Preparation and handling of the samples

The aim of sample preparation for IR spectroscopy is to obtain thin, plane-parallel layers of the substance under investigation. This is almost always successful the state of aggregation of the substance plays no decisive 

Of all the techniques of preparation used, a 10- to 20-per cent solution of the substance provides the best IR spectra with the least disturbance. It proves most practical to fill the solution into detachable, sealed liquid cuvettes. The path length is set by means of spacers. The windows consist of plane-parallel NaCl plates (16 pm transmittance limit) or KBr plates (25 pm transmittance limit). In quantitative analysis, the plane-parallelism of the layers and plates must be checked and the path length measured by interferometry with an empty cuvette. 

There is no suitable solvent for IR spectroscopy which is sufficiently transparent over the whole range from 4000 to 400 cm . Between 4000 and 900 cm carbon tetrachloride is frequently used, and between 1400 and 400 cm l carbon disulphide. Below 400 cm i or 25 pm all the main solvents are sufficiently transparent. If the substance does not dissolve to an adequate extent, solvents such as n-hexane, chloroform or dimethyl formamide may be used. Water should be avoided as a solvent. In cases where this proves impossible, spectroscopy may also be performed with aqueous solutions with the aid of certain manipulative measures. The simplest is to saturate the solution with NaCl or KBr, the same material as the windows. Should this be chemically undesirable, the analysis should be conducted without cuvettes. 

According to H. Hausdorff s method the surface energy of the aqueous 

Good results in the IR spectroscopy of aqueous samples are given by the ATR technique (attenuated total reflection) and a further development of this, the MIR technique (multiple internal reflection) a prerequisite is that the sample space of the IR spectrophotometer is sufficiently large to accommodate the supplementary MIR device. The MIR technique likewise involves measurement of the absorption spectrum of the substance. All of the same molecule-specific and analytical conclusions can therefore be drawn as with an IR spectrum in the transmission of the same substance, even though the two spectra are physically not quite identical. 

---

A more comprehensive discussion of individual examples of decomposition reactions will not be given here. Experimental measurements can be affected quite markedly by the preparation and handling of the sample as well as by the surrounding gas atmosphere. Consequently, it can be very difficult to achieve reproducibility. Therefore, it is not an easy matter to study the elementary atomic steps of the decomposition reaction. In order to obtain an idea of the difficulties involved in interpreting the experimental results, reference may be made to review articles [8,55,56], and especially to the work of Haul [47] who has made a very thorough study of the decomposition of dolomite. 

---