Analytical services: δ2H and δ18O in natural waters

The oxygen (δ18O) and hydrogen (δ2H) isotopic composition of water can be determined by isotopic equilibration of sample water with an appropriate gas mix in a headspace vial in a Gasbench II. Sample water is added to 12 ml exetainer headspace vials and then flushed with an appropriate gas mixture. A period of isotopic equilibration allows atoms to exchange with the headspace flush gas until the headspace gas has taken on the isotopic composition of the sample water (after  correcting for liquid-gas phase fractionation). Because the two elements need different equilibrations gases, two separate analyses must be performed to analyze both elements. The SPA lab can analyze the same aliquot of water for both analyses at a small loss of precision (mostly for δ2H).  However, the small amount of water required for each analysis (200 μL) usually allows for analysis of separate aliquots of water. After this equilibration period, the headspace gas is entrained into a He stream, water vapor is removed, passed through a gas chromatographic (GC) column and finally introduced into the MAT-253 mass spectrometer for isotopic analysis.

During analysis, two different calibrated in-house water standards are interspersed between samples to normalize and correct for possible drift during the analysis run. Two calibrated laboratory standards are run "as a sample" for quality control and to monitor long-term performance. We use a light in-house standard derived from winter precipitation and a heavier in-house standard derived from summer precipitation. These laboratory standards, which span most of the δ18O isotopic range found in nature, have been previously calibrated against NIST Standard Reference Materials (SMOW, SLAP, and GISP). We use a two-point linear normalization approach and report delta values relative to Standard Mean Ocean Water (SMOW).

δ18O in natural waters

The δ18O value of water can be easily determined by equilibrating the water with a helium flush gas containing 3% CO₂. The vials are then allowed to equilibrate for 18 hours (usually overnight). Minimum sample size is 200 μL.

δ2H in natural waters

The δ18O value of water can be determined by equilibrating the water with a helium flush gas containing 2% H2.  A platinum catalyst is added to vial to facilitate isotopic exchange in the gas phase. Copper and charcoal can be added for increased precision should the researcher prefer, though we have not seen a large increase in precision for most natural waters with medium to low DOC. The vials are then allowed to equilibrate for 3 hours. Minimum sample size is 200 μL.

δ2H and δ18O on same water aliquot

Separate analyses are required to obtain both δ2H and δ18O on the same aliquot of water. First, the δ18O is determined per the above procedures. Next, the exetainer caps are removed, a platinum catalyst rod is added and vials are sealed with new caps, flushed and analyzed per the procedures for δ2H outlined above. The analyses take 3 days to complete. Minimum sample size is 200 μL.

δ2H and δ18O of water rates

Isotope	Phase	Quantity	Measurement Precision	Internal	External
δ18O	Liquid	0.2–1 mL	± 0.2 ‰	$6	$10
δ2H	Liquid	0.2–1 mL	± 1 ‰	$6	$10
δ18O and δ2H	Liquid	0.2–1 mL	± 0.2 and ±3 ‰	$11	$18

Sample types: natural waters

Sample requirements: Water samples for δ2H and/or δ18O should be submitted in vials with no head space. As noted above, we usually need at least 200 μL for each analysis. For small sample amounts, we use 2 ml shorty vials with conical-bottomed caps. If you ship extracted water samples in glass tubes and need part of your samples to be saved, please provide us with appropriate vials for return shipment. While we can extract water from plant material using an extraction line, this is only practical for researchers very nearby to Skidmore College.