Temperature trapping

Removal of water from gases may be by physical or chemical means, and is commonly by adsorption on to a drying agent in a low-temperature trap. The effectiveness of drying agents depends on the vapour pressure of the hydrated compound - the lower the vapour pressure the less the remaining moisture in the gas. 

Molybdenum hexafluoride [7783-77-9] M 209.9, b 35"/760mm. Purified by low-temperature trap-to-trap distillation over predried NaF. [Anderson and Winfield J Chem Soc, Dalton Trans 337 1986.] Poisonous vapours. 

Such vessels can also be baked at a temperature of several hundred degrees, to drive off any gas adsorbed on metal surfaces. The pumping function of an ion gauge was developed into efficient ionic pumps and turbomolecular pumps , supplemented by low-temperature traps and cryopumps. Finally, sputter-ion pumps, which rely on sorption processes initiated by ionised gas, were introduced. A vacuum of 10 "-10 Torr, true UHV, became routinely accessible in the late 1950s, and surface science could be launched. 

AG jg g- - (-104.9 kJ)- (298 K) (-290.2 J/K) (10 kJ/J) - -18.4 kJ Thus, anhydrous calcium sulfate acts as a chemical sponge at room temperature, trapping water vapor spontaneously to form calcium sulfate dihydrate. 

Water Purged at room or elevated temperature trapped in closed loop injected into GC GC/ECD 0.2 ppb 104 Wang and Eenahan 1984... 

Second, no work has yet been done on the application of the transpiration method to the preparation of samples for chemical analysis. In this area the same strictures on odour sampling apply, even where sub-ambient-temperature trapping techniques are used. Especially where cryogenic trapping is proposed, preparation of the odour sample in the laboratory is a considerable advantage. 

The hydrolysis of peptide bonds catalyzed by the serine proteases has been the reaction most extensively studied by low-temperature trapping experiments. The reasons for this preference are the ease of availability of substrates and purified enzymes, the stability of the proteins to extremes of pH, temperature, and organic solvent, and the existence of a well-characterized covalent acyl-enzyme intermediate. Both amides and esters are substrates for the serine proteases, and a number of chromo-phoric substrates have been synthesized to simplify assay by spectrophotometric techniques. 

Tris(4-hthiophenyl)methane (76), a l,9,9 -trilithiated compound, was obtained by Oda and coworkers . The starting tribromo compound 75 reacts with three equivalents of w-butyUithium in THF at low temperatures. Trapping reactions with various electrophiles resulted in yields between 59 and 90% (Scheme 27). 

Methanol or dimethyl ether feeds were fed from an ISCO positive-displacement pump. Liquid products were collected in a room-temperature trap and gaseous products were analyzed by on-line gas chromatography and volumes measured by a wet-test meter. The catalyst could be regenerated in-situ by switching to a nitrogen/air mixture. 

Falandysz, J., W. Kotecka, and K. Kannan. 1994. Mercury, lead, cadmium, manganese, copper, iron and zinc concentrations in poultry, rabbit and sheep from the northern part of Poland. Sci. Total Environ. 141 51-57. Fernandez, T.V. and N.V. Jones. 1990. Studies on the toxicity of zinc and copper applied singly and jointly to Nereis diversicolor at different salinities and temperatures. Trap. Ecol. 31 47-55. 

Purified by fractional distillation to remove HF. Also purified by low temperature trap-to-trap... 

Fig. 1.2. Manifold for medium vacuum and inert gas. A low-temperature trap, on the right side of the figure, is used in the vacuum line to protect the pump from harmful vapors. When the apparatus is being filled with gas or purged with inert gas, the valve on the pressure release bubbler (which contains a check valve to prevent oil from backing up into the line) is opened to avoid excess pressure which would blow the apparatus apart. Often a mineral oil bubbler (not shown here) is connected in line with the inert-gas source to provide visual indication of the inert-gas flow.

A vacuum pump should be scrupulously protected from corrosive vapors and materials which will be absorbed in the pump oil or condense in the pump. For most laboratory operations a low-temperature trap is employed for this purpose, and in the case of fluorine handling systems a soda-lime trap is used to neutralize the corrosive gases. Despite these precautions, the pump oil does eventually break down and become contaminated. Regular oil changes should be scheduled for a pump at about yearly intervals for a well-protected pump and more often for pumps which are not well protected. 

Fig. 6.4. Cross-section of a metal diffusion pump. The upper stage in this pump has a wide annular opening (A) which provides a good ultimate vacuum. The lower stage has a small annular opening (A ) so the pump will operate against a high fore pressure. (B) High-vacuum connection to the low-temperature trap and vacuum line. (C) Connection to rotary oil-sealed pump. This pump is cooled by means of water tubes (D). Air-cooled versions have fins in place of these tubes and a fan is installed to blow air over these fins. (E) Electrically heated oil reservoir.

The preceding discussion has made clear that trapping is inefficient both at low temperatures (for a dose of <51022 ions m 2) and at high temperatures. The low temperature limit is determined by the low diffusion rate of the gas in the solid leading to saturation of the surface layers of the metal. The high temperature limit is determined by the decomposition pressure of deuterium in the metal deuteride. However, at intermediate temperatures, trapping efficiencies greater than 90% may be obtained. 

Purge and trap injectors are equipped with a sparging device by which volatile compounds in solution are carried into a low-temperature trap. When sparging is complete, trapped compounds are thermally desorbed into the carrier gas by rapid heating of the temperature-programmable trap. 

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