The void volume corresponds to the amount of mobile phase in an LC system. In most separation modes the void volume would be equivalent to the amount of mobile phase needed to elute a compound that doesn’t interact with the stationary phase and is hence unretained.
In size exclusion, analytes are not separated based on chemical interactions with the stationary phase but rather their ability, based on size, to enter pores in the stationary phase. Therefore, in size exclusion the terms total permeation and total exclusion volume have greater significance than void volume. The total permeation volume corresponds to the amount of mobile phase in the system, including that in the pores of the stationary phase, whereas the total exclusion volume does not include the mobile phase within the pores of the stationary phase.
Consequently, the total permeation volume corresponds to the amount of mobile phase needed to elute a compound that is small enough to completely enter into the stationary phase pores and the total exclusion volume is the amount of mobile phase needed to elute a compound that is entirely excluded from the stationary phase pores.
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