Isotopic Analysis of Water δ18O

Sample Material: Freshwater or seawater
(no brines)

Sample Size: 2 to 5ml (for a single analysis)

Sample Collection: See below

Sample Preparation: None
(filtration and desalination are not required)

Precision: ±0.05 permil for δ18O

Method:
The δ18O of water samples is measure using the water-CO2 equilibration method modified from Epstein and Mayeda (1953). In this method 5ml of sample (or standard) water is pipetted into a glass bottle. In total 24 glass bottles are filled with a combination of samples and standards. Note, standards are selected so that the range of expected sample δ18O values is bracketed. Vials are connected to the equilibration line (Photo at right), placed in a water bath (18°C), and allowed to temperature equilibrate for 15 minutes. After temperature equilibration the headspace of the vials is pumped out for ~10 minutes, and then refilled with CO2 gas. Samples are then allowed to isotopically equilibrate for 12 hours. Throughout the entire process samples are slowly shaken to aid isotopic equilibration. Finally the water is analyzed by dual inlet mass spectrometry using the DeltaPlus XL.

Suggested Reading:

Epstein, S. and Mayeda, T. (1953). Variations of 18O content of waters from natural sources. Geochimica et Cosmochimica Acta, 4, 213-224.

Water Collection Method (for δ18O analysis):

1. Start with a clean and dry glass container (>10 ml capacity) that has a tight fitting lid preferably with a plastic seal of some sort (e.g., 20ml scintillation vials with polyseal cap). Avoid using plastic vials. If samples are to be stored for a long period before analysis (> month) do not use caps with septa due to the potential for evaporation through the septa over time.

2. Carefully fill container with sample while preventing turbulence. If possible, use a tube to fill sample bottle from the bottom and flush bottle 1 or 2 times. Fill bottle completely and make sure there is no headspace or air bubbles on the sides of the bottle before capping. Once capped, no air should be in the sample bottle. Ideally, the sample should have limited exposure to air while sampling.

3. Wrap parafilm or tape around the cap to prevent the cap from loosening. When applying the parafilm (or tape) wrap it in the direction of tightening the cap otherwise it may inadvertently loosen the cap.

4. If possible, store sample in a larger plastic bottle that can be capped and sealed. Add a little sample water to this larger plastic bottle to keep the humidity high.

5. If possible store in a cool location. Do NOT freeze the sample.