Rationale: 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.
Methods: 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.
Results: 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‰.
Conclusions: 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.
Copyright © 2017 John Wiley & Sons, Ltd.