A unified method for measuring noble gas isotope ratios in air, water, and volcanic gases via dynamic mass spectrometry

Abstract

We describe a new method for the purification, transfer, and analysis of heavy noble gas isotope ratios in water, air, or volcanic gas samples using a dynamic dual-inlet isotope-ratio mass spectrometer (IRMS). IRMS noble gas analysis offers the potential for orders-of-magnitude higher precision than traditional (static) noble gas mass spectrometry. However, due to both inherent IRMS requirements and challenges associated with sample collection and purification, measuring noble gases via IRMS has typically been limited to samples with an air-like elemental composition. Our new method allows for highly accurate and precise measurements of Ar, Kr, and Xe isotope ratios via IRMS, independent of sample composition, thereby opening the door to air, water, or volcanic gas analyses all made with the same technique. This approach exploits the fact that purified heavy noble gas samples are dominated by 40Ar, such that matching 40Ar ion beams between sample and reference gas streams results in pressure balancing. We further introduce new techniques for precisely quantifying and correcting for (i) Kr and Xe isotope non-linearity and matrix effects, (ii) interference of the low energy 40Ar tail on 36Ar and 38Ar ion beams, and (iii) the impact of short-term background fluctuations on trace Xe isotope measurements. We also present results from adsorption-desorption experiments with multiple adsorbent materials across a range of temperatures. For adsorption on silica gel, we find small equilibrium isotopic fractionation of Kr (εeq = −0.37 ± 0.04‰ amu−1, ±2 SE) and Xe (εeq = −0.06 ± 0.02‰ amu−1, ±2 SE) that may be relevant to studies of terrestrial Xe isotope signatures. We demonstrate that adsorption and desorption of heavy noble gases on silica gel at 77 K and 303 K, respectively, is highly reproducible and thus constitutes a viable and efficient method for transferring gases without reliance on cryostats or liquid helium.