Passive sampling performance/limitations

Ambient air samples are collected on adsorbents such as Tenax (Wallace 1987), or multisorbent (Heavner et al. 1992 Oliver et al. 1996), or in passivated canisters (EPA 1988a). Tenax traps are thermally desorbed, concentrated cryogenically, and analyzed by capillary GC/MS (Wallace et al. 1987). Recovery is good (81-110%), precision for side-by-side samples is acceptable (9-45% RSD), and the detection limit is 1 g/m (Wallace 1987). Multisorbent traps may be solvent desorbed and analyzed by capillary GC/MS. Recovery and precision are good and detection limits as low as 0.019 ppb have been reported (Oliver et al. 1996). Collection of air samples in passivated stainless steel canisters is also widely utilized (EPA 1988a), but performance data are unavailable. Passive sampling devices are also widely used, due in part to their ease of use and small size (Lewis et al. 1985). 

The advantages of the in situ techniques include an intact blood supply multiple samples may be taken, thus enabling kinetic studies to be performed. A fundamental point regarding the in situ intestinal perfusion method is that the rat model has been demonstrated to correlate with in vivo human data [46 19], Amidon et al. [36] have demonstrated that it can be used to predict absorption for both passive and carrier-mediated substrates. However, the intestinal luminal concentrations used in rat experiments should reflect adequately scaled and clinically relevant concentrations to ensure appropriate permeability determinations [50], There are limitations of the in situ rat perfusion models. The assumption involved in derivation of these models that all drug passes into portal vein, that is drug disappearance reflects drug absorption, may not be valid in some circumstances as discussed below. 

H2O adsorption from water vapor has been extensively studied, first on powdered samples, more recently on single crystals. Most experiments on powders were carried out at high vapor pressures (> 1 torr). Experiments on single crystals are usually performed at low gas pressures (< 10 torr) because of the limitations of the modem surface science techniques. Powdered samples are highly defective and contain various crystalline orientations and stmctures. They are more representative of the behavior of passive films formed in aqueous solution. Studies on well-characterized surfeces of single crystals make it possible to investigate the role of surface defects and surface orientation on the mechanisms of chemisorption. 

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