Automated calibration of laser spectrometer measurements of δ18O and δ2H values in water vapour using a Dew Point Generator. (14th May 2018)

• Record Type: Journal Article
• Authors: Munksgaard, Niels C.; Cheesman, Alexander W.; Gray‐Spence, Andrew; Cernusak, Lucas A.; Bird, Michael I.
• Is Part Of: Rapid communications in mass spectrometry. Volume 32:Number 12(2018)
• Page Start: 1008
• Page End: 1014
• Publication Date: 2018-05-14
• Subjects: Mass spectrometry -- Periodicals; 543.65
• DOI: 10.1002/rcm.8131
• Languages: English
• ISSNs: 0951-4198

Abstract

Abstract : 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 δ 18 O and δ 2 H 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 δ 18 O and δ 2 H 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.