Direct thermal desorption of semivolatile organic compounds from diffusion denuders and gas chromatographic analysis for trace concentration measurement.

A novel method for collection and analysis of vapor-phase semivolatile organic compounds (SOCs) in ambient air is presented. The method utilizes thermal desorption of SOCs trapped in diffusion denuders coupled with cryogenic preconcentration on Tenax-TA and analysis by high resolution gas chromatography (GC)-electron-capture detection (ECD). The sampling and analysis methods employ custom-fabricated multicapillary diffusion denuders, a hot gas spike (HGS) apparatus to load known quantities of thermally stable standards into diffusion denuders prior to sample collection, a custom-fabricated oven to thermally desorb SOCs from the diffusion denuder, and a programmable temperature vaporization (PTV) inlet containing a liner packed with Tenax-TA for effective preconcentration of the analytes and water management. High flow rates into the PTV inlet of 750mLmin(-1)during thermal desorption are ca. a factor of ten greater than typically used. To improve resolution and retention time stability, the thermal desorption and PTV inlet programming procedure includes three steps to prevent water from entering the analytic column while effectively transferring the analytes into the GC system. The instrumentation and procedures provide virtually complete and consistent transfer of analytes collected from ambient air into the GC evidenced by recovery of seven replicates of four internal standards of 90.7+/-4.0-120+/-23% (mean+/-95% confidence interval, CI). Retention time based compound identification is facilitated by low retention time variability with an average 95% CI of 0.024min for sixteen replicates of eight standards. Procedure details and performance metrics as well as ambient sampling results are presented.

Evaluation of novel techniques for measurement of air-water exchange of persistent bioaccumulative toxicants in Lake Superior.

We report initial measurements of concentrations and net air-water exchange fluxes of target persistent bioaccumulative toxicants (PBTs) in Lake Superior utilizing techniques not previously applied for this purpose. Gaseous PBTs are collected in diffusion denuders containing sections of commercial chromatography columns and subsequently thermally extracted into the cooled injection inlet of a high-resolution gas chromatograph. The PBT sampling/analytical methods enable accurate determination of gas-phase PBT concentration and micrometeorological measurement of fluxes to be carried out. PBT fluxes are measured by the modified Bowen ratio technique in which sensible heat flux is related to PBT flux, with the assumption of identical transfer velocities of heat and PBTs between two heights in the atmospheric surface layer. Micrometeorological measurement of flux accounts for all sources of resistance to mass transfer, including atmospheric stability effects, surface films, waves, sea spray, and bubbles. The sensible heatflux, PBT concentration, and PBT flux measurements carried out in 14 2- or 3-h periods during seven sampling events in Lake Superior in summer and fall 2002 and spring 2003 demonstrate advantages under the constraints of the techniques. The uncertainty of the flux measurements was typically in the range from 1% to 160%. Gaseous concentrations of a-hexachlorocyclohexane (alpha-HCH) and hexachlorobenzene (HCB) over Lake Superior were in the range from 6 to 170 and 12-95 pg/m3, respectively. Fluxes out of Lake Superior were measurable in 75% of the cases in which a concentration gradient was measured, and were in the range from -0.17 to +0.064 ng/m2 x h for alpha-HCH and from -0.60 to -0.093 ng/m2 x h for HCB.