A new negative and positive gas ion source for isotope ratio mass spectrometry

The goal of this research was to construct an effective gas ion source for isotope ratio mass spectrometry, which generates sufficiently strong and stable ion beams for multiple isotope ratio analysis of sulfur and oxygen from readily available gases such as SO₂ , CO₂ , CO, O₂ and chlorine from CH₃ Cl gas.

By means of a 60° sector mass spectrometer equipped with a new type of sensitive ion source and with a dual-inlet system and triple collector assembly, we have measured ³⁴ S/³² S and challenging ³⁶ S/³² S isotope ratios using positive sulfur ions generated from SO₂ gas. The new ion source differs from the Nier-type ones by the electron beam being replaced with a tungsten filament. The obtained isotope signals of ³² S⁺ , ³⁴ S⁺ and ³⁶ S⁺ were from 1 to 5 V (depending on the pressure of admitted gas) on high-ohm resistors of 200 MΩ, 5 GΩ and 500 GΩ, respectively. The variable ³³ S peak was not recorded as useful due to interference from H³² S⁺ ions.

Although the ionizing electrons in the new ion source have significant energy dispersion the obtained peak profiles are flat-topped and they lead to good resolving power for the mass spectrometer (M/∆M ≈ 100). The achieved precision of δ³⁴ S and δ³⁶ S determination was better than ±0.01 and ±0.1‰, respectively. The determination of ³⁷ Cl/³⁵ Cl in a volatile organic compound CH₂ Cl₂ was easily achieved on Cl^- negative ions with a precision of ±0.005‰.

The new ion source has great potential in the quantitative analysis of gases on positive ion mass spectra as well as, in the case of gases with high electron affinity such as CH₃ Cl, CH₃ Br, SF₆ , SiF₄ , on negative ones. It usefulness was demonstrated on an example of multiple sulfur isotope analysis and ³⁷ Cl/³⁵ Cl ratio determination with great precision. A negative ion source of this type will probably be useful in a more complicated instruments used in SIMS or AMS. Copyright © 2017 John Wiley & Sons, Ltd.