Reference Materials: We are tied to the VSMOW international scale via internal laboratory reference waters measured against VSMOW and SLAP for our water isotope measurements.
Sample Submission: If you would like to submit water samples for δ18O analysis, we prefer a minimum of 5 mL of frozen sample in a labeled water tight plastic container that is large enough and strong enough to withstand water expansion upon freezing. Simple legible sample IDs are appreciated. We need to know if it is fresh or salt water. Please include in your shipment container the following form (pdf or doc) and email Andy.
Exhaustive description of analysis: Vials containing 200 µL of water samples and standards are evacuated and filled with carbon dioxide (CO2) and allowed 8 hours at 35 °C to exchange oxygen in the water with oxygen in the CO2. The CO2 is then expanded through a cold trap held at -90 °C for water removal and into a variable volume bellow. Mass / charge 44,45,46 of the sample CO2 gas are measured alternately with a reference CO2 gas in a Micromass Isoprime dual-inlet isotope ratio mass spectrometer. Samples are interspersed with internal laboratory reference waters calibrated against VSMOW (δ18O = 0 ‰) and SLAP (δ18O = -55.5 ‰), using GISP (δ18O = -24.8 ‰) as a quality control reference.
Calculations: The Masslynx software provided δ18O values relative to the reference gas (along with all other selected data) are imported into a matlab script for data reduction. A linear regression is completed using the raw vial δ18O values for two of our internal reference waters and the accepted values. All data are corrected to VSMOW using this linear equation. Precision and accuracy are estimated for each run using a third internal reference water that is treated as an unknown. Reference waters are chosen to flank the sample range in δ18O values. All reference water δ18O values can be seen here.