Quantitative measurements infrared spectra

Pyrazin-2-one (124) has been shown to exist predominantly as such by comparison of its ultraviolet spectrum with those of the fixed alkylated derivatives and by its infrared spectrum. The pK measurements support this conclusion but cannot yield quantitative results since cations of a common type are not formed. ... 

Many characteristic molecular vibrations occur at frequencies in the infrared portion of the electromagnetic spectrum. We routinely analyze polymers by measuring the infrared frequencies that are absorbed by these molecular vibrations. Given a suitable calibration method we can obtain both qualitative and quantitative information regarding copolymer composition from an infrared spectrum. We can often identify unknown polymers by comparing their infrared spectra with electronic libraries containing spectra of known materials. 

An attempt to use the infrared spectrum of materials collected at the sea surface for a quantitative measure of composition has been made by Baier et al. [285]. They dipped a germanium crystal through the surface film, then ran an internal reflectance spectrum on the material clinging to the crystal. From the spectrum, they concluded that the bulk of the material present in the surface film was there as glycoproteins and proteoglycans. 

One of the problems which must be solved for quantitative measurements by emission is the need for a blackbody source at the temperature of measurement. And a variety of blackbody references have been used including a V-shaped cavity of graphite 164), a metal plate covered with a flat black paint1S6 160) and a cone of black paper l53). However, none of these methods of producing a blackbody reference spectrum are adequate. In most cases the efficiency of the reference has not been established. The most recent recommendation 1S0) is an aluminium cup painted with an Epley-Parsons solar black lacquer which has an emittance of greater than 98% over the infrared spectral range. 

For each series of measurements about 50 g of solvent was transferred quantitatively in the dry box to the cell by pouring it into the dilution bulb this was the minimum amount required to fill the cell bulb. The cell was removed from the dry box, placed in the oil bath, and connected to the bridge. Time was allowed for the attainment of thermal equilibrium then at least three resistance measurements were made at five-min intervals, and the average value was calculated. The cell was removed from the bath and returned to the dry box. Dilute stock solution was quantitatively added to the cell by means of a weighing buret. The contents of the cell were carefully mixed, and the resistance of the solution was measured as before. The procedure just described was repeated several times with the dilute stock solution and then with the concentrated stock solution. About ten concentrations with a hundredfold range were obtained. A portion of the final solution in the cell (the most concentrated solution) was removed, and the infrared spectrum taken no absorption band indicative of traces of water was observed at 3600 cm-1. It was necessary to obtain the densities of... 

Infrared spectroscopy is an important technique for studying acidity. Acidic OH groups can be studied directly. Probe molecules such as pyridine may be used to study both Bronsted and Lewis acidity since two forms of adsorbed probes are easily distinguished by their infrared spectra. Quantitative infrared spectroscopy may be performed by measuring the spectrum of acidic OH or probes adsorbed on thin, self-supporting wafers of the acidic solid. Other spectroscopic methods which may provide information in specific cases include Fourier Transform Raman spectroscopy, electron spin resonance spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. 

An infrared spectrum can be analysed quantitatively by studying the variations in absorption wave-numbers, which reflect a change in environment, or the variations in the line intensities. To measure the latter it is necessary to use, depending on the transparency of the sample to the radiation, respectively, the Beer-Lambert law for transmission measurements and the Kubelka-Munk law for measurements using diffuse reflectance. 

Even in those cases where an aiialysis is qualitative, quantitative measures are employed in the processes associated with signal acquisition, data extraction, and data processing. The comparison of, say, a sample s infrared spectrum with a set of standard spectra contained in a pre-recorded database involves some quantitative measure of similarity in order to find and identify the best match. Differences in spectrometer performance, sample preparation methods, and the variability in sample composition due to impurities will all serve to make an exact match extremely unlikely. In quantitative analysis the variability in results may be even more evident. Within-laboratory tests amongst staff and inter-laboratory round-robin exercises often demonstrate the far from perfect nature of practical quantitative analysis. These experiments serve to confirm the need for analysts to appreciate the source of observed differences and to understand how such errors can be treated to obtain meaningful conclusions from the analysis. 

A technique for applying infrared measurements to insulating oil is available (ASTM D-2144), and considerable information about mineral oil composition can be gained from infrared spectroscopy. Oxygenated bodies formed when oil deteriorates can be recognized, and hence this procedure can be used for surveillance of oils in service. An infrared spectrum can also give information as to the aromaticity of an oil and can detect antioxidants such as 2,6 di-tertiary butyl p-cresol. A chemical test for the latter is, however, available and is preferable for quantitative purposes (ASTM D-1473). 

Carbon dioxide in water absorbs at 2343.5 cm" . The observed small shift for bound CO 2 was attributed to a solvent effect. The absence of a larger spectral shift showed that this substrate is not appreciably distorted upon binding by the enzyme. The data of Riepe and Wang (1968) indicated that at a pcoz of 1 and 25°C the active site of the enzyme was about 25% saturated with CO2. Consequently, the measurement of peak height from the difference infrared spectrum was much less reliable than the measurement of frequency for the bound COj. On the other hand, carbonic anhydrase can be completely saturated by azide at fairly low total con-concentration of the latter. Therefore, quantitative infrared measurements of the concentration of the enzyme-azide complex were more easily carried out. 

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