interparticle porosity, ɛ in chromatography, in chromatography

https://dx.doi.org/10.1351/goldbook.I03115
The interparticle volume of a packed column per unit column volume:
ɛ = V 0 V c
It is also called the interstitial fraction of the column.
Source:
PAC, 1993, 65, 819 (Nomenclature for chromatography (IUPAC Recommendations 1993)) on page 832