Concentration effects on laser‐based δ¹⁸O and δ²H measurements and implications for the calibration of vapour measurements with liquid standards

Abstract

Recently available isotope ratio infrared spectroscopy can directly measure the isotopic composition of atmospheric water vapour (δ(18) O, δ(2) H), overcoming one of the main limitations of isotope ratio mass spectrometry (IRMS) methods. Calibrating these gas-phase instruments requires the vapourisation of liquid standards since primary standards in principle are liquids. Here we test the viability of calibrating a wavelength-scanned cavity ring-down spectroscopy (CRDS) instrument with vapourised liquid standards. We also quantify the dependency of the measured isotope values on the water concentration for a range of isotopic compositions. In both liquid and vapour samples, we found an increase in δ(18) O and δ(2) H with water vapour concentration. For δ(18) O, the slope of this increase was similar for liquid and vapour, with a slight positive relationship with sample δ-value. For δ(2) H, we found diverging patterns for liquid and vapour samples, with no dependence on δ-value for vapour, but a decreasing slope for liquid samples. We also quantified tubing memory effects to step changes in isotopic composition, avoiding concurrent changes in the water vapour concentration. Dekabon tubing exhibited much stronger, concentration-dependent, memory effects for δ(2) H than stainless steel or perfluoroalkoxy (PFA) tubing. Direct vapour measurements with CRDS in a controlled experimental chamber agreed well with results obtained from vapour simultaneously collected in cold traps analysed by CRDS and IRMS. We conclude that vapour measurements can be calibrated reliably with liquid standards. We demonstrate how to take the concentration dependencies of the δ-values into account. Copyright © 2010 John Wiley & Sons, Ltd.

Measuring variations of δ<sup>18</sup>O and δ<sup>2</sup>H in atmospheric water vapour using two commercial laser-based spectrometers: an instrument characterisation study

“…This uncertainty of the water vapour mixing ratio correction propagates into the isotope measurement uncertainty. Similar dependencies were found by Schmidt et al (2010) for the L1102-i version of the water vapour isotope instrument by Picarro. Since the correction can be different for every instrument, this characterisation step has to be done individually for every instrument.…”

“…Many recent studies indicate similar performance of laser and conventional IRMS systems in terms of precision (Kerstel et al, 2006;Gupta et al, 2009;Sturm and Knohl, 2010). Equally relevant for atmospheric field applications, however, is the overall measurement uncertainty resulting from a range of factors like calibration, sensitivity to variations in water concentration, and retention effects from the tubing (Brand et al, 2009;Schmidt et al, 2010;Johnson et al, 2011;Rambo et al, 2011). Thus, a detailed assessment of the novel laser instruments and testing of their performance in field conditions is necessary to fully characterise the measurement uncertainty and to correct for biasing effects.…”

Characterizing moisture exchange between the Hawaiian convective boundary layer and free troposphere using stable isotopes in water

“…A LEAP Technologies PAL (Prep and Load) autosampler was used to inject each standard by syringe into a Picarro vaporization module, which flash evaporated the liquid injection before delivery to the Picarro instrument. Although Schmidt et al [2010] found some difference in the way Picarro vapor isotopic analyzers measure δD when the sample is liquid versus vapor, no significant difference was found in δ 18 O. To reduce cross-contamination between standards, each standard was injected 15 times and isotope ratios from the last four injections were averaged to determine the final measurement.…”

“…To reduce cross-contamination between standards, each standard was injected 15 times and isotope ratios from the last four injections were averaged to determine the final measurement. A known concentration bias in the Picarro vapor isotopic analyzers causes the instrument to measure isotope ratios differently at both very low and very high humidity [Schmidt et al, 2010;Tremoy et al, 2011;Aemisegger et al, 2012]. To account for this, the five waters were sampled at up to seven mixing ratios (approximately 0.3, 0.6, 1.2, 2.4, 3.3, 6.5, and 11.9 g/kg) that were generated by controlling the amount of liquid injected into the vaporizer.…”

“…While the effects on δ 18 O measurements were found to be negligible, effects on δD boundary layer measurements were evident for one profile. As a result, and because δD exhibits a slower response time to ambient changes (a larger "memory effect") and larger relative errors with calibration [Schmidt et al, 2010;Aemisegger et al, 2012], δ 18 O measurements are used preferentially throughout the analysis.…”