external image h20_equil.jpg
Note: This system as shown is interfaced to a Delta Plus isotope ratio mass spectrometer with an installed dual inlet system at the Cornell Isotope Laboratory.
This system is designed to equilibrate liquid samples at set unit temperatures and rotation rates. Equilibration flasks are loaded with a specific volume of liquid and then backfilled with either hydrogen or carbon dioxide gas.
Below is an outline with set unit parameters and for the successful completion of these isotope measurements.
The unit shown can hold 24 equilibration flasks so the analysis is quite slow in being able to turn over large number of samples, particularly when taking equilibration time into account.

GFL unit parameters for the measurement of d18O from water:
Gas used for backfilling: CO2
Temperature: 30C
Rotation rate: 80 rotations per minute
Equilibration time: 10 hours (36000 seconds)

Typical precision for this isotope measurement is below 0.1 per mil standard deviation. Normally we use 4-5 of the 24 flask spaces for our in-house standard. The in-house standard is calibrated against three international reference waters (VSMOW, GISP, and SLAP).
Tips for success: Before beginning analysis we check to make sure that water or liquid samples are within a pH range that would not be detrimental to the dual inlet system (generally water samples that have a pH between 3-10 pose no problems). We also check for any organic matter that may interfere with the analysis. Samples are filtered in-house before analysis. If this is done we also run our in-house standard through the same process to check for any background effect that the filtering may cause (usually this is not a factor). One last tip is to make sure that the water trap on the equilibration unit is working properly. We use a mix of methanol and dry ice which is stirred into a slurry (can also use methanol and liquid nitrogen) and then place this on the trap. We have found that by heating the trap every so often (every 3-4 sample runs) prevents a buildup of ice within the trap which can cause gas flow problems from the equilibration unit to the dual inlet system.