Gas injection membrane extraction for fast on-line analysis using GC detection

Membrane extraction has been interfaced with gas chromatography and mass spectroscopy for the analysis of volatile organics in water. The vacuum in a mass spectrometer provides fast mass transport. The time required to complete permeation in a GC interface can be fairly long, because the positive pressure of the carrier gas on the permeate side slows down the analyte permeation. The aqueous boundary layer formed on the membrane is considered to be the biggest contributor to the resistance to mass transfer. Another issue is the dispersion of analyte in the aqueous stream, which broadens the input pulse to the membrane. The overall effect of these two factors is to increase the analysis time. Gas injection of aqueous samples is presented in this paper to address these issues. Gas injection reduces the formation of boundary layer, and increases the overall diffusion coefficient seven times. Axial mixing of the sample with a gaseous eluent is minimal, and this eliminates the tailing in permeation profiles. The overall membrane extraction is found to be significantly faster when a gas is used to inject an aqueous sample. This method is also simpler in terms of instrumentation and operational procedures.