LAUSR

Abstract The isotopic composition of water (2H/1H and 18O/16O) 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 isotopic composition in-situ and at high temporal resolution. Here we explore the use of Nafion membranes as a potential Membrane Introduction (MI) inlet system for high frequency isotope analysis. As yet, the fractionation behaviour of 2H/1H and 18O/16O isotopic ratios in water transported through Nafion membranes 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 isotopic ratios and different membrane thicknesses. Nafion membranes showed no fractionation effects on reported isotope ratios for natural waters. Also no fractionation effects were detected with salinity. Membrane thickness affected slightly the precision and accuracy of the isotopic ratio analysis and our tests showed that thinner Nafion membranes provide better results. However, for samples contaminated with organic matter, the thicker membranes performed better for the 2H/1H isotopic ratio, while for samples contaminated with pollutants, the thicker membranes performed better for the 18O/16O isotopic ratio. Overall, Nafion membranes appear well suited to MI inlet use and our work suggests that the optimal Nafion membrane thickness is 50–150 µm.