Window materials, sample handling

Osland has described a heated press for the preparation of plastic films for analysis by infi ared spectroscopy. The press can produce films as thick as 500 um and of reproducible thickness. Earlier workers have used the hot press film method, where the sample is heated imtil molten, pressed to a thin film and allowed to solidify. Sampling handling accessories are now available with which films of constant thickness can be prepared. Alternatively, the sample material can be dissolved in a suitable organic solvent and a film cast onto glass or a cell window. 

Sample handling also presents a number of problems in the infrared region. For example, there is no rugged window material for cuvettes that is transparent and also inert over this region. The alkali halides are widely used, particularly sodium chloride, which is transparent at wavelengths as long as 625 cm. For frequencies less than 600 cm, polyethylene cells are frequently used. 

Clean and dry cells before use. Disposable sample cups are not to be reused. Window material usually is 6 pm polyester or polycarbonate film. Handling of the fllm must be kept to an absolute minimum to prevent contamination. Renewal of the window is essential for the measurement of each sample. 

Solids are usually ground with a material such as potassium bromide and compressed into a pellet. Moisture must be absent, and the transparent disk is placed in the window of the spectrometer. In general, however, measurement of intensity of absorption in the solid phase is unreliable due to scattering and reflection losses, and a uniform distribution of sample in the pellet is difficult to achieve. One method of handling solutions is to allow them to soak into a KBr wedge, evaporate the solvent, and compress the tip into a microdisk or pellet (C26) alternatively the sample (1 /tl) may be either placed directly on the KBr disk (B13) or mixed with a little KBr powder and subsequently incorporated in the disk. These microdisk techniques can be used for the examination of gas chromatograph effluent. For multiple analyses, an enclosed turntable system loaded with the disks can be used. 

Multichannel instruments are widely used for the determination of several components in industrial materials such as steel, other alloys, cement, ores, and petroleum products. Both multichannel and singlechannel insirument.s are equipped to handle samples in the form of mct ils, powdered solids, evaporated films, pure liquids, or solutions. When necessary, the materials are placed in a cell with a Mylar or cellophane window. 

There are several injection methods currently used in GC monomer determinations. However, all of these techniques fall into two categories direct injection and injection of the vapour above the polymer, i.e., headspace analysis or polymer solution. Direct injection of a polymer, or its precipitates, is perhaps the most common method. This technique requires both an inert material in the injection port to trap the polymer, and a narrow window of acceptance on the injection port temperature. High temperatures lead to polymer decomposition while low temperatures prevent fast volatilisation of the monomer. In addition, the sample viscosity must be low enough to allow handling with a syringe. 

This class of solids is an extension of the sample types already discussed, and many of the procedures already highlighted may be used here. If the material dissolves in a suitable solvent, then a cast film may be prepared on an IR transmitting window or on the surface of an ATR element. Moldable materials, such as polymer pellets, may be prepared as hot-pressed films, with care taken to ensure that material does not thermally degrade. Grind-able materials can be handled as previously discussed for powders using the compressed halide pellet, mineral oil mull, or diffuse reflectance methods to acquire the spectrum. 

In the past, solution-based measurements were popular mainly because sampling was limited to the simple procedures involving transmission measurements. One possible benefit is that both solids and liquids may be studied in solution. In this case, the optical effects caused by differences between these two types of sample phase are removed. The problem has always been the selection of an appropriate solvent. All liquids have an infrared spectrum, and almost all liquids have relatively intense and complex spectra. There are few materials that have simple spectra that have clear windows for broad-range transmission measurements. As a general comment, the materials that are the best solvents, particularly for polar compounds, by nature are strong infrared absorbers. Therefore, it is very difficult to find convenient solvents that have good solvent characterisitics, that are relatively involatile, and that are convenient to handle (e.g., are nontoxic). 

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