Vapor-phase trapping

Johnson reported at the 59th TSRC, in 2005, on the results of the identification of spin-trapped vapor-phase free radicals in MSS via an ESI-triple quadrupole tandem mass spectrometry analysis method. Four alkoxyl free radicals were unequivocally identified (OC2H5, OC3H7, OC4H9, and OCjHjj). The superoxide free radical was also tentatively identified (27A56). 

The method is based on the determination and the analysis of an amount of trapped vapor phase, which has been stripped from the liquid phase by a known volume of inert gas. The carrier gas has to pass several saturators filled with the same liquid mixture of known composition, in order to be saturated with vapors. Then, the collected amount of condensate corresponds directly to the partial pressure(s). The typical experimental setup (applicable to systems of electrolytes, too) is described by Linek and Hala (67LIN1). Other stills can be found elsewhere, e.g. (910VE1) or (75ANA1). A semimicromethod based on gas chromatography was presented by (63WIC1)... 

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]. 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C and is usually a vapor phase reaction. In the laboratory this is done by using a glass tube in the heating zone of a small furnace. The vapors of the reactant are swept through the hot chamber by an inert gas and into a cold trap. Similar reactions occur with esters derived from long-chain acids. If the boiling point of the ester is above the decomposition temperature, the reaction can be carried out in the liquid phase, with distillation of the pyrolysis product. 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

Other methods explored internally to alter the form of nicotine delivered included the use of base- or acid-coated filters. For example, researchers at RJR applied sodium hydroxide-coated filters to a cigarette yielding only 0.06 mg of nicotine in order to heighten sensory impact (Shannon et al. 1992). Alternately, a filter coated with an acid (lactic, levulinic, citric) was used to reduce the impact of a high nicotine sheet, either by trapping the nicotine or by changing the pH of the smoke so there is not as much lucotine in the vapor phase (Shannon et al. 1992). The researchers noted that ... 

To collect a sample, the probe with a SPME fiber installed is inserted into the soil. Air is pumped through the probe, drawing subsurface soil vapors into the probe tip and across the SPME fiber. Pumping air across the fiber increases uptake of target analytes by the SPME fiber relative to what is collected by molecular diffusion alone. Once a sample is collected, the SPME fiber is removed from the probe for analysis. To analyze the sample, the SPME fiber is inserted into a modified inlet system attached to the Fido sensor. The modified inlet serves to heat the SPME fiber, causing rapid and quantitative desorption of trapped molecules of analyte. The vapor-phase analyte is then swept into the sensor for analysis by a flow of carrier gas. 

Finally, in the context of the overall vapor-phase mutagenicity of ambient air, we note that significant fractions of the two powerful human cell and bacterial mutagens discussed earlier, cyclopen ta[c<7]pyrene (XXVIII) and 2-nitrodibenzopyranone (XI), have been found in the gas phase (i.e., trapped on PUF plugs) in samples collected during hot weather at sites in southern California (Fraser et al., 1998, and Arey et al., 1994, respectively). Flence the contributions of such species, which are normally considered to be primarily in the particle phase, to the gas-phase mutagenicity at high ambient temperatures should also be considered. 

Atmospheric Pressure Chemical Ionization (APCI) is coupled to Tandem Mass Spectrometry (MS/MS) for the purpose of analyzing vapor phase perfume mixtures. Air-borne fragrances are analyzed directly by APCI/ MS/MS without the need for time consuming and potentially adulterating trapping and chromatography steps. Volatile fragrance chemicals have been rapidly identified by this novel technique as they emanate from vials or directly from skin. 

The vapor-phase pyrolysis (680°C) of 2-diazoindane-l,3-dione, followed by trapping of the intermediate ot-oxo ketene 11 in alcohols such as methanol, propan-2-ol or tert-butyl alcohol, gave 31-51 % yields of the corresponding j3-oxo esters 12.49... 

Polybrominated Diphenyl Ethers. Like PCBs, air samples containing PBDEs are usually collected by pumping air through a sampler containing a glass fiber filter and adsorbent trap to separate the particle bound and vapor phase fractions, respectively (Dobber et al 2000a Hillery et al 1997). The filters and adsorbants are then Soxhlet extracted with acetone/hexane, and the extracts are cleaned-up and analyzed by high resolution GC techniques. 

Molecules that enter the vapor phase in an open container can escape the liquid and drift away until the liquid evaporates entirely, but molecules in a closed container are trapped. As more and more molecules pass from the liquid to the vapor, chances increase that random motion will cause some of them to return occasionally to the liquid. Ultimately, the number of molecules returning to the liquid and the number escaping become equal, at which point a dynamic equilibrium exists. Although individual molecules are constantly passing back and forth from one phase to the other, the total numbers of molecules in both liquid and vapor phases remain constant. 

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. 

Analyte in aqueous samples extracted by purge and trap a measured volume of sample purged with helium volatile analytes transferred into the vapor phase and trapped on a sorbent trap analyte thermally desorbed and swept onto a GC column for separation from other volatile compounds detected by HECD, ECD, or MSD. 

Purge and trap method ethylbenzene transferred from aqueous to vapor phase under helium purge analyte adsorbed on a sorbent trap thermally desorbed out from the sorbent trap backflushed with He onto a GC column for separation from other volatile compounds determined by PID, FID or a mass spectrometer. 

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