid bold line shows optical path of specular reflection. Reprinted, by permission, from B. Li and R. D. Gonzalez, Appl. Spectrosc. 52, 1488-1491 (1998), p. 1489, Fig. 1. Copyright 1998 Society for Applied Spectroscopy.

Another mechanical solution to the problem of Fresnel specular reflectance is cutting off the specular component at the exit aperture of the accessory, as shown in Fig. 4.25 [154]. The waveguide that filters the KM component from the scattering radiation in the cup-on-the-saucer DRIFTS accessory (Fig. 4.19) may also be regarded as a mechanical device. 

It is possible to analyse the filler in an undiluted state using DRIFTS, however, specular reflection and excessively strong absorption can lead to poor quality spectra featuring negative peaks. It is now widely accepted that the filler sample must be diluted with an IR transparent powder such as KBr or KF. The latter must be finely ground and ideally the same batch of ground diluent must be used for a whole sample series. The level of dilution should be such that there is no compression or saturation of the spectrum. This can be observed by comparison of the DRIFTS spectrum with a transmission spectrum of the untreated filler in a KBr disk. Usually 1-10% w/w filler in the diluent will give uncompressed spectra. 

Figure 3.15 Schematic diagram of DRIFTS sample holder with specular reflection blocker...

In addition to specular reflectance, diffuse reflectance can also be used to study interfaces. Six compounds adsorbed on pretreated aluminum surfaces were studied by Kollek.(26) Figure 18 is the DRIFT spectrum of dicyandiamide in potassium bromide and adsorbed on CAA aluminum 2024-T3 clad alloy. The intensity of the nitrile peaks at 2207 and 2164 cm - in the dicyandiamide in a potassium bromide sample (upper spectrum) decreased in the dicyandiamide adsorbed onto CAA aluminum sample (lower spectrum), suggesting that a chemical reaction occurred between the nitrile group in the dicyandiamide and the aluminum oxide with new peaks resulting from this reaction assigned to an amide group. 

Figure 5.33 Simplified DRIFTS. The sample is mixed with KBr and placed in a cup in a DRIFTS accessory. Light from the interferometer hits the surface at an angle, and the specularly reflected light is blocked while the diffusely reflected light is captured by a curved mirror and directed toward the detector. The sample spectrum is ratioed against KBr. 

PA-FTIR depth profiling results are consistent with the known layer stmcture of a packaging laminate film and an adhesive label [477]. Doublelayered PET/PET, PP/PET and PET/PP laminates were studied by PA-FTIR [478]. PA-FTIR and DSC identified a skin layer and a core in injection moulded PET plates [479]. Plastic-coated paper was analysed by both PA-FTIR and DRIFTS allowing for shallow- and deep-sampling, respectively [453]. PA-FTIR is also a suitable tool for the analysis of polymer films used as a barrier coating on beverage and food containers at variance to specular reflectance measurements surface flatness is not critical. 

Specular reflectance (Fresnel) Diffuse reflectance (DRIFTS) Reflection-absorption (RA) Grazing angle... 

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