Polymers, vapor trapping

Procedures for trapping accelerant vapors in the headspace of a closed container on charcoal that is either encased in a porous pouch or impregnated into a flexible membrane have been described (124). Trace amounts of explosive compounds can be trapped from hplc effluents onto a porous polymer microcolumn for confirmatory gc examination (125). 

The efficacy of RF in stabilizing small rings is well illustrated by the fact that the azete from trifluoro-l,2,3-triazine is considerably more reactive. Trapping experiments were unsuccessful and a polymer was isolated at room temperature. The dimer (41) forms an observable anion with CsF, which confirmed the endo structure [87CC1699 90JCS(P1)975, 90JCS(P1)983], In contrast, trifluoro-l,2,4,-triazine is resistant to vapor phase photoysis and flow pyrolysis [87JCS(P1) 1251]. 

Physical Properties. The size, molecular weight, and vapor pressure determine the pore size necessary to trap and release a substance efficiently. Large molecules such as pesticides cannot be collected and recovered on molecular sieves nor will small molecules like gaseous hydrocarbons be retained well by porous polymers. 

Exemplary water-repellent treatments for masonry surfaces include metal stearates, oils, waxes, acrylates (both polymers and monomers), silicones (solvent-based and emulsion), siliconates, silanes and, fluorochemicals. In contrast, to waterproofing coatings, water-repellent coatings, because they are permeable to water vapor, do not trap moisture and, therefore they can reduce spalling. In addition most water-repellent coatings do not alter the appearance of a porous masonry. 

An inert gas is bubbled through the sample. The volatile hydrocarbons are transferred into the vapor phase and trapped over a sorbent bed containing 2,6-diphenylene oxide polymer (Tenax GC). A methyl silicone (3% OV-1 on Chromosorb-W, 60/80 mesh) packing protects the trapping material from contamination. Other adsorbents such as Carbopack B and Carbosieve S III may also be used. If pentane and other low boiling hydrocarbons need to be detected, the sorbent trap should be filled with activated charcoal, silica gel, and Tenax, respectively, in equal amounts. 

To imprint more complex structures, a Si master with positive relief structures was first created. Then the master was used to imprint the channel pattern on a PMMA plate (135°C for 5 min) (see Figure 2.22) [1011]. This method is called compression molding or embossing. In using this method, a vacuum chamber is needed to remove air bubbles trapped at small features and to remove water vapor from the polymer [7]. 

Thermally desorbed vapor concentrators are conceptually quite simple. They rely on a pump to pull a large quantity of ambient air (presumably contaminated with the vapor-phase analyte) over a polymeric sorbent that traps the vapors onto the large surface area of the polymer. These vjq)ors are then released from the trap by applying heat and backflushing additional air over the hot sorbent to remove the vapors. Vapor concentration is enriched by virtue of the fact that the volume of air required to absorb the vapor sample onto the cool trap is much larger than the volume of air required to remove the vapor sample from the hot trap. Vapor concentrators can give impressive increases in sensitivity (e.g., a factor of 1000). 

A final advantage of the vapor concentrator is that it can enhance the selectivity of the sensor. Clearly, the method is only effective for compounds that can be trapped on a sorbent polymer. Low molecular weight vapors such as methane, ethane, and propane are not readily trapped and thus will not be enriched. Likewise, very high molecular weight vapors will not be easily desorbed from the trap and thus will actually be diminished in concentration. 

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