dc.description.abstract | New sorptive organic-inorganic hybrid materials, i.e., polydimethylsiloxane (PDMS) and mesoporous silica molecular sieves MCM-41, were employed in this research for sampling volatile organic compounds (VOCs) in ambient air due to their high thermal stability, surface area, and homogeneity.
The sol-gel technique was employed to synthesize PDMS coated onto a glass rod to form a large volume sorption medium for sampling and analysis of VOCs when coupled with a gas chromatograph (GC). In order to accommodate the PDMS rod for thermal desorption, a regular GC injector was modified. Since desorption from the large volume PDMS was not as rapid as the commercial solid phase microextraction (SPME) due to its large volume, a micro-sorbent trap was installed downstream of the injector for re-focusing the VOCs prior to column separation. The modified injection system was tested for linearity and reproducibility by introducing various amounts of pure benzene, toluene, ethylbenzene, m-, o-xylenes (BTEX) with R2 coefficient and RSD% better than 0.99 and 2%, respectively, demonstrating the validity of the injection system.
To standardize the extraction condition for testing sorption ability of the PDMS rod, a headspace bottle was designed which held a fixed amount of BTEX standard solution to produce a constant BTEX concentration in the headspace. Reproducibility of 2-14% for BTEX was observed based on 5 repeated headspace extraction of 10 ?g/L (ppb) BTEX standard solution.
Increase coating thickness of PDMS on the glass rod and hence the sorption ability was attempted by repeatedly immersing the glass rod into fresh sol solution. The increase in film thickness as confirmed by SEM resulted in dramatic enhancement of the extraction amount of BTEX.
End capping was proven to be a crucial step in determining the sorption ability of PDMS. Our study indicates that addition of end-capping reagent into sol solution at a proper timing is necessary to prevent PDMS gel from excessive cross-linking and to preserve the sorption ability of PDMS.
Our study also showed that the desorption temperature for BTEX from PDMS rod required a minimum temperature of 80 ?C with a residual of only less than 0.2%, suggesting a full desorption and therefore little carry-over was possible.
Another sorption material, meso-porous silica MCM-41 with a pore size of 46.6Å, was also tested for sorption ability of VOCs using the same extraction and injection devices. Increase of sorption amount over a prolonged exposure of BTEX was observed for both PDMS and MCM-41 suggesting their potential applications as a passive sampler. | en_US |