| Summary: | 碩士 === 國立中央大學 === 化學研究所 === 97 === The research includes two parts: the characteristic study on porous materials and the preparation of standard gas mixtures.
In the first part, the adsorption efficiency of porous materials for volatile organic compounds (VOCs) was investigated by gas chromatography (GC) with flame ionization detection (FID). Comparison of sorption characteristics was made between self-prepared silicates and commercial carbon adsorbents via trapping a ppm level gas mixture containing compounds from C2-C10. These self-prepared silicates included microporous silicate-ZSM-5, as well as mesoporous silicates-MCM-41 and SBA-1. To quantitatively describe adsorption efficiency, the term “recovery” was defined and assessed for each silicate by referencing to a well-studied multi-carbon sorbent combination. When trapping with silicates at room temperature the recovery was found to be above 90% for compounds heavier than C7, but was poor for higher boiling compounds. It was also noticed that the poor recovery for high boiling compounds can be improved by increasing the sample pressure.
The second part describes the preparation of standard gas mixtures. Standard gas mixtures containing 21 VOCs from C2-C12 at low parts per million by volume (ppmv) level was prepared by a gravimetric method developed in-house. Capillary tube containing these compounds were prepared and accurately weighed. These tubes then were crushed and flushed by high purity N2 to a 29.5 L evacuated aluminum cylinder, filling to approximately 1000 psi.
Unlike conventional gravimetric method where a large-scale balance is used for weighing filled cylinders to determine final mixing ratios, this study deliberately avoided using the large-scale balance due to its high cost and inaccessibility to average laboratories. Instead, a traceable parts per billion by volume (ppbv) level commercial gas standard was then used to calibrate the concentrations of selected compounds within the self-prepared mixture and to estimate the dilution factor during the dilution process. Because the mole ratios between the 21 compounds were known from weighing, final concentrations of other compounds in the cylinder can then be derived. In this process, GC’s electronic pneumatic controllers (EPCs) were employed as the flow device for diluting the standard mixture from the ppmv to the ppbv level.
Although the gravimetric method is suited for preparing standard mixtures with high accuracy for volatile non-polar compounds, it has limitations for polar or unstable compounds due to possible wall effects. As a result, techniques for making permeation tube were also investigated for on-line calibration of ambient VOCs.
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