| Membrane extraction with a sorbent interface (MESI) is a new technique for the analysis of volatile or semi-volatile organic compounds. The method has been developed to enable rapid routine analysis and short- or long-term on-site continuous monitoring. A MESI system comprises a membrane extraction module, a sorbent interface, a gas chromatograph (GC), and a computer. This study employed a silicone hollow fiber membrane, which can selectively extract some organic compounds and exclude water from the GC system. The sorbent interface enables on-line preconcentration and injection and the computer-controlled system enables the automatic operation of trapping and injection, which makes the technique flexible in extraction time and sensitivity. MESI has many advantages including no use of solvent, ease of automation, simplicity, efficiency, low cost, the possibility of on-line and on-site monitoring, and good selectivity and sensitivity.; Two mathematical models have been developed to describe the processes of extraction from air and water. The models have been found to be in good agreement with experimental results. Some important parameters which effect the extraction efficiency have been theoretically discussed and experimentally investigated. These include membrane length, membrane wall thickness, stripping gas flow rate, temperature, pressure, humidity, sample size, and agitation. The partition coefficient and the diffusion coefficient of the analyte are the most important parameters in membrane extraction. Their influence on extraction rate and response time have been extensively investigated and are discussed. The partition coefficients and diffusion coefficients of some analytes (in the membrane) were experimentally determined.; Estimation of air concentration without additional external calibration, on the basis of the mathematical model for air extraction, was investigated. The experiment showed that the method has advantages of simplicity, speed, and reasonable accuracy. Quantitation based on different extraction processes was studied and the steady-state extraction process is highly recommended for this application. To increase the limit of detection, various approaches were used, including microwave heating, stop-flow extraction, membrane probe cooling, and heating.; For on-site headspace monitoring a practical field sampling device was designed and coupled to MESI. The extraction was assisted by use of a microfan and by magnetic stirring, and improved extraction efficiency was observed. Headspace monitoring was also performed by placing the extraction module under water. Real-time headspace monitoring of a fermentation process demonstrates the potential use of MESI. |
