Vapor pressures of the polychlorinated dibenzodioxins (PCDDs) and the polychlorinated dibenzofurans (PCDFs)

A dynamic gas saturation technique was used to measure the solid-phase vapor pressures ( p S o) of six polychlorinated dibenzodioxins (PCDDs) and seven polychlorinated dibenzofurans (PCDFs) at 25°C. Three polycyclic aromatic hydrocarbons (phenanthrene, anthracene, and chrysene) were also examined to validate the method. The p S o values of the three PAHs at 25°C were all within a factor of 2 of values previously obtained by a similar technique. The measured p S o values were converted to sub-cooled liquid vapor pressure ( p L o) values using entropy of fusion information. For eight of the PCDD/F congeners, which included four toxicologically significant 2,3,7,8-substituted PCDD/F congeners, the p L o values obtained here at 25°C are significantly different than the values that can be extrapolated from higher temperatures using previously available p S o values. The log p L o values at 25°C obtained in this study were regressed vs.: (1) corresponding values of gas chromatography retention index (GC-RI); (2) number of chlorines; (3) molecular weight; and (4) melting point. The best correlation was obtained using GC-RI, and was used together with information on the temperature-dependent molar enthalpy of vaporization to develop equations for predicting temperature-dependent p L o values for all 210 PCDD/Fs.