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Analytical Methods

d2H and d18O of waters by equilibration

(Note that we currently use the TC/EA method for waters but this equilibration method is available for specific needs)

  1. d18O of water: The analyst places 0.5mL of water into a 12mL exetainer vial. The vial is flushed with a blend of 0.5% CO2 in He for 15 minutes. This is then allowed to equilibrate on a heating block at 25.0C for greater than 20 hours. Two equilibrium reactions are occuring here:

     

    • CO2(g) <=> CO2 (aq)                 (CO2 is exchanged between the gaseous and aqueous phase)

    • CO2 (aq) + H2O <=> H2CO3     (dissolved CO2 reacts with water to form carbonic acid)

     

    It takes about 20 hours at 25.0C for the isotope exchange between gaseous CO2 and water to reach equilibrium. Some literature indicates that this equilibrium time can be dramatically decreased by using an ultrasonic bath. We may experiment with this in the future.

    Samples are run once (unless otherwise requested) along with 6 replicates of 3 different standards. Two of the standards used should bracket the isotope ratio of the samples and the third standard is used as a check of the normalization. Although there is isotopic fractionation between gaseous CO2 and dissolved CO2, as well as between dissolved CO2 and H2CO3, this same fractionation occurs to the same extent with all of the samples and standards. Hence, by normalizing the results with respect to the working standards, fractionation associated with these phase changes and reactions cancels out in the calculations.

    The standard deviation reported for the samples is the mean of the standard deviation of the results for the standards.

    It should also be noted that carbonic acid has a pKa of 6.4 and consequently in alkaline conditions it will dissociate to form bicarbonate:

     

    • H2CO3 + OH- <=> HCO3- + H2O

     

    As a result of this, water samples with a relatively high pH will remove more of the CO2 from the headspace, making it is necessary to verify that there is still enough CO2 gas to get a good signal for the analysis.

    This file gives a step by step account of how do process the results from a full sequence of samples. This is an example of the results we get for d18O of waters and how we process them.

     

  2. d2H of water: The analyst places 0.3mL of water into a 12mL exetainer vial. The vial is flushed with a blend of 2% H2 in He for 15 minutes. This is then allowed to equilibrate on a heating block at 25.0C for greater than 4 hours. A Pt catalyst is added (a plastic rod with a Pt dust coating on it, called a Hokku bead). Gaseous H2 adsorbs onto the Pt surface and dissociates and reassociates. Water vapor does the same. Isotopic equilibrium between H2 and H2O occurs within about one to two hours, however, we allow a minimum of 4 hours so that no doubt can be cast on whether or not equilibrium had been reached.

     

    As is the case with d18O analysis, a minimum of 6 replicates of 3 different standards is run along with the individual samples. Two of the standards are chosen to bracket the samples and the third standard is used as a check of the quality of the normalization curve.

    The standard deviation reported for the samples is the mean of the standard deviation of the results for the standards.

    This file gives a step by step account of how do process the results from a full sequence of samples using d18O. This is an example of the results we get for d2H of waters and how we process them.

 

Page last updated: Aug 9, 2006

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