An evaluation of low vapor pressure liquids for membrane introduction mass spectrometry

Abstract

Four liquids of low vapour pressure have been examined for use as semi-permeable membranes in the sampling and analysis of volatile organic compounds by the technique of membrane introduction mass spectrometry (MIMS). The chosen liquids are inert and hydrophobic and can be formed to any desired thickness or shape. The selected polymers—polyphenyl ether, alkylated cyclopentane, perfluorinated ether and silicone oil—were supported on a microporous substrate and mounted in a direct insertion membrane probe. Polyphenyl ether, alkylated cyclopentane and silicone oil each formed stable semi-permeable barriers which passed the analytes of interest while discriminating strongly against the water solvent. These highly stable liquids also showed no significant loss of mass or contribution to the background of the mass spectrometer. Optimal injection volumes (2.25 ml) and membrane temperatures (90°C) gave 10%–90% rise times of 44–55 s for the three liquids compared with 35 s for a reference silicone membrane. Comparable detection limits at the low-ppb level were observed for benzene, toluene, trans-1,2-dichloroethylene, chlorobenzene, carbon tetrachloride and nitrobenzene using the silicone reference and alkylated cyclopentane membranes. © 1997 John Wiley & Sons, Ltd.