Transmittance qualitative analysis

Although a transmittance IR absorption spectrum measurement evaluates the bulk of the alignment material, the distribution of the alignment of the molecules can he determined hy a qualitative analysis by changing the polar and azimuthal angles of the sample. The relationship between the dichroism of the polyimide and its thickness was studied using polarized infrared absorption measurements, and it was found that the dichroism was consistently less than 17 nm and decreased when the polyimide becomes thicker [22]. From these results and retardation measurements, a molecular distribution model was proposed which shows uniform anisotropy to some depth, lower anisotropy in deeper areas, and isotropy in areas which are even deeper [22]. 

If the reflectance of a sample is low, as it is with gaseous samples, e(v), is approximately equal to 1 — r(. Thus, for any sample for which a transmittance spectrum with discrete absorption bands can be measured, the emittance spectmm should yield equivalent information. As a result, qualitative analysis of the components of hot gases by infrared emission spectroscopy can be as easy as it is by transmission spectrometry. The problem of obtaining quantitative information by infrared emission spectroscopy is more difficult, since not only must the temperature of the sample be known if the radiant power from the blackbody is to be calculated, but the instrument response function must also be taken into account [1]. 

Optical techniques have provided valuable information on many important properties of surface sihcides. The most common optical experiments include reflectance (R), transmittance (T), and elipsometric spectroscopy (SE). Although the physical process is the same, the method used to extract the optical functions from the experimental R, T, or SE measurements is different in the case of thin films, where the properties of the film itself and its purity and crystalline quality play a relevant role in the reliability of the optical constants determined. In particular, the results obtained from optical measurements on thin films can be affected by several contributions originating from the presence of the substrate. For qualitative analysis, it is sufficient to identify the spectral features of the substrate and normalize the measured spectra to this contribution. However, interference effects may also play... 

NIR spectroscopy is probably the most successful technique for the development of qualitative and quantitative methods in the pharmaceutical industry. NIR spectra contain both chemical and physical information from samples (solid and liquid). Spectra can be acquired off-line in three different modes transmittance, reflectance and transflectance. In all cases, the spectra are obtained in a few seconds without or minimum sample pretreatment. Multivariate data analysis techniques are usually needed for the development of the... 

Modern infrared (IR) spectroscopy is a versatile tool applied to the qualitative and quantitative determination of molecular species of all types. Its applications fall into three categories based on the spectral regions considered. Mid-IR (MIR) is by far the most widely used, with absorption, reflection, and emission spectra being employed for both qualitative and quantitative analysis. The NIR region is particularly used for routine quantitative determinations in complex samples, which is of interest in agriculture, food and feed, and, more recently, pharmaceutical industries. Determinations are usually based on diffuse reflectance measurements of untreated solid or liquid samples or, in some cases, on transmittance studies. Far-IR (FIR) is used primarily for absorption measurements of inorganic and metal-organic samples. 

Solvents employed in paint manufacture can be qualitatively and quantitatively determined using liquid cells or from the spectra obtained from a few drops of the solvent, previously distilled, deposited between two crystals of an alkyl halide (in this case, for quantitative purposes an internal standard or the bands quotient approach must be employed). From these spectra the presence of several solvents in the formulation can be established and, if the mixture is not too complex, quantitative determination of all solvents can be carried out without chromatographic separation. Attenuated total reflectance (ATR) measurements offer a simple and fast alternative to the use of transmittance cells for qualitative and quantitative analysis. For this purpose, the solvent fraction obtained from the paint is placed over the surface of an ATR crystal (usually ZnSe). Figure 4 shows the spectra obtained for pure solvents and the solvent fraction of one polyester type paint. 

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