Enhancement of extraction efficiency and reduction of boundary layer effects in pulse introduction membrane extraction

Membrane separation has emerged as an attractive alternative for interfacing an extraction step directly to a gas chromatograph or to a mass spectrometer. In pulse introduction (or flow injection type) membrane extraction, a sample pulse is introduced onto an eluent stream that transports it onto the membrane. Since a fixed sample volume is injected, the detector response is directly proportional to the extraction efficiency. This in turn depends on membrane module design, flow conditions, etc. Also, when water contacts a membrane, a static boundary layer is formed at the membrane surface that serves as an additional diffusional barrier to the permeation process. Consequently, permeation slows down, which lowers the speed of analysis. In this paper, methods of increasing the extraction efficiency and decreasing boundary layer effects are presented. The goal is to have higher sensitivity at a shorter analysis time. A stream of nitrogen is introduced into the membrane after sample elution to eliminate the aqueous boundary layer. This technique is found to be effective not only for faster analysis, but also for increasing extraction efficiency.