Applications of a microtrap for on-line monitoring of volatile organics.

摘要

Microtrap is made by packing a narrow metal tubing with adsorbents. The advantage of a microtrap is that it can be heated and cooled in the order of seconds. It has been used previously as a concentration cum injection device for on-line gas chromatography and also monitoring non-methane organic carbon in air emissions. In this research breakthrough and desorption characteristics of the microtrap were studied. A two-stage microtrap system was developed to reduce breakthrough while making sharp injection for GC separation. Microtrap was also used as a concentrator cum injector in on-line mass spectrometry. Finally, a microtrap based, continuous non-methane organic carbon analyzer was field tested at an industrial site.;Breakthrough characteristics of the microtrap were studied as a function of analyte concentration. The logarithm of breakthrough volume decreased linearly with the logarithm of adsorbate concentration at low concentration. At high concentration, breakthrough volume remained constant. The adsorption isotherms illustrated that retention of methanol and acetone on Carbopack B was by monolayer adsorption while those of benzene and acetone on Carbopack C were by multilayer adsorption. Microtrap temperature was measured using an infrared thermocouple. Desorption efficiency at a given temperature depended upon the analyte as well as the adsorbent. The desorption peak width decreased with increasing desorption temperature and sample flow rate.;A two-stage microtrap system was developed by connecting two microtraps in series. The first microtrap, packed with relatively more adsorbent, prevented breakthrough of small molecules, and served as the retention trap. The second, smaller diameter trap provided rapid desorption and served as the injection trap. Two-stage microtrap increased the breakthrough time for large volume sampling without decreasing chromatographic resolution.;Microtrap was used as an interface for mass spectrometry. The objective was to provide preconcentration and elimination of background molecules such as CO2 and H2O. Different configurations combining the microtrap with a gas sampling valve were studied. On-line microtrap with backflush desorption was found to be most effective in direct sampling mass spectrometry. Due to the elimination of background gases, the detection limit was as low as the parts per trillion level. Emission from a catalytic incinerator was monitored using this technique.;A previously developed continuous non-methane organic carbon (C-NMOC) analyzer was field tested at a coating facility in North Carolina. The C-NMOC analyzer demonstrated high accuracy and high precision in the field study. The advantages of real-time monitoring, such as immediate response for transient events were also demonstrated. Continuous monitoring was possible in the presence of high concentrations of moisture and carbon dioxide.