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

Abstract

Abstract. A versatile vapor generation module has been developed for both field-based water vapor isotope calibrations and laboratory-based liquid water isotope measurements. The vapor generation module can generate a stream of constant vapor at a wide variety of humidity levels spanning 300 to 30 000 ppmv and is fully scalable, allowing in principle an unlimited number of standards or samples to be connected to a water vapor isotope analyzer. This versatility opens up the possibility for calibrating with multiple standards during field deployment, including examining instrument humidity–isotope dependence. Utilizing the ability to generate an uninterrupted constant stream of vapor, we document an Allan deviation for 17O-excess (Δ17O) of less than 2 per meg for an approximate 3 h averaging time. For similar averaging time, the Allan deviations for δ17O, δ18O, δD, and d-excess are 0.004 ‰, 0.005 ‰, 0.01 ‰, and 0.04 ‰, respectively. Measuring unknown samples shows that it is possible to obtain an average standard deviation of 3 per meg for Δ17O and an average standard error (95 % confidence limit) of 5 per meg. Using the vapor generation module, we document that an increase in the Allan deviation above the white noise level for integration times between 10 min and 1 h is caused by cyclic variations in the cavity temperature, which if improved upon could result in an improvement in liquid sample measurement precision of up to a factor of 2. 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. The vapor generation module as a calibration system has been documented to generate a constant water vapor stream for more than 90 h, showing the feasibility of being used to integrate measurements over much longer periods than achievable with syringe-based injections as well as allowing the analysis of instrument performance and noise. Using clean in-house standards, we have operated the vapor generation module daily for 1–3 h for more than 6 months without the need for maintenance, illustrating its potential as a field-deployed autonomous vapor isotope calibration unit. When operating the vapor generation module for laboratory-based liquid water isotope measurements, we document a more than 2 times lower memory effect compared to a standard autosampler system.