interstitial volume

in gas chromatography
https://doi.org/10.1351/goldbook.I03122
The volume V G of the column not occupied by the liquid phase and its solid support, or by the active solid. It does not include any volume external to the column, such as the volume of the sample injector or of the detector.
Source:
Orange Book, 2nd ed., p. 99 (http://media.iupac.org/publications/analytical_compendium/)