Pressed disc method

Porous materials, such as silica and alumina, have thermal diffusion lengths of approximately 10 m, which is much less than the typical thickness of pressed discs. The small thermal diffusion length gives photoacoustic spectroscopy a larger dynamic range than transmission methods when applied to powdered samples. An additional advantage is the ease of sample preparation, since photoacoustic spectroscopy uses powdered samples with no special preparation required. 

The densities of polymers can be determined by the pyknometer technique or by the flotation method. In the pyknometer technique the liquid volume displaced by the polymer sample is determined by weighing. Most polymers have a density larger than that of water, which can, therefore, be used as the liquid. Polymers in the form of powders or pressed discs tend to adsorb or occlude air bubbles, which can lead to serious errors. This can be largely prevented by careful degassing of the pyknometer and polymer sample under vacuum before filling with liquid, and/or by addition of a small amount (0.1%) of commercial detergent to lower the surface tension of the water. 

In the pressed disc technique a known weight of sample is intimately ground with pure, dry potassium bromide and the mixture inserted into a special die and subjected to pressure under vacuum. The concentration of sample in the disc is usually in the region of 1.0 per cent. The disc so produced may be mounted directly in the sample beam path of the spectrophotometer and the spectrum recorded. This method has the advantage that the spectrum so produced is entirely due to the sample since pure dry potassium bromide is infrared transparent in the 2-25 /xm region. To eliminate the possibility of impurities in the potassium bromide, however, a blank disc (no sample) can be made and mounted in the reference beam path of the spectrophotometer. Care should be taken to ensure that both discs are of equal thickness otherwise inverse peaks may occur if the potassium bromide is damp or impure, and this will be particularly noticeable if the reference disc is thicker than the sample disc. 

An interesting phenomenon has been observed in applying the XRF method to the determination of parts per million of chlorine in hot-pressed discs of low-pressure polyolefins. In these polymers the chlorine is present in two forms, organically bound and inorganic, with titanium chloride compounds resulting as residues from the polymerisation... 

The most common means of recording the spectrum of solid samples, the pressed halide disc method, relies on containing the material to be analysed in a self-supporting matrix of infrared transparent material. KBr is by far the most popular salt to use, although other metal halides, such as Csl or KCl, have their own particular advantages. Csl, for instance, transmits down to 180 cm whereas KBr cuts off at 350 cm ... 

McLaughlin, J.L. Crown gall tumours on potato discs and brine shrimp lethality two simple bioassays for higher plant screening and fractionation, in Methods in Plant Biochemistry, Hostettmann, K., Ed., Vol. 6, Academic Press, London, 1-32, 1991. 

The amino acids were analyzed from the press juices of the berries by an amino acid analyzer with ion-exchange columns as previously reported (17, 18). We described in detail the methods of disc electrophoresis and the isoelectrical focusing in polyacrylamid gels in another publication (8). 

A conventional steady-state guarded hot-plate method for thermal conductivity measurement was used by Cook and Leaist (1983). Their apparatus was used to perform an exploratory measurement of methane hydrate to within 12%. A sample of methane hydrate was made externally, pressed, and placed in the hot-plate cell at the Sample Disc. The lower sample heater had thermocouples contacting the top and the bottom of the sample to determine the temperature gradient. 

Refs. [i] Heyrovskf J, Kuta J (1966) Principles of polarograpy. Academic Press, New York [ii] Bond AM (1980) Modern polarographic methods of analysis. Marcel Dekker, New York [Hi] Kissinger PT, Heineman WR (eds) (1996) Laboratory techniques in analytical chemistry, 2nd edn. Marcel Dekker, New York [v] Pleskov YuV, Filinovskii VY (1976) The rotating disc electrode. Plenum Press, New York [vi] Galus Z (1994) Fundamentals of electrochemical analysis, 2nd edn. Ellis Horwood, New York, Polish Scientific Publ PWN, Warsaw... 

Spiral or worm stirrers are suitable for agitating the contents of test-tubes and boiling tubes. A large sphere of molten glass is pressed into a thin disc by the method described for type (b). A rod is then attached to the disc and the disc heated in a soft flame and stretched and twisted to the spiral form shown in Fig. 8.5i. The first attempt to make such a spiral may not succeed, but repeated efforts will soon produce surprisingly long and regular worm stirrers. 

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