A discussion regarding the contribution of intrapulmonary gas mixing to O2 isotope fractionation by respiration using experimental data for 36Ar and 40Ar

We determined the argon (Ar) isotope ratio in samples of expired alveolar gas gathered during Ar washout from residual gas relative to this ratio in samples of expired alveolar gas gathered just before the beginning of this washout in 13 young, healthy human subjects at rest. These data were determined for a limited number of breaths in early washout and were used to calculate the relative difference between the alveolar ventilations of 36Ar and 40Ar (ΔrelV˙A(36Ar,40Ar)). Mean ΔrelV˙A(36Ar,40Ar) amounted to 1.6‰ (S.D. = 1.3‰). This result was then used to discuss the contribution of intrapulmonary gas mixing by diffusion to oxygen isotope fractionation of alveolar gas by respiration. On the basis of our finding for ΔrelV˙A(36Ar,40Ar) and further theoretical considerations we arrived at the conclusion that this contribution for subjects at rest is small (about 1‰) and that this contribution is negative irrespective of the level of exercise.