A new automated method for measuring noble gases and their isotopic ratios in water samples

Abstract

A method is presented for precisely measuring all five noble gases and their isotopic ratios in water samples using multiple programmed multistage cryogenic traps in conjunction with quadrupole mass spectrometry and magnetic sector mass spectrometry. Multiple automated cryogenic traps, including a two-stage cryotrap used for removal of water vapor, an activated charcoal cryotrap used for helium separation, and a stainless steel cryotrap used for neon, argon, krypton, and xenon separation, allow reproducible gas purification and separation. The precision of this method for gas standards is ±0.10% for He, ±0.14% for Ne, ±0.10% for Ar, ±0.14% for Kr, and ±0.17% for Xe. The precision of the isotopic ratios of the noble gases in gas standards are ±1.9‰ for 20Ne/22Ne, ±2.0‰ for 84Kr/86Kr, ±2.5‰ for 84Kr/82Kr, ±0.9‰ for 132Xe/129Xe, and ±1.3‰ for 132Xe/136Xe. The precision of this method for water samples, determined by measurement of duplicate pairs, is ±1% for He, ±0.9% for Ne, ±0.3% for Ar, ±0.3% for Kr, and ±0.2% for Xe. An attached magnetic sector mass spectrometer measures 3He/4He with precisions of ±0.1% for air standards and ±0.14% for water samples.