After 10 days, 3-??mol CO2 samples in containers having glass stopcocks with Viton?? or FETFE O-rings were enriched in 18O by 1.5% as a result of absorption by the elastomer; this amount of enrichment is ???20 times greater than the precision of ??18O measurements of most laboratories. No change in 13C content was observed. Increasing the sample size to 100 ??mol resulted in an 18O enrichment of 0.2% and did not affect the 13C content. Caution needs to be exercised in selecting sample containers for CO2 isotope-ratio samples of < 200 ??mol. If stopcocks are used in construction of containers for such samples, the use of all-glass stopcocks with Apiezon N?? hydrocarbon-based grease will eliminate the fractionation of oxygen isotopes. ?? 1991.
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Caution on the use of Viton?? or FETFE?? O-rings in carbon dioxide sample containers for ??180 analysis