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

摘要

Here we present an experimental setup for water stable isotope (δ18O and δD) continuous-flow measurements and provide metricsdefining the performance of the setup during a major ice core measurementcampaign (Roosevelt Island Climate Evolution; RICE). We also use themetrics to compare alternate systems. Our setup is the first continuous-flowlaser spectroscopy system that is using off-axis integrated cavity outputspectroscopy (OA-ICOS; analyzer manufactured by Los Gatos Research, LGR) incombination with an evaporation unit to continuously analyze water samplesfrom an ice core.A Water Vapor Isotope Standard Source (WVISS) calibration unit,manufactured by LGR, was modified to (1) enable measurements on severalwater standards, (2) increase the temporal resolution by reducing theresponse time and (3) reduce the influence from memory effects. Whilethis 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 and18 s for 2013 and 2014 setup, respectively) compared to the original WVISSunit (~ 62 s), which is an improvement in measurementresolution. Another improvement compared to the original WVISS is that thecustom setups have a reduced memory effect.Stability tests comparing the custom and WVISS setups were performed andAllan deviations (σ_Allan) were calculated to determineprecision at different averaging times. For the custom 2013 setup theprecision after integration times of 103 s is0.060 and 0.070 ‰ for δ18O and δD, respectively. The corresponding σ_Allan values for the custom 2014 setup are 0.030, 0.060 and 0.043 ‰ for δ18O, δD and δ17O, respectively. For the WVISSsetup the precision is 0.035,0.070 and 0.042 ‰ after 103 sfor δ18O, δD and δ17O, respectively. Boththe custom setups and WVISS setup are influenced by instrumental drift withδ18O being more drift sensitive than δD. The σ_Allan values for δ18O are 0.30 and0.18 ‰ for the custom 2013 and WVISS setup, respectively,after averaging times of 104 s (2.78 h). Using response timetests and stability tests, we show that the custom setups are more responsive(shorter response time), whereas the University ofCopenhagen (UC) setup is more stable. More broadly,comparisons of different setups address the challenge of integratingvaporizer/spectrometer isotope measurement systems into a CFA campaign withmany other analytical instruments.