Trapping efficiency

The adsorbent also needs to be spiked directly with the chemical to determine the recovery from the adsorbent, and this recovery value can be used to adjust the recovery from the adsorbent after air sampling. For compounds with extremely low volatilities, the chemical deposit can be heated to promote volatilization and to minimize the time required for the trapping experiments. 

The trapping efficiency of polymeric, microporous adsorbents [e.g., polystyrene, polyurethane foam (PUF), Tenax] for compound vapors will be affected by compound vapor density (i. e., equilibrium vapor pressure). The free energy change required in the transition from the vapor state to the condensed state (e.g., on an adsorbent) is known as the adsorption potential (calories per mole), and this potential is proportional to the ratio of saturation to equilibrium vapor pressure. This means that changes in vapor density (equilibrium vapor pressure) for very volatile compounds, or for compounds that are gases under ambient conditions, can have a dramatic effect on the trapping efficiency for polymeric microporous adsorbents. 

Screen containing pesticide deposit on glass wool 

The concept of adsorption potential comes from work with high-purity, synthetic microporous carbon, which relies solely on van der Waals dispersive and electrostatic forces to provide the energy for adsorption. The polymeric microporous adsorbents that operate solely through van der Waals dispersive and electrostatic forces often cannot provide the surface potential energy needed to trap compounds that are gases under ambient conditions, and for very volatile compounds the trapping efficiency can be low for similar reasons. 

For the charcoal, XAD, and PUF adsorbents discussed above, solvent extraction techniques have been developed for the removal and concentration of trapped analytes. Although thermal desorption has been used with Tenax-GC in some specialized air sampling situations [primarily with sampling volatile organic compounds (EPA, Method TO-17 )], this approach is not a viable alternative to solvent extraction for the charcoal, XAD, and PUF adsorbents. The polystyrene and PUF adsorbents are thermally unstable and the charcoal chemisorption bonding is more easily broken by 

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H. Daimon and Y. Hirata, Trapping efficiency and solute focusing in on-line supercritical fluid exti action/capillai y supercritical fluid chi omatography , 7. Microcolumn Sep. 5 531-535 (1993). 

Reports of the generation and subsequent electrophile trapping of nonstabilized metalated aziridines appeared before those for metalated epoxides. Desulfinylation of sulfinylaziridine 250 with EtMgBr gave metalated aziridine 251, which, remarkably, could be kept at 0 °C for 1 h before quenching with D2O (Scheme 5.64). The deuterated aziridine 252 (E = D) was obtained in excellent yield, but acetaldehyde was the only other electrophile found to be trapped efficiently [90],... 

As illustrated above there exist a large variety of techniques for preparing liposomes. From a pharmaceutical point of view, optimum liposome preparation techniques would avoid the use of organic solvent and detergents (which are difficult to remove), would exhibit a high trapping efficiency, would yield well-defined vesicles which can be produced in a reproducible way, and would be rapid and amenable to scale-up procedures (see Sec. VIII). 

Mayer, L. D., Hope, M. J., Cullis, P. R., and Janoff, A. S. (1985a). Solute distributions and trapping efficiencies observed in freeze-thawed multilamellar vesicles, Biochim. Biophys. Acta, 817, 193-196. 

Figure 5 Example of sampler configurations for (a) low-volume (0.1-5L min ) and (b) high-volume (>5-100Lmin ) determination of trapping efficiency for semi-volatile pesticides, and (c) low-volume (0.1-5 L min ) trapping efficiency for volatile pesticides...

Table 2 Trapping efficiency related to vapor density for four common pesticides...

Saturation vapor pressure at 25 °C (Pa) Adsorption potential (cap moP ) Trapping efficiency (%) ... 

Percentage difference in trapping efficiency relative to the 1 p.g m result. MITC = methyl isothiocyanate. 

Vxh can then be compared to Pph to assess the magnitude of lateral °Thxs advection, or with the measured sediment-trap °Thxs flux to assess the trapping efficiency. 

Scholten JC, Fietzke J, Vogler A, Rutgers van der Loeff MM, Mangini A, Koeve W, Waniek J, Staffers P, Antia A, Kuss J (2001) Trapping efficiencies of sediment traps from the deep Eastern North Atlantic The °Th calibration. Deep-Sea Res II 48 2383-2408... 

Yu E-F, Francois R, Bacon MP, Honjo S, Fleer AP, Manganini SJ, Rutgers van der Loeff M. M., Ittekot V. (2001b) Trapping efficiency of bottom tethered sediment traps estimated from the intercepted flttxes of °Th and Pa. Deep-Sea Res Part 1 48 865-889... 

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