Automated calibration of laser spectrometer measurements of δ¹⁸O and δ²H values in water vapour using a Dew Point Generator

Rationale: Continuous measurement of stable O and H isotope compositions in water vapour requires automated calibration for remote field deployments. We developed a new low-cost device for calibration of both water vapour mole fraction and isotope composition. 

Methods: We coupled a commercially available dew point generator (DPG) to a laser spectrometer and developed hardware for water and air handling along with software for automated operation and data processing. We characterised isotopic fractionation in the DPG, conducted a field test and assessed the influence of critical parameters on the performance of the device. 

Results: An analysis time of 1 hour was sufficient to achieve memory-free analysis of two water vapour standards and the δ¹⁸O and δ²H values were found to be independent of water vapour concentration over a range of ≈20,000–33,000 ppm. The reproducibility of the standard vapours over a 10-day period was better than 0.14 ‰ and 0.75 ‰ for δ¹⁸O and δ²H values, respectively (1 σ, n = 11) prior to drift correction and calibration. The analytical accuracy was confirmed by the analysis of a third independent vapour standard. The DPG distillation process requires that isotope calibration takes account of DPG temperature, analysis time, injected water volume and air flow rate. 

Conclusions: The automated calibration system provides high accuracy and precision and is a robust, cost-effective option for long-term field measurements of water vapour isotopes. The necessary modifications to the DPG are minor and easily reversible.