Vapor Concentration Enrichment

Occasionally, one needs to measure vapor concentrations that are below the detection limit of the sensor. In these situations, enrichment of the vapor concentration can provide substantial increases in the sensor s apparent sensitivity. Vapor-enrichment schemes for sensors based on sorbent trapping and thermal 

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. 

Overall, the addition of a vapor-concentrator device to a chemical vapor sensor can produce dramatic enhancements in performance. Significantly lower vapor concentrations can be reliably detected from the combined effects of source concentration enrichment, which increases the apparent sensor signal, and baseline drift compensation, which reduces the apparent noise produced by the sensor. Unfortunately, these performance enhancements come with a fairly heavy price in the form of additional pumps, valves, traps, and increased energy consumption requirements. 

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Carbon. Isotopic fractionation between CO2 and dissolved carbon in melts has been estimated by varions anthors to vary between 2 and 4%c (as summarized by Holloway and Blank 1994), the vapor being enriched in C relative to the melt. This fractionation can be nsed to interpret the carbon isotope composition of glasses and CO2 in volcanic gases and to estimate the initial carbon concentration of nndegassed basaltic melts. 

UF6 is a solid (mp 64 °C) but has a high vapor pressure (15.3 kPa at 25 °C). Since 19F is the only stable isotope of fluorine, the only molecular species by mass in UF6 are 235UF6 and 238UF6. Repeated diffusion of UF6 (g) through porous plugs (or centrifugation of the vapor) concentrates 235 UF6 relative to 238 UF6, since the speed of diffusion varies inversely as the square root of the molecular mass. This enrichment is not needed for... 

The tie line between liquid and vapor concentration shows that the vapor is enriched in the more volatile component. This is the basis for the process of... 

Besides offering enhanced sensitivity, the vapor concentrator can also act as a sample modulator for baseline drift compensation. As illustrated in Figure 6.17 (page 389), the enriched vapor concentration discharged from the concentrator exhibits a characteristic maximum at a predictable time after the desorptimi cy-... 

Analyte Separation/Concentration Enrichment, separation, isolation, concentration, matrix or compound separation, physico-chemical separation, hydride generation, cold vapor generation (Hg), solvent extraction (complexation), precipitation/coprecipitation, chromatography (including extraction, ion exchange, adsorption), distillation, volatilization, electrolysis, electrodeposition... 

Oleum, which is found infrequently in chlor-alkali plants, is an enriched form of sulfuric acid containing fiee SO3. It is characterized by its SO3 concentration or referred to as X% acid with X > 100. It has all the hazards of H2SO4 in addition to that of an objectionable vapor concentration of SO3. 

Physical Form. Liquid gasoline is a complex mixture of at least 150 hydrocarbons with about 60-70% alkanes, 25-30% aromatics, and 6-9% alkenes. The small-chain, low-carbon-numbered components are more volatile and thus in higher percentages in the vapor phase than the larger and heavier molecules. The concentrations of aromatics, the more toxic of the components, are depleted to about 2% in the vapor phase. The light alkanes, the less toxic components, are enriched to about 90%. Benzene is also present and represents a component of major concern. 

Finally, corrosion of internal metal surfaces, constituents of the absorbing and water-conditioning agents, and condensation of vapor components further contribute to the concentrations of Cr and Mn, Ca and P, and Se and S in the scrubbing solution and lead to their enrichment in fine aerosol emissions. 

Considerable effort has been made to examine the volatiles and trace components that contribute to food flavors. Sone early techniques for measuring the volatile components in food products by gas chromatography consisted of analyzing headspace vapors to detect vegetable and fruit aromas (5) and volatiles associated with other food materials ( ). AlTo, sample enrichment has been used in the analysis of Tome food products. However, these techniques require steam distillation or extraction and concentration, or both, before the volatile mixture can be introduced into a gas chromatograph (, 9, 10). Besides being... 

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