Abstract
The widespread method for measuring Δ17O (17O-excess) is an offline CoF3 (Cobalt tri-fluoride) conversion of water to molecular oxygen with subsequent isotope determination by dual inlet mass spectrometry. High precisions for Δ17O measurements, using CoF3 water conversion, are so far only possible with off-line methods. Here we report on an improved and modified online continuous flow method intended for high precision triple oxygen isotope analysis. This method is improved by optimizing the reactor (site for conversion of H2O into oxygen through the chemical reaction) compositions, size of the fused silica capillary, flow regulator, and data treatment. Our modified online continuous method was further compared with the recently developed cavity ring down measurement principle. The precision is significantly better for the commercially available laser-based system than our current version of improved online CoF3 conversion method using mass spectrometry. Factors identified for limiting precision in our continuous flow system are: (i) compaction of the reactor with time that leads to the restriction of flow rate of carrier gas, (ii) the CoF3 treatment, (iii) the amount of CoF3 inside the reactor, (iv) the pore size of the steel frit, and (v) the metallic tube. Changes in all of these items as well as the dimension of the fused silica capillary, the positioning of the fused silica capillary in the open split, and the memory effect can also lead to a declining precision. These limiting factors for precision still provide us enough space for further improvement of our improved online method which will be worthwhile for the measurement of smaller aliquot samples as fluid inclusions for palaeoclimatic applications. With present improvement, multiple injections (n = 15 or even more) should be applied to obtain a precision better than 10 per meg for Δ17O. Furthermore, a comparison of the laser-based system with an improved conventional equilibration method has been made on precipitation samples originating from Jungfraujoch.
Highlights
Water stable isotopes are being used as an ideal tracer to reveal the underlying processes in the hydrologic cycle as well as past climatic information through their sensitive character during evaporation, condensation, mixing, and moisture transport (Dansgaard, 1964; Galewsky et al, 2016)
To characterize potential influences of the carrier gas on the eluted O2 sample signal, we performed an experiment where a 2 μl H2O sample was allowed to react with CoF3 and pass through the cryogenic cool trap without any carrier gas (Supplementary Figure 3)
Twothird of the sample signal were obtained within 30 min from the sample injection, while a small percentage of the sample was still obtained at the end of measurements which indicates the sticky nature of water that can lead to fractionation effects within our system line and reduce precision. These results document the importance of using a carrier gas in an online system to obtain a Gaussian shape sample signal that significantly helps to minimize fractionation effects and reducing measurement time
Summary
Water stable isotopes (primarily oxygen isotopic ratio) are being used as an ideal tracer to reveal the underlying processes in the hydrologic cycle as well as past climatic information through their sensitive character during evaporation, condensation, mixing, and moisture transport (Dansgaard, 1964; Galewsky et al, 2016). Concomitant differences in mass and symmetry of the stable isotopologues (HH16O, HH17O, HD18O, etc.) of the water during phase changes result in isotopic fractionations (mass dependent) that is being used as a tool to understand the hydrological cycle and its link to climate (Epstein and Mayeda, 1953; Dansgaard, 1954; Craig, 1961). The isotopic ratios of 18O/16O or 17O/16O in samples are denoted with 18R or 17R which is commonly reported to an international reference (i.e., VSMOW) standard through eqs. The isotopic ratios of 18O/16O or 17O/16O in samples are denoted with 18R or 17R which is commonly reported to an international reference (i.e., VSMOW) standard through eqs. 1 and 2 where delta values have become a standard tracer for environmental studies generally denoted in permil ( )
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
