Absorption of semi-volatile compounds in oiled impaction substrates: Measurement of pyrene absorption

Oiled impaction substrates have been used to prevent particle bounce during the collection of size-segregated aerosol samples, which have been analyzed for trace-level airborne organic compounds, including polycyclic aromatic hydrocarbons (PAHs). The use of the oiled impaction substrates, however, may introduce another sampling artifact-the absorption of semivolatile species from the gas phase which could artificially increase the amount of PAHs attributed to the aerosol. In this article, laboratory measurements of the absorption of a particular PAH, pyrene, from the gas phase to impaction substrates of polytetrafluoroethylene membranes impregnated with dibutyl phthalate are reported. Overall mass transfer coefficients are determined from the data. These results are used to calculate the absorption of gas phase PAHs during sampling of size-segregated atmospheric particles. Criteria are developed to determine if the absorption artifact is negligible. The first criterion requires that the analyte be negligibly soluble in the oil; this criterion is met by none of the impaction oils reported in the literature. The second criterion is that species do not have time to reach an equilibrium distribution between the gas phase and impaction oil; this criterion is met for nonvolatile species, those with vapor pressures equal to or less than that of benzo[a]pyrene (3.5 x 10(-6) Pa). We recommend that oiled impaction substrates be used only if the absorption artifact is expected to be negligible on the basis of these criteria.