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 25.0°C for
greater than 20 hours.
Two equilibrium reactions are occuring here:
It takes about
20 hours at 25.0°C
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:
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.
