Revision of an open-split-based dual inlet system for elemental and isotope ratio mass spectrometers with a focus on clumped isotope measurements

Abstract. In this work we present a revision of an open-split-based dual inlet system for elemental and isotope ratio mass spectrometers (IRMS), which was developed by the Climate and Environmental Physics Division of the University of Bern two decades ago. Besides discussing the corresponding improvements we show that with this inlet system (NIS-II) external precisions can be achieved that are high enough to perform measurements of multiply-substituted isotopologues (clumped isotopes) on pure gases. For the clumped isotope ratios 35/32 and 36/32 of oxygen we achieved standard deviations of 3.4 ⋅ 10⁻⁹ and 4.9 ⋅ 10⁻⁹, respectively, that we calculated from 60 interval means (20 s integration) of pure oxygen gas measurements.

Moreover, we report various performance tests and show that with the NIS-II delta values of various air components can be measured with precisions of order tenths of per meg and higher. In addition, we demonstrate that our new open-split-based dual inlet system allows to measure some of these delta values with significantly higher precisions than a NIS-I (precursor of NIS-II) and a conventional changeover-valve-based dual inlet system (tests performed with an Elementar iso DUAL INLET). The greatest discrepancies between the NIS-II and the iso DUAL INLET were observed for δ_32/28 and δ_44/28; the differences in the external precisions were 4 per meg and 35 per meg (10 SA/STD measurements), respectively. With respect to the reproducibility of δ_32/28 means, the deviations from a reference value were even larger, namely around 0.1 ‰.

Due to the successful preliminary tests regarding measurements of clumped isotope ratios, we will continue our work in this area to perform clumped isotope studies according to common practices.