Specular-reflection spectra analysis

Many techniques are based on this principle and can be used for the analysis of all types of samples. The spectrum obtained from reflected light is not identical to that obtained by transmittance. The spectral composition of the reflected beam depends on the variation of the refractive index of the compound with wavelength. This can lead to specular reflection, diffuse reflection or attenuated total reflection. Each device is designed to favour only one of the above. The recorded spectrum must be corrected using computer software. 

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... 

The thin films or coatings can be studied nondestructively, with no sample preparation other than deposition on a polished metal surface if necessary. Specular reflectance has been used to study lubricant films on computer disks, oxide layers on metal surfaces, paint curing as a function of time, and molecules adsorbed on surfaces. For example, the IR absorption spectrum of proteins adsorbed onto a polished gold surface can be studied. This spectrum from an adsorbed layer can form the basis of sensors for compounds that will bind to the proteins and change the spectrum. Use of a polarizer in conjunction with grazing angle analysis can provide information about the orientation of molecules adsorbed onto surfaces. 

Sloane and co-workers [129] described a specular reflectance system for the infrared analysis of micro-sized samples. They compare the advantages and limitations of this technique with other micro infrared techniques. Samples are mounted on small metal mirrors (Figure 4.20), which reflect the light beam back through the sample. A transmission spectrum is obtained but the effective path length is twice that of the actual sample thickness and a given absorption band consequently has twice the absorbance obtained by conventional transmission measurements. This system was applied successfully to gas chromatographic fractions, and is particularly useful for the examination of non-volatile liquids such as, for example, dioctyl phthalate. Crystalline solids are easily deposited and... 

Further, the techniques of Scanning Electron Microscopy (SEM), Specular Reflectance Infrared Spectroscopy (SRIS), and Electron Spectroscopy for Chemical Analysis (ESCA) have proved complementary in this investigation of the relationships between adherend surfaces and adhesive properties. As SRIS results have shown the presence of adhesive on all fracture samples, ESCA and SEM results further clarified the nature of the fracture surface through the presence or absence of a Ti ESCA spectrum and the observation or lack of observation of the substrate structure in the SEM photomicrographs. It is concluded from the results of the three techniques that for the Set I samples, cohesive failure was noted for 219D2 whereas adhesive failure was noted for 220D3. Cohesive failure was noted for samples lm2-517 and lmp2-516 and adhesive failure was noted for 2m2-515 in Set II. 

Sample handling is the same for both types of spectrometer and use is made of salts such as potassium bromide and sodium chloride which do not absorb in the IR region. Solid polymers usually are analysed in the form of either (i) discs pressed from finely powdered dilute (1-2 per cent) dispersions of polymer in potassium bromide, or (ii) melt-pressed or solution-cast thin films. Liquid polymers are analysed as thin films between the polished faces of two blocks (known as plates) of sodium chloride. Analysis of polymers in solution tends to be avoided where possible because a significant proportion of the IR spectrum of the polymer is obscured by the IR absorptions of the solvent. IR spectra of polymer surfaces can be recorded using techniques such as attenuated total reflectance and specular reflectance, and their use has grown with the increasing importance of polymer surface chemistry. 

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

There have been many new developments in color measurement systems technology in recent years. A major breakthrough is in the area of portable color measurement techniques. Newly developed portable spectrophotometers (Figure 6-13h) now make it possible to measure and analyze data on the production floor. Some portable spectrophotometers are also capable of displaying an actual spectral reflectance curve. Bench-top spectrophotometers have been updated to allow more computer control of the lens, UV filter, and specular port. The ability to calibrate the UV component of the color spectrum is an important feature for measuring fluorescent colors accurately. Advances in windows-based software has improved the capabilities for statistical analysis of color measurements, color corrections, and color formulation. 

Owing to these improvements, infrared spectroscopy has undergone a marked development in the last few years because of the possibility of adapting new accessories to the spectrometers and, therefore, of analysing samples whose infrared spectrum was impossible to obtain some years ago. Among these accessories are those which can measure specular and diffuse reflection, attenuated total reflectance and microscopes. One area in which the technique has improved dramatically is forensic analysis. 

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