SPMD Description and Rationale

Although SPMDs are simple in design, the mechanisms governing their performance as passive samplers of HOCs can be quite complex (see Chapter 3). The underlying principle of molecular-size discrimination in the uptake and loss of chemicals by SPMDs is shown in Eigure 2.1. The sizes of the molecules shown in the illustration are scaled to the postulated 10 A diameter of the transient pores in the membrane. Temperature and the presence of plasticizers/solvent will affect the effective pore sizes. 

Ions of organic and inorganic chemicals are not sampled by SPMDs because charged species are hydrophilic and are essentially insoluble in nonpolar LDPE. Water qnality variables, snch as pH and salinity (Huckins et al., 1999), may affect the dissolved concentrations of some compounds in environmental waters (e.g., the residne concentrations of organic compounds with p/faS 4 and 9). 

Conceptually, SPMD data fills a gap between exposure assessments based on direct analytical measurement of total residues in water and air, and the analysis of residues present in biomonitoring organisms. SPMDs provide a biomimetic approach (i.e., processes in simple media that mimic more complex biological processes) for determining ambient HOC concentrations, sources, and gradients. Residues accumulated in SPMDs are representative of their environmental bioavailability (see Section 1.1.) in water and air and the encounter-volume rate as defined by Landrum et al. (1994) is expected to be proportional to the uptake rate. SPMD-based estimates of water concentrations can be readily compared to aquatic toxicity data (generally based on dissolved phase concentrations) and SPMD extracts can be used to screen for toxic concentrations of HOCs using bioassays or biomarker tests. 

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