Passive sampling diffusion barrier

An important performance characteristic of passive samplers that operate in the TWA regime is the diffusion barrier that is inserted between the sampled medium and the sorption phase. This barrier is intended to control the rate of mass transfer of analyte molecules to the sorption phase. It is also used to define the selectivity of the sampler and prevent certain classes (e.g., polar or nonpolar compounds) of analytes, molecular sizes, or species from being sequestered. The resistance to mass transfer in a passive sampler is, however, seldom caused by a single barrier (e.g., a polymeric membrane), but equals the sum of the resistances posed by the individual media (e.g., aqueous boundary layer, biofilm, and membrane) through which analyte diffuses from the bulk water phase to the sorption phase.19 The individual resistances are equal to the reciprocal value of their respective mass transfer coefficients and are additive. They are directly proportional to the thickness of the barrier... 

The ceramic dosimeter is a time-integrative passive sampling device which is based on a ceramic tube as diffusion-limiting barrier enclosing a receiving phase that consists of solid sorbent beads. It has been successfully applied to the extraction of PAHs from groimdwater using Amberlite as sorbent bead [194]. 

The weight collected on the sorbent is equal to the product of the diffusion coefficient, the dimensions of the static air column (area divided by path length) and the concentration gradient. is the concentration at the velocity barrier which is assumed to closely approximate the ambient concentration. Cn is the concentration inside the monitor at time t 0 The D (A/L) term is the Monitor Sampling Rate and has the dimensions of flow rate (cm /sec.). The exposure time is measured by t. Therefore, the weight collected, W, is equal to the flow or sampling rate times the ambient concentration times the time. This is a dynamic, nature driven type of monitoring rather than a passive system. 

The first decision to make is whether the analyte in question is actively pumped or whether it diffuses passively. There is always the caveat of any possible rostro-caudal gradients, in which case the volume removed is important and it is always prudent to measure the levels of analyte in blood plasma (or serum). For proteins with passive diffusion one must correct simultaneously for both the serum level and for an independent measure of barrier function. The quotient of CSF analyte/serum analyte divided by CSF albumin/serum albumin can be expressed as an index value, but if there are alterations in any of the barriers there may be a nonlinear relationship that is particularly characteristic of large molecular weight proteins. This relationship is thus best expressed by a nonlinear function. The golden rule must be to compare like with like, and make appropriate corrections for known differences due to inhomogeneous samples. 

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