Do SPMDs sample only dissolved or vapor phase chemicals?
Nonporous polymeric films such as low density polyethylene (membrane of choice for SPMDs) contain transient cavities having maximum diameters of about 10 Å. These cavities are far too small to accommodate colloids or macromolecular dissolved organic carbon such as humic acids. Also, comparisons of chemical concentrations determined by traditional analytical methods in ultra filtered river water (colloids and DOC > 50 Å diameter were removed) to those estimated from SPMDs exposed to unfiltered water, appear to confirm that SPMDs sample only dissolved residues, which are readily bioavailable. Finally, note that water quality criteria and threshold limit values (airborne chemicals) are based on dissolved and vapor phase concentrations, respectively, and that toxicity data bases are as well.
