Applications

Sublimation is applicable to a wide range of organic and inorganic compounds in an equally wide range

Figure 4 Apparatus for sublimation at reduced pressure. Coolant is circulated through the cold finger, CF, whilst a vacuum is applied to the sample chamber.

Figure 5 Improved sublimation apparatus proposed by Eisenbraun et al. (1978). Sample, S, sublimes from the lower to the upper chamber where condensation takes place on the cooled surface. A vacuum is applied to the apparatus and cooling coils, C, improve the condensation process. A specially formed spatula, SP, can be used to help remove the sublimate afterthe upper part of the apparatus is removed.

Figure 5 Improved sublimation apparatus proposed by Eisenbraun et al. (1978). Sample, S, sublimes from the lower to the upper chamber where condensation takes place on the cooled surface. A vacuum is applied to the apparatus and cooling coils, C, improve the condensation process. A specially formed spatula, SP, can be used to help remove the sublimate afterthe upper part of the apparatus is removed.

of different matrices. Sublimable substances include ice, iodine, arsenic(III) oxide, cadmium sulfide, ammonium chloride and a large number of organic compounds. Common matrices from which substances are sublimed include biological fluids, plant materials, carbonaceous materials, samples of crude organic solids and samples of rocks and ores.

Sublimation as a method of applying substances to thin-layer chromatography (TLC) plates involves the sample being sublimed and the vapour produced being directed by means of a drawn capillary onto the surface of a TLC plate which is slowly moved in one dimension. This results in the sublimed materials being deposited upon the TLC plate in a differential mode - the most easily sublimed compounds are deposited first whilst those requiring a higher temperature are deposited later. The TLC plate is then developed in the normal way to give what has been termed a 'thermofractogram' in which the substances are separated as a function of their heats of sublimation along one axis and as a function of their chromatographic characteristics along the other axis. This approach has been applied to a wide range of substances including pharmaceutical preparations, plant components and foodstuffs.

Only a very limited number of standard methods have been reported in which sublimation is an important aspect. These comprise an ASTM standard for measurement of sublimation from thermionic emitters, and two standards from Germany and Japan testing the stability of dyes and printing inks to sublimation. The first covers the determination of the quantity, rate, and identity of sublimed, evaporated, or sputtered materials, whilst the latter two are concerned with textile materials and semi-manufactured products.

Sublimation is often the mechanism by which pre-concentration of an analyte is effected, although this fact is frequently not appreciated. Dynamic head-space concentration from solid samples such as plant materials, foodstuffs or polymeric materials occurs by sublimation of the volatile components. Indeed, given appropriate apparatus that can be operated at different temperatures for dynamic

Figure 6 Typical freeze-drying apparatus. The frozen sample, S, is attached to the condenser assembly and a vacuum is applied. A cold trap, CT, protects the pump as ice sublimes from the sample and subsequently condenses in the refrigerant condensers, RC. On completion of the drying the sample is removed and the collected ice melts and drains from the system through the stopcock, SC.

Figure 6 Typical freeze-drying apparatus. The frozen sample, S, is attached to the condenser assembly and a vacuum is applied. A cold trap, CT, protects the pump as ice sublimes from the sample and subsequently condenses in the refrigerant condensers, RC. On completion of the drying the sample is removed and the collected ice melts and drains from the system through the stopcock, SC.

headspace concentration, the heat of sublimation can be determined for various compounds.

Derivatization procedures carried out on crude samples can produce materials with improved sublimation characteristics. This technique has been used to produce volatile compounds of lanthanides and actinides which have then been sublimed prior to analytical determinations. Derivatives have been made using $-diketones (hexafluoroacetylacetone or acetylacetone), benzoyltrifluoroacetone and thenoyl-trifluoroacetonates.

Low-temperature sublimation, which in some circumstances is termed freeze-drying, has been used to separate water, as ice, from biological fluids such as serum, urine or saliva. The technique has been particularly useful in paediatric cases where sample volumes are extremely low. Determinations have then been accomplished using IR spectroscopy or mass spectro-metry. Preparation of physiological samples for determination of deuterium oxide has included sublimation techniques prior to spectrophotometric determinations.

Low-temperature sublimation has been used to prepare samples for cryo-scanning electron microscopy (SEM) analysis in order to examine herbicide particles in a water suspension. The sublimation of herbicide-containing frozen water droplets provides a suitable etching of the surface for the SEM technique.

High-temperature sublimations are often the methods of choice in sample preparations from mineral ores, particularly in the case of trace enrichment of noble metals and the actinides and lanthanides prior to activation methods. Temperatures of 800-1200°C are typical. The procedure is carried out in silica tubes with entrainment gases, for example air or argon, being used to increase the sublimation process.

Polycyclic aromatic compounds have been separated using sublimation techniques from a variety of samples including coal, solids derived from oil, coal and petroleum processing, and residues (soots) resulting from the use of such fossil fuels.

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Solar Panel Basics

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