Use of Nafion as a Membrane Separator in Membrane Introduction Mass Spectrometry

Abstract

Nafion is a commercially available perfluorosulphonate cation exchange membrane commonly used as a perm-selective separator in chlor-alkali electrolysers and as the electrolyte in solid polymer fuel cells. In our experiments, a Nafion sheet membrane serves as the interface between the aqueous sample and the vacuum in membrane introduction to the mass spectrometer (MIMS). The penetration by volatile polar compounds (VOC-methanol, ethanol, 1-propanol), volatile non-polar compounds (VOC-benzene, toluene and p- xylene), semi-volatile low polar compounds (SVOC-fluorobenzene, chlorobenzene and bromobenzene) and non-volatile polar compounds (o-chlorophenol, m-chlorophenol and p-chlorophenol) in aqueous solution through the Nafion membrane to the mass spectrometer was studied. In all cases, a simple fragmentation pattern of the intact molecule was observed, typically with m/z = nominal mass + 1 as the most intensive ion current, which suggests that the ionisation process takes part in which water acts as the chemical ionisation reagent. No additional gases were needed for chemical ionisation. We also measured detection limits and linear dynamic ranges of all observed compounds with Nafion membrane MIMS. The observed detection limits were in the order of ppb for the alcohol and aromatic groups and for the halogenbenzene and monochlorophenol groups they were in the order of ppm. Linear dynamic ranges for all tested compounds were one order of magnitude.