Caution on the use of Viton® or FETFE® O-rings in carbon dioxide sample containers for δ¹⁸0 analysis

After 10 days, 3-μmol 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 δ¹⁸O measurements of most laboratories. No change in ¹³C content was observed. Increasing the sample size to 100 μmol 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 μmol. 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.