mean free path, λ in chromatography

https://dx.doi.org/10.1351/goldbook.M03778
The average distance a molecule travels between collisions. For a molecule, λ = ( 2 π n d m 2 ) −1, where n is the number of molecules per unit volume and d m is their mean diameter. For O2 at one atmosphere and 25 °C, this distance is only 9.7 × 10 −6 cm; at 10 −6 atmospheres and 25 °C it is 9.7 cm. For an aerosol particle, the mean free path, λ B in the Stokes region (see Stokes law) is given by: λ B = 3 k T m m B where m is the mass of the particle, k is the Boltzmann constant (1.381 × 10 −23 J K −1), T is the temperature (K) and B is the mobility.
Source:
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2201