The use of Nafion membranes to measure 2H/1H and 18O/16O isotopic ratios in water

Abstract The isotopiccomposition of water ( 2 H/ 1 H and 18 O/ 16 O) has been widely used in hydrology, ecology, paleoclimatology and forensic science. However, sampling frequency limits many such studies and there is now a clear need for field-portable mass spectrometer and laser-based spectrometer devices to measure water isotopiccomposition in-situ and at high temporal resolution. Here we explore the use of Nafionmembranes as a potential Membrane Introduction (MI) inlet system for high frequency isotope analysis. As yet, the fractionation behaviour of 2 H/ 1 H and 18 O/ 16 O isotopicratios in water transported through Nafionmembranes has not been investigated. We quantify this behaviour for water samples with different matrices (organic matter and pollutants) and salinity concentrations across a wide range of isotopicratios and different membrane thicknesses. Nafionmembranes showed no fractionation effects on reported isotoperatios for natural waters. Also no fractionation effects were detected with salinity. Membrane thickness affected slightly the precision and accuracy of the isotopicratio analysis and our tests showed that thinner Nafionmembranes provide better results. However, for samples contaminated with organic matter, the thicker membranes performed better for the 2 H/ 1 H isotopicratio, while for samples contaminated with pollutants, the thicker membranes performed better for the 18 O/ 16 O isotopicratio. Overall, Nafionmembranes appear well suited to MI inlet use and our work suggests that the optimal Nafionmembrane thickness is 50–150 µm.