The total volume of mobile phase in the column regardless of where it exists; V_M = V_e + V_i, where V_e is the interstitial volume and V_i is the intraparticle volume. Also called the column void volume. IUPAC indicates that use of the term dead volume should be eliminated for this concept. The use of dead volume is limited to regions not swept by the flowing mobile phase system. Holdup volume is measured by injecting an unretained species that fits in all the pores. See also interstitial porosity and intraparticle porosity.