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 δ32/28 and δ44/28; 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 δ32/28 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.
Stephan Räss et al.
Status: open (until 20 Jun 2023)
- RC1: 'Comment on amt-2023-96', Anonymous Referee #1, 25 May 2023 reply
Stephan Räss et al.
Stephan Räss et al.
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For more than seven decades, relative isotope-ratio-delta values have been measured precisely by mass spectrometry by switching back and forth between two gases rapidly. Figure 6 of McKinney et al. (1950, https://doi.org/10.1063/1.1745698) shows magnetically operated glass-to-glass sealing valves for introducing two gas samples alternatively to the source of the mass spectrometer and a waste pump. In the intervening seven decades, dual inlet introduction systems have advanced substantially. This manuscript by Räss et al. (2023) presents the state-of-the-art for a dual inlet system in which the amount of sample gas is large (not limited). The performance of this inlet system for clumped isotopes is excellent as shown in Table 4, which is a performance comparison between an Elementar isoprime precision having this new inlet system and a Thermo Scientific 253 Ultra HR.
Perhaps the biggest limitation with this new inlet system is the relatively large amount of sample gas required. Readers will want to know the minimum amount of sample that can be analyzed. Additionally, it would be useful if the authors indicated the nominal length of the glass capillary between the NIS-II container and the IRMS. Are there constraints or concerns about its length?
I would encourage the authors to review the first sentence in this manuscript (Most isotope ratio mass spectrometers (IRMS) are fed by means of a changeover-valve-based dual inlet system). Manufacturers can provide sales data, but I suspect the majority of isotope ratio mass spectrometers are now sold for analysis of environmental and biological samples using continuous-flow introduction peripherals, and they do not require dual inlet systems.
This manuscript would benefit from English grammar editing. Addition of fifty or more commas would improve readability.