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

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.