Detection, vapor-phase organic compounds

Detection. Nearly all of the vapor-phase organic compounds will respond when added to a flame ionization detector, Consequently, this detector is most commonly used. Other special-purpose detectors include photoionization, mass spectrometry, atomic emission, ion mobility, mercury oxide reduction, and chemiluminescence detectors. 

J. H. Krech, S. L. Rose-Pehrsson, Detection of volatile organic compounds in the vapor phase using solvatochromic dye-doped polymers, Chimi. Acta34, 53-62 (1997). 

In Ref. 11, IT voltammograms of NH4 and N03 were obtained when a 0-pipette was exposed to vapors of ammonia and nitric acid, and linear dependence of the voltam-metric response on concentration of vapor-generating solution has been demonstrated. The surface liquid layer in all pipettes used in that work was aqueous, and only the detection of water-soluble gases was discussed. However, the detection of organic compounds in the gas phase may also be possible using a 0-pipette with a nonaqueous sensing film. 

In the thermal methods, the sample is heated to increasingly higher temperatures, with most steps being carried out in the presence of 02. The basis of this method is that volatile organics will vaporize first and then other organic compounds will be oxidized. Only at the highest temperatures will graphitic carbon oxidize. The carbon thus ejected into the vapor phase at various temperatures is detected in the form of C02 or, alternatively, after catalytic reduction, as CH4. 

The vapor phase from heated waste water often enriches the organic compound. This enriched vapor partly condenses at the membrane on the IRE. Additionally, the enrichment factor increases because of the lower temperature of the membrane. Therefore the head space method has mainly two advantages first, protecting the IRE from contamination and second, improving the detection limit. 

The vapor pressure of 2,4-DNP is 1.49x1 O 5 mm Hg at 18 °C (Mabey et al. 1981). Organics with vapor pressures of 10" to 10" mm Hg at ambient temperature should exist partly in the vapor and partly in the particulate phase in the atmosphere (Eisenreich et al. 1981). Nitrophenols were detected experimentally in the particulate phase in air (Nojima et al. 1983), although the method used to collect atmospheric particulate matter was not suitable for collecting vapor-phase dinitrophenols. The distance of atmospheric transport of dinitrophenols will depend on atmospheric residence times. The residence time of dinitrophenols, based on the estimated rates of various reactions, is long enough to allow atmospheric transport (see Section 5.3.2.1). The removal and transport of atmospheric dinitrophenols to land and water by physical processes, such as wet and dry deposition, will depend on the physical states of these compounds in the atmosphere. Since dinitrophenols have been detected in rain, snow, and fog (Alber et al. 1989 Capel et al. 1991 ... 

Special techniques have been developed to measure critical temperature, pressure and density. The most common manner to observe the critical temperature is to heat a sample in a closed tube and measure the temperature at which the boundary (meniscus) between liquid and vapor disappears. This method produces an accuracy of about 0.5 degree in most cases. More sophisticated methods for detecting the merging of the two phases are available, but achieving a reproducibility of better that 0.1 degree is difficult. Some properties of a substance change rapidly in the vicinity of the critical point and many organic compounds decompose at or below the critical temperature. Rapid methods of observation have been developed for these compounds. 

Transport of organic compounds from the solid or aquatic phases to the gas phase (and back again) is now known to be a highly important process for the dispersion of chemical compounds around the globe. Dissolution into and volatilization from the aqueous phase is an elaborate process that depends on solubility, vapor pressure, turbulence within the two phases, and other physical and chemical factors. Volatilization of materials from the earth s surface into the troposphere can result in their long-range transport and redeposition, with the outcome being that measurable quantities of such substances can be detected far from their point of release. 

In addition to the examples discussed above, a number of other xenobiotics are measured by their phase I reaction products. These compounds and their metabolites are listed in Table 20.1. These methods are for metabolites in urine. Normally, the urine sample is acidified to release the phase I metabolites from phase II conjugates that they might have formed, and except where direct sample injection is employed, the analyte is collected as vapor or extracted into an organic solvent. In some cases, the analyte is reacted with a reagent that produces a volatile derivative that is readily separated and detected by gas chromatography. 

GC-MS is used to detect both common and emerging explosive compounds. A review of GC-MS methods used to detect organic explosive compounds is available [129]. T vo common GC-MS sample introduction techniques are solid-phase microextraction (SPME) and headspace vapor collection [130-133], TTiese sample introduction methods are employed in the analysis of water and soil samples with suspected explosive residue contamination [134-137]. The U.S. EPA Method 8095, Explosives by Gas Chromatography, is recommended as a resource for sample preparation of soil and water samples analyzed for the common nitroaromatic, nitra-mine, and nitrate ester explosive compounds [138]. 

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