Passive Sampler Fundamentals and Terminology

Some introductory comments on the conceptual basis of SPMD uptake (ku) and release (ke) rate constants and the associated sampling rates (i.e., Rs) are in order. The can be conceptualized as the volume of air or water cleared of chemical per unit sampler mass or volume per unit time (e.g., mL g d or mL mL d ) and Rs is the volume of air or water cleared per unit time (e.g., L d ). Thus, the only difference between ku and Rs is that Rs is not normalized to a unit mass or unit volume of sampler. In the context of organism exposure (see Section l.L), the SPMD is equivalent to the encounter volume times the fractional bioavailability of the chemical (which excludes dietary uptake). The release rate constant (d ) is equal to kuK J. 

If the aim of an investigator is to determine equilibrium concentrations in samplers, then the residence time (tm) is a logical parameter to compare among samplers. The tm is the mean length of time that a molecule spends in a passive sampling device, where solute exchange follows first-order kinetics. Residence time is given by 

For example, under low flow conditions and at a temperature of 18 °C, the SPMD residence time for phenanthrene is 45.4 d and ke values for benzo[, /f,/]perylene are too small to measure, which suggests a residence time of 10 d. 

The lag time l is a closely related parameter to tr but is generally used for diffusional processes under membrane control. This term is given by 

Alvarez, D.A. Petty, J.D. Huckins, J.N. Jones-Lepp,T.L. Getting, D.C. Goddard, J.P. Manahan.S.E. 2004, Development of a passive, in situ, integrative sampler for hydrophilic organic contaminants in aquatic environments. Environ. Toxicol. Chem. 23 1640-1648. 

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