The infra-red analysis of solid substances. A review

Abstract

OF CONTENTS Introduction Physical factors affecting spectra Chemical factors affecting spectra Adsorption of substance on particles of dispersing medium Chemical reaction of substance with medium Polymorphism of substance Mixed-crystal formation Distortion of the crystalline structure Formation of complexes between substance and medium THE fundamental principles of the analysis of gases, liquids and solutions by infra-red spectroscopy are well established, and the techniques that have to be used are generally well known.Very early on, infra-red spectroscopists were faced with the problem of finding a method of recording solid-phase spectra. The method based on mulling the sample with Nujol, or with fluorinated or chlorinated hydrocarbons, has been in use for several years and is still standard practice for many purposes, such as quantitative analysis and structural diagnosis.There are, however, some practical difficulties in the use of solid-phase spectra for quantitative analysis: accurate control of sample thickness, reflectivity and scatter is much more difficult to achieve than with solutions. The internal-standard methodl ,2 has been used with success, and a good accuracy has been attained in the analysis of multicomponent mixtures by combining the internal-standard method with the method of compensated extinctions ; however, the method runs rapidly into some difficulties owing to Beer's law not being applicable.The potassium bromide technique, in which the infra-red absorbing solid, finely ground and dispersed in a powdered halide, is pressed into a clear disc, proposed in 1952 by Stimson3 and Schiedt,* was received with much enthusiasm; the absence of adsorption bands in the dispersing medium and the possibility of controlling the sample weight accurately are certainly two important advantages of the disc technique over the mull method. However, I would like to draw attention to the difficulties that are encountered, both in general in quantitative measurements of absorption spectra of solids on the one hand (physical factors), and on the other hand, to the difficulties that arise through the use of alkali halide discs (the chemical and physico-chemical factors).

Effect of Swelling on the Infrared Absorption Spectrum of Montmorillonite

“…In order to interpret the latter observation, it will be instructive to derive an equation for the transmittance of radiant energy by clay particles dispersed in water. Although satisfactory equations are available for the transmittance of radiant energy by dispersions of spheres and cubes (Otvos et al, 1957;Duyckaerts, 1959), they are not strictly applicable to dispersions of planar clay particles. Our derivation follows that of Otvos et al (1957).…”

Quantitative IR-ATR spectrometry of asbestos fibers on membrane filters

“…creased geometric size will eventually causk peak broadening and degradation of the signal, due to light dispersion and reflection, as was elegantly shown for calcite particles by Duyckaerts (23). Since aspect ratios 1/2r --200 are not common, fibers almost 1 mm long may be encountered.…”

Nondestructive FT-IR Sampling Technique to Study Glass Fiber Composite Interfaces

“…The peaks in this region are inverted. This is unusual for KBr spectra because alkali halide salts are used to correct for the large difference in the refractive indexes between the samples and air [14,15]. Spectrum C showr the result of subtracting spectrum A from spectrum B.…”

“…A strong broad peak near 3500 cm in figure 2 shows this. It is possible that some of the frequency differences observed between the subtraction spectra in figures 1 and 2 are due to differences in hydrogen bonding interactions with water, and also differences in the refractive index which are known to cause frequency shifts in the spectrum [15].…”

“…Although the KBr pellet technique is very widely used, it has certain disadvantages [14,15] for the study of composites which must be considered carefully. First, grinding the composite into a powder of small particles and exposing the resulting mixutre to tremendous pressures when making the .4| pellet could certainly influence the structure of the coupling agent.…”