Noble gas isotope analysis (He to Xe) with an iterative trapping method on a split flight tube mass spectrometer

Abstract

Noble gas isotopes are being increasingly applied in groundwater and fugitive gas tracing and dating studies. Analyzing all five stable noble gases is important to acquire as much geological information as possible. However, Ar–Kr separation has been a main challenge for noble gas analysis because Ar is over eight thousand times more abundant than Kr. The residual Ar released with Kr is considerable and interferes with the Kr measurement. This study explores a simple and iterative trapping method for Ar–Kr separation. This iterative trapping method improves both noble gas trapping efficiency and time of the whole procedure. The Helix SFT is a static vacuum gas-source mass spectrometer for the high-precision analysis of He by the simultaneous collection of ³He and ⁴He using a split flight tube. It is also capable of measuring isotopes of Ne up to Xe through peak jumping, although encumbered by the issue of remnant magnetism in the pole pieces which offsets the He peaks after measuring heavier noble gases. This study tested the performance of the Helix SFT using a new inversed tuning protocol that resolves the issue of remnant magnetism of the pole pieces when jumping back from higher masses for He analysis. With a newly designed pneumatic processing line and the iterative trapping method, noble gases have been successfully separated and yield satisfying results. Sensitivities of this analysis system have been determined for He (1.72 × 10⁻⁴ A Torr⁻¹) at a trap current of 400 μA, Ne (9.79 × 10⁻⁵ A Torr⁻¹) at 200 μA, Ar (2.28 × 10⁻⁴ A Torr⁻¹) at 125 μA, Kr (5.09 × 10⁻⁴ A Torr⁻¹) and Xe (1.01 × 10⁻³ A Torr⁻¹) at 200 μA. The 1σ precision of the isotopic ratios in air standards is ³He/⁴He (±1.39%), ²⁰Ne/²²Ne (±0.26%), ⁴⁰Ar/³⁶Ar (±0.19%), ⁸⁶Kr/⁸⁴Kr (±0.09%), and ¹³⁶Xe/¹³⁰Xe (±0.22%). The iterative trapping method and the inversed tuning protocol allow the Helix SFT to measure beyond He despite the remnant magnetism of the pole pieces.