Passive sampling sorption phase

Passive sampling can be defined as any sampling technique based on the movement (by diffusion) of analyte molecules from the sampled medium to a receiving phase contained in a sampling device. This mass transfer process is driven by a difference in chemical potentials of the analyte in the two media. This process continues until equilibrium is reached in the system, or until the sampling process is stopped.14 Analytes are retained in a suitable medium within the device, known as a receiving or sorption phase. This can be a solvent, chemical reagent, absorbent, or... 

FIGURE 3.1 Concentration profiles in a passive sampling device. The driving force of accumulation is the difference in chemical potentials of the analyte between the bulk water and the sorption phase. The mass transfer of an analyte is governed by the overall resistance along the whole diffusional path, including contributions from the individual barriers (e.g., aqueous boundary layer, biofilm layer, and membrane). 

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... 

Passive samplers are widely used in monitoring volatile organic chemicals (VOCs) in groundwater. Such samplers have the potential to reduce costs of monitoring from the high levels associated with the use of pumps to sample the test wells. Moreover, the risk of loss of volatile analytes during sample transport and storage is substantially reduced once the compounds are accumulated in the sampler sorption phase. 

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