Role of conducting airways in partial separation of inhaled gas mixtures.

A positive (hollow) cast of the bronchial tree was made from a pig's lung. Gas mixtures containing sulfur hexafluoride (SF6) and helium (He), and SF6 and argon (Ar), were blown down the cast at two different flows, the cast having first been filled with air. Gas was sampled by a mass spectrometer probe from 1-mm-diam branches situated on short, medium, and long pathways. The front of the SF6 appeared in advance of the fronts of the He and the Ar. This relative advancement was greater a) with the SF6/He mixture than with the SF6/Ar mixture; b) at slower flows; and c) on longer pathways. With reverse flow up the cast using SF6/He there was little differences between the arrival times of the two gas fronts at either flow. These results could be explained by the effects of Taylor dispersion on gases having different diffusion coefficients.