Water isotopes

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Sample material: rain water, sea water, snow, glacier ice, ground water, river runoff 

Precision: typically < 0.1 permil for δD and < 0.025 permil for δ18

Instrument: Picarro L2140-i with Vapourizer and Autosampler 

Sample requirements: min. 1.5 ml sample liquid in evaporation-proof container 

Routines: options from fast method (6 injections per sample) to low memory (12 injections per sample) for high-precision d-excess measurements 

For analysis of δ18O, δ17O, and δD in water, samples are filtered, if needed, with 25mm Nylon filters with a 0.2 μm PTFE membrane (part #514-0066, VWR, USA), and transferred to 1.5 ml glass vials with rubber/PTFE septa (part #548-0907, VWR, USA). An autosampler (A0325, Picarro Inc) transferred ca. 2μl per injection into a high-precision vapourizer (A0211, Picarro Inc, USA) heated to 110°C. After blending with dry N (< 5 ppm H2O) the gas mixture was directed into the measurement cavity of a Cavity-Ring Down Spectrometer (L2140-i, Picarro Inc) for about 7 min with a typical water concentration of 20 000 ppm. 

Memory effects are reduced by two times measuring a vapour mixture at a mixing ratio of about 50 000 ppm, obtained from 2 injections of about 2 μl of sample for 5 min at the beginning of each new sample vial. Thereafter, another 6-12 injections of about 2 μl per sample are measured individually as described above, and averages of the last 3 to 4 injections are used for further processing. 

Several laboratory standards are measured at the beginning and end of each batch. Batches consist typically of 20 samples, including a drift standard measured every 5 samples. For calibration according to IAEA recommendations, at least two laboratory standards matching the range of the samples are used. During calibration, linear drift of the analyser during the time it takes to run the batch is assumed. 

Long-term measurement precision is 0.15 ‰ for δD and 0.02 ‰ for δ18O, resulting in a measurement precision of 1.0 ‰ for d-excess. 

The method for δ17O-excess is still under development.