A large-radius high-mass-resolution multiple-collector isotope ratio mass spectrometer for analysis of rare isotopologues of O2, N2, CH4 and other gases

Abstract

We describe a unique and novel isotope ratio mass spectrometer (IRMS), the Panorama, developed explicitly for high-mass-resolution analysis of isotopologue ratios of gas samples. The double-focussing instrument routinely operates at a mass resolving power of 40,000 with a maximum useful MRP of ∼80,000. The instrument achieves this exceptional MRP for a multi-collector using a Matsuda ion optical design with an ESA radius of 1018mm and a magnetic sector radius of 800mm. Collectors comprise 9 Faraday cups and a single channel of ion counting each with continuously variable collector slits. First results demonstrate both accuracy and precision comparable to, and in some cases, surpassing, other gas-source multi-collector IRMS instruments for singly-substituted species. For example, accurate bulk D/H and 13C/12C for methane gas measured with CH4 as the analyte are measured simultaneously with internal precision of 0.02–0.04‰ (1 std error) and ∼0.006‰ (1 se), respectively. Ion counting with continuous rebalancing of sample and standard gases permits high-precision measurements of rare, multiply-substituted isotopologues with relative abundances as small as ∼0.1ppm. In the case of methane, both 13CH3D/12CH4 and 12CH2D2/12CH4 ratios are measured with precision of ∼0.1‰ and ∼0.5‰, respectively. Accuracy of the multiply-substituted species measurements is demonstrated using isotope ratio mixing experiments. The ability to measure both Δ13CH3D and ΔCH2D2 (‰ variations relative to the stochastic reference frame) provides heretofore unmatched capabilities to identify kinetic reaction pathways, isotope fractionation during transport, mixing, as well as temperatures of formation for methane gas. The high-resolution instrument can be used for a wide variety of applications. For example, it easily resolves 36Ar+ from 18O18O+ for oxygen bond-ordering studies. It also easily resolves 14N16O+ from 15N15N+ for measurements of the doubly-substituted N2 species.