High-resolution continuous-flow analysis setup for water isotopic measurement from ice cores using laser spectroscopy

Here we present an experimental setup for water stable isotope (δO and δD) continuous-flow measurements and provide metrics defining the performance of the setup during a major ice core measurement campaign (Roosevelt Island Climate Evolution; RICE). We also use the metrics to compare alternate systems. Our setup is the first continuousflow laser spectroscopy system that is using off-axis integrated cavity output spectroscopy (OA-ICOS; analyzer manufactured by Los Gatos Research, LGR) in combination with an evaporation unit to continuously analyze water samples from an ice core. A Water Vapor Isotope Standard Source (WVISS) calibration unit, manufactured by LGR, was modified to (1) enable measurements on several water standards, (2) increase the temporal resolution by reducing the response time and (3) reduce the influence from memory effects. While this setup was designed for the continuous-flow analysis (CFA) of ice cores, it can also continuously analyze other liquid or vapor sources. The custom setups provide a shorter response time (∼ 54 and 18 s for 2013 and 2014 setup, respectively) compared to the original WVISS unit (∼ 62 s), which is an improvement in measurement resolution. Another improvement compared to the original WVISS is that the custom setups have a reduced memory effect. Stability tests comparing the custom and WVISS setups were performed and Allan deviations (σAllan) were calculated to determine precision at different averaging times. For the custom 2013 setup the precision after integration times of 10 s is 0.060 and 0.070 ‰ for δO and δD, respectively. The corresponding σAllan values for the custom 2014 setup are 0.030, 0.060 and 0.043 ‰ for δO, δD and δO, respectively. For the WVISS setup the precision is 0.035, 0.070 and 0.042 ‰ after 10 s for δO, δD and δO, respectively. Both the custom setups and WVISS setup are influenced by instrumental drift with δO being more drift sensitive than δD. The σAllan values for δ O are 0.30 and 0.18 ‰ for the custom 2013 and WVISS setup, respectively, after averaging times of 10 s (2.78 h). Using response time tests and stability tests, we show that the custom setups are more responsive (shorter response time), whereas the University of Copenhagen (UC) setup is more stable. More broadly, comparisons of different setups address the challenge of integrating vaporizer/spectrometer isotope measurement systems into a CFA campaign with many other analytical instruments.