11 Sep 2023
 | 11 Sep 2023
Status: this preprint is currently under review for the journal AMT.

A versatile water vapor generation module for vapor isotope calibration and liquid isotope measurements

Hans Christian Steen-Larsen and Daniele Zannoni

Abstract. A versatile vapor generation module has been developed for the purpose of both field water vapor isotope calibrations and laboratory liquid water isotope measurements. The vapor generation module is fully scalable allowing in principle an unlimited number of standards or samples to be connected, opening up the possibility for calibrating with multiple standards during field deployment. Compared to a standard autosampler system, the vapor generation module has a more than 2 times lower memory effect. The vapor generation module can in principle generate a constant stream of vapor with constant isotopic composition indefinitely. We document an Allan Deviation for 17O-excess (Δ17O) of less than 2 per meg for an approximate 3 hour averaging time. For similar averaging time the Allan Deviation for 𝛿17O, 𝛿18O, 𝛿D, d-excess is 0.004, 0.006, 0.01, 0.03 ‰. Measuring unknown samples show that it is possible to obtain an average standard deviation of 3 per meg leading to an average standard error (95 % confidence limit) using 4–5 replicates of 5 per meg.

Using the vapor generation module we document that an enhancement in the Allan Deviation above the white noise level for integration times between 10 minutes and 1 hour is caused by cyclic variations in the cavity temperature. We further argue that increases in Allan Deviation for longer averaging times could be a result of memory effects and not only driven by instrumental drifts as it is often interpreted as.

The vapor generation module as a calibration system have been document to generate a constant water vapor stream for a period of more than 90 hours showing the feasibility of being used as an autonomous field vapor isotope calibration unit for more than 3 months.

Hans Christian Steen-Larsen and Daniele Zannoni

Status: open (until 16 Oct 2023)

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Hans Christian Steen-Larsen and Daniele Zannoni

Hans Christian Steen-Larsen and Daniele Zannoni


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Short summary
The water vapor generation module is completely scalable allowing autonomous calibrations to use N standards and measurements to use an integration time only restricted by sample availability. Using this module, we document an improved reproducibility in 17O-excess liquid measurements of nearly a factor two compared to previously reported measurement capabilities using laser spectroscopy analyzers. This improvement is making spectroscopy measurements comparable isotope-ratio mass spectrometry.