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

Membrane extraction has been interfaced with gas chromatography and mass sp ectroscopy for the analysis of volatile organics in water. The vacuum in a mass spectrometer provides fast mass transport The time required to complet e permeation in a GC interface can be fairly long, because the positive pre ssure of the carrier gas on the permeate side slows down the analyte permea tion. 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 inpu t pulse to the membrane. The overall effect of these two factors is to incr ease the analysis time. Gas injection of aqueous samples is presented in th is paper to address these issues. Gas injection reduces the formation of bo undary layer, and increases the overall diffusion coefficient seven times. Axial mixing of the sample with a gaseous eluent is minimal, and this elimi nates the tailing in permeation profiles. The overall membrane extraction i s 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 operati onal procedures.