Water vapor δ<sup>2</sup>H and δ<sup>18</sup>O measurements using off-axis integrated cavity output spectroscopy

Abstract

Abstract. We present a detailed assessment of a commercially available water vapor isotope analyzer (WVIA, Los Gatos Research, Inc.) for simultaneous in-situ measurements of δ 2 H and δ 18 O in water vapor. This method, based on offaxis integrated cavity output spectroscopy, is an alternative to the conventional water trap/isotope ratio mass spectrometry (IRMS) techniques. We evaluate the analyzer in terms of precision, memory effects, concentration dependence, temperature sensitivity and long-term stability. A calibration system based on a droplet generator is used to characterize the performance and to calibrate the analyzer. Our results show that the precision at an averaging time of 15 s is 0.16‰ for δ 2 H and 0.08‰ for δ 18 O. The isotope ratios are strongly dependent on the water mixing ratio of the air. Taking into account this concentration dependence as well as the temperature sensitivity of the instrument we obtained a long-term stability of the water isotope measurements of 0.38‰ for δ 2 H and 0.25‰ for δ 18 O. The accuracy of the WVIA was further assessed by comparative measurements using IRMS and a dew point generator indicating a linear response in isotopic composition and H 2 O concentrations. The WVIA combined with a calibration system provides accurate high resolution water vapor isotope measurements and opens new possibilities for hydrological and ecological applications.

Monitoring CO2 concentration and δ13C in an underground cavity using a commercial isotope ratio infrared spectrometer

“…A linear approximation for the concentration dependence law is commonly used for IRIS analyzers [3,7,18]. The concentration dependence law obtained in this study is consistent with this approximation, as least in the limited range of concentration that was investigated.…”

“…Third, the δ 13 C dependence on CO 2 concentration is known to be one of the largest sources of uncertainty for IRIS analyzers [3,7,9,18,19] and needs to be carefully checked. Reference gases with 17,800 as well as 2,002 ppm CO 2 concentrations (F and G, Table 1) were stepwise diluted by CO 2 -free synthetic air with 78 % N 2 , 21 % O 2 (Air Products), using two mass flow controllers (SLA5850S, Brooks, range 1 L min −1 ).…”

Water vapor δ2H, δ18O and δ17O measurements using an off‐axis integrated cavity output spectrometer – sensitivity to water vapor concentration, delta value and averaging‐time

“…The water vapor isotopic ratios were expressed in δ-notation as a deviation from a reference ratio [23] ,…”

“…First, in order to study the temporal and spatial dynamics of water vapor δ 17 O values in field settings where different water vapor concentrations and various delta values often occur, we need to evaluate the accuracy and precision of laser-based instruments in complex and volatile environments. Second, in previous observations, the trends in accuracy and precision of δ 2 H and δ 18 O measurements with increasing water vapor concentrations were different for different measuring techniques and instruments [10,[23][24][25] , and even for the same sample for different isotopes [23,25] . Third, the fundamental principle of LAS techniques (e.g., OA-ICOS and CRDS techniques) is measuring the mixing ratio of different isotopes.…”

“…In order to utilize δ [23,24] , delta range [9,26] and averaging-time [10,27] ) on instrument performance will improve our capacity to interpret observations of δ 2 H, δ 18 O and δ 17 O from complex environments.…”