Cold trapping of volatiles

Pankow, J.F. Cold-trapping of volatile organic compounds on fused-silica capillary columns. Journal of High Resolution Chromatography and Chromatography Communications 1981,4,156-163. 

Sunesson et al. have performed a multivariant optimization of parameters for the thermal desorption-cold trapping of volatiles (16). Their conclusions, although based on the use of a specific instrument, are generally applicable for all direct thermal desorption devices ... 

An explosion was experienced dining work up of an epoxide opening reaction involving acidified sodium azide in a dichloromethane/dimethyl sulfoxide solvent. The author ascribes this to diazidomethane formation from dichloromethane [1]. A second report of an analoguous accident, also attributed to diazidomethane, almost certainly involved hydrogen azide for the cold traps of a vacuum pump on a rotary evaporator were involved this implies an explosive more volatile than dichloromethane. It is recommended that halogenated solvents be not used for azide reactions [2]. 

Liquid nitrogen is used in the cold-trapping of materials such as carbon dioxide and volatile organic carbons (VOCs) from gas streams, as a coolant for electronic equipment, for pulverizing plastics or rubber material, for deflashing of rubber tires, and for simulating the conditions... 

Reamer et al have discussed the applicability of a gas chromatograph coupled with a microwave plasma detector (GC-MPD) for the determination of tetraalkyllead species in the atmosphere. The tetra-alkyllead species are collected by a cold trap. The volatile lead species are concentrated within an organic solvent, separated by a gas chromatographic column and determined by an MPD system which measures the emission intensity of the lead 405.78 nm spectral line. Previous workers have used an acidic solution of hydrochloric acid > activated charcoal , Apiezon L on a silanized universal sup-port the chromatographic support OV-1 and silicone rubber on Chromosorb to collect alkyllead compounds from the atmosphere. 

Niobium Penta.fIuoride, Niobium pentafluoride is prepared best by direct fluorination of the metal with either fluorine or anhydrous hydrofluoric acid at 250—300°C. The volatile NbF is condensed in a pyrex or quartz cold trap, from which it can be vacuum-sublimed at 120°C to yield colorless monoclinic crystals. It is very hygroscopic and reacts vigorously with water to give a clear solution of hydrofluoric acid and H2NbOF ... 

Methylenecyclopropane, b.p. 11° (760 mm.), is volatile at room temperature all adapter fittings must be carefully checked. The checkers recommend the use of two cold traps in series. 

The kinetics of many decompositions are conveniently studied from measurements of the pressure of the gas evolved in a previously evacuated and sealed constant volume system. It is usually assumed, and occasionally confirmed, that gas release is directly proportional to a, so that the method is most suitable for reactants which yield a single volatile product by the irreversible breakdown of a substance that does not sublime on heating in vacuum. A cold trap is normally maintained between the heated reactant and the gauge to condense non-volatile products (e.g. water vapour) and impurities. The method has found wide application, notably in studies of the decomposition of azides, permanganates, etc., and has been successfully developed as an undergraduate experiment [114—116]. 

Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

To add to the silver salt (Eq. 19) and peroxymercuration (Eq. 34-36) methods of preparing dialkyl peroxides, a third mild alkylation procedure has been developed that involves the use of alkyl trifluoromethane sulphonates (triflates)55). The peroxide transfer reaction between bistributyltin peroxide and the bistriflate 58 of cis-1,3-cyclopentanediol provided one of the first syntheses of 2,3-dioxabicyclo[2.2.1]heptane 9 (Eq. 44, H for D Tf = 02SCF3)56). Because of the sensitivity of 9, it was necessary to carry out the reaction in vacuo with rapid transfer of the volatile products to a cold trap to avoid decomposition a yield of 22% was achieved. 

This method was similar to that used by Hiteshue et al (3). In this method sand (50 g, mesh 0.42 - 0.15 mm) was mixed with the coal (25 g, mesh 0.5 - 0.25 mm). The addition of sand to the coal helped to prevent agglomeration (4). All the experiments used an aqueous solution of stannous chloride impregnated on the coal as a catalyst. The amount of catalyst added on a tin basis was 1% of the mass of the coal. These mixtures were placed in a hot-rod reactor and heated to 500°C at a heating rate of 200°C per minute. Residence time at temperature was 15 minutes. Hydrogen at a flow rate of 22 liters/minute and a pressure of 25 MPa was continously passed through the fixed bed of coal/sand/catalyst. The volatile products were collected in high-pressure cold traps. A schematic of the apparatus used is shown in Figure 2. 

It is possible to collect the volatiles in a cold trap [22]. A more favoured technique is the collection of the gases by adsorption on some support such as one of the Chromosorbs, or Tenax GC [13,22,23]. The volatiles are then desorbed by heating and injected into a gas chromatograph. 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

MacKinnon [91] and Wangersky [180] have made direct determinations of the volatile fractions from a variety of depths and stations in the North Atlantic. The volatile fraction as defined by MacKinnon s method is that fraction which can be removed from solution by purging with an inert gas at 80 °C and a pH of 8 for 10-12 hours, then at 65 °C for a further 10-12 hours. The inert gas stream is flushed through an ice-packed condenser to remove water, then into a trap packed with Tenax GC followed by a U-shaped stainless steel cold trap held at -78 °C. 

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