Thin Pervaporation Membranes for Improved Performance in On-line Flow Injection Analysis Membrane Introduction Mass Spectrometry

Abstract

Characterization of some thin (10–50 μm) and selective (zeolite filled) sheet pervaporation membrane materials for the on-line analysis of volatile compounds in solution by membrane introduction mass spectrometry and flow injection analysis is reported. A comparison of the membrane performance, both in aqueous (normal) and organic (reverse) phase, is made using a seven-component test mixture and a triple quadrupole mass spectrometer. While no significant difference is observed in the normal phase experiments, reverse phase experiments with the thin membranes showed an improvement in rise times and analysis times by a factor of two. Thus, the thin membranes are more suitable for chemical and petrochemical on-line process monitoring where the frequency of sampling is of great importance. The thin hydrophobic membranes allow enough water to permeate the membrane for chemical ionization to be performed as an alternative to electron impact provided an ion trap mass spectrometer is used. The detection limits observed for the compounds studied using water as a chemical ionization reagent are similar to those achieved by a two-stage enrichment microporous membrane/jet separator device interfaced to an ion trap mass spectrometer and are in the lower parts-per-billion range for some of the compounds studied (5 ppb for methyl ethyl ketone, 5 ppb for acetone). Thus the thinner hydrophobic membranes offer the unique combination of rapid permeation and low detection limits in chemical ionization experiments with an ion trap.