A Mass Spectrometric System for the Analysis of Noble Gases and Tritium from Water Samples

The design, setup, and performance of a mass spectrometric system for the analysis of noble gas isotopes (3He, 4He, 20Ne, 21Ne, 22Ne, 36Ar, 40Ar, 84Kr, 136Xe) and tritium (3H) from water samples are described. The 3H concentration is measured indirectly by the 3He ingrowth from radioactive decay. After extraction, purification, and separation, the noble gases are measured in two noncommercial double-collector 90° magnetic sector mass spectrometers. We present a new approach for the analysis of the heavy noble gas isotopes that enables, in principle, simultaneous measurement of Ar, Kr, and Xe. Typical precisions of the measurements of 3H, He, Ne, Ar, Kr, and Xe concentrations are ±2.7%, ±0.3%, ±0.9%, ±0.3%, ±0.8%, and ±1.0%, respectively. For the isotopic ratios 3He/4He, 20Ne/22Ne, and 40Ar/36Ar the typical precisions are ±0.7%, ±0.3%, and ±0.2%. These values express the reproducibility of the measurement of an internal freshwater standard and include the overall stability of the system as well as of the extraction procedure. To verify the method, the noble gas concentrations of air-saturated water samples prepared under controlled conditions are compared with noble gas solubility data. The 20Ne/22Ne and 36Ar/40Ar fractionation during solution is estimated from 70 surface water samples to be −2.0 ± 0.2‰ and −1.3 ± 0.2‰, respectively.