Slight pressure imbalances can affect accuracy and precision of dual inlet-based clumped isotope analysis

Abstract

It is well known that a subtle nonlinearity can occur during clumped isotope analysis of CO₂ that – if remaining unaddressed – limits accuracy. The nonlinearity is induced by a negative background on the m/z 47 ion Faraday cup, whose magnitude is correlated with the intensity of the m/z 44 ion beam. The origin of the negative background remains unclear, but is possibly due to secondary electrons. Usually, CO₂ gases of distinct bulk isotopic compositions are equilibrated at 1000 °C and measured along with the samples in order to be able to correct for this effect. Alternatively, measured m/z 47 beam intensities can be corrected for the contribution of secondary electrons after monitoring how the negative background on m/z 47 evolves with the intensity of the m/z 44 ion beam. The latter correction procedure seems to work well if the m/z 44 cup exhibits a wider slit width than the m/z 47 cup. Here we show that the negative m/z 47 background affects precision of dual inlet-based clumped isotope measurements of CO₂ unless raw m/z 47 intensities are directly corrected for the contribution of secondary electrons. Moreover, inaccurate results can be obtained even if the heated gas approach is used to correct for the observed nonlinearity. The impact of the negative background on accuracy and precision arises from small imbalances in m/z 44 ion beam intensities between reference and sample CO₂ measurements. It becomes the more significant the larger the relative contribution of secondary electrons to the m/z 47 signal is and the higher the flux rate of CO₂ into the ion source is set. These problems can be overcome by correcting the measured m/z 47 ion beam intensities of sample and reference gas for the contributions deriving from secondary electrons after scaling these contributions to the intensities of the corresponding m/z 49 ion beams. Accuracy and precision of this correction are demonstrated by clumped isotope analysis of three internal carbonate standards. The proposed correction scheme can be easily applied if the slit width of the m/z 49 Faraday cup is bigger than that of the m/z 47 cup.

Keywords:

• background correction
• carbon-13
• carbon dioxide
• carbonates
• clumped isotopes
• heated gas correction
• isotope measurements
• methods and equipment
• oxygen-18
• pressure baseline correction

Acknowledgements

We like to thank Cedric John and an anonymous reviewer for their helpful comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was enabled through DFG grants FI-948/4-1 and FI-948/7-1. The authors also acknowledge support through the LOEWE funding programme (Landes-Offensive zur Entwicklung wissenschaftlich-ökonomischer Exzellenz) of Hesse's Ministry of Higher Education, Research, and the Arts.

Supplemental data

Supplemental data for this article can be accessed http://dx.doi.org/10.1080/10256016.2015.1010531.