| dc.description.abstract | Ambient volatile organic compounds (VOCs) are known to induce various health or environmental concerns, as most of which are either toxic or precursors of some other secondary air pollutants. Monitoring of these species in the air with adequate time resolution constitutes the first step towards the understanding their impact on the environment. An automated gas chromatographic system aiming at performing unattended hourly measurement of VOCs and ozone precursors was developed in house. To encompass volatile organic compounds of a wide range of volatility, two different designs in sample enrichment and delivery were explored in order to simultaneously separate VOCs of C3-C12 in ambient air. In our first design, dual traps and dual columns was adopted to cope with the large volatility difference between these VOCs, in which the C3-C6 species are separated on one set of trap and column, whereas the C6-C12 species are separated on the other. Two detectors are needed for these two channels of separation, which has been the basic design concept found in many ozone precursor monitoring stations. These two detectors are usually flame ionization detectors (FID), plus the two sets of hardware of valves and traps, hence imposing great demands on gas consumption and routine maintenance for keeping the systems running in a continuous mode. To reduce the complexity of the system, a different design employing the heat-cut technique was developed to simplify the system by using only single trap and FID for the two channels. In such a design, plumbing was designed to allow C6-C12 peaks from the DB-1 column following C2-C5 peaks from the PLOT column resulting in a single combined chromatogram. Furthermore, stop flow technique was configured to manipulate peak retention times into peak groups, so that chunks of peaks from one column can be surgically inserted into the gaps of the other chromatogram to make the synthesized chromatogram more condensed but without losing resolution.
This innovative “peak tailoring” technique is simply in design, less costly, and extremely robust for prolonged use as no moving parts are used, which is beneficial for maintaining remote monitoring stations. | en_US |