Measurement of dissolved neon by isotope dilution using a quadrupole mass spectrometer

We describe an analytical method with improved precision for measuring the concentration of neon dissolved in water. Measurements were calibrated by isotope dilution, with addition of a 22Ne spike directly to the evacuated sample flask prior to water collection. The use of glass flasks for water sampling, with flushing of air from the system by CO2, greatly reduced the chance of contamination. Dissolved gases were extracted from the water sample by equilibration with a headspace and cryogenically processed to remove interferences. The 22Ne/20Ne ratio was then measured on a quadrupole mass spectrometer operated in static mode. Mass fractionation by the system was accounted for by frequent analyses of the 22Ne/20Ne ratio in air. We routinely obtained precisions of ±0.13% based on the standard deviation of duplicate field samples. Accuracy of the method, in the form of possible systematic offsets that could skew all the data, was assessed to be ±0.18%, excluding a possible ±0.2% error in the estimate of the Ne concentration of air.