Fig. 5.

Chromatographic traces of ¹²C¹⁶O¹⁶O (A) and ¹³C¹⁶O¹⁶O (B) from a 25-μL mixture of methane, ethane, and propane from which the δ¹³C value is calculated for each component (C). The large amount of noise in the baseline of C arises mainly from the noise in the measurement of ¹³C¹⁶O¹⁶O. The noise in the ¹³C¹⁶O¹⁶O measurement is approximately independent of its concentration within the pressure regime we operate, which leads to significant fluctuations in δ¹³C at baseline levels. As the sample passes through the instrument, the higher concentrations of CO₂ allow for relatively more precise measurements that results in the δ¹³C calculation to be made with a precision of <1‰.