CAUTION ON THE USE OF VITON OR FETFE O-RINGS IN CARBON-DIOXIDE SAMPLE CONTAINERS FOR DELTA-O-18 ANALYSIS

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