The LC Column Dead Volume

The internal volume of a column is occupied by three substances, the mobile phase, the stationary phase and the support. The term mobile phase is a misnomer as it impfies that all the mobile phase is moving, which is not so. The mobile phase within the pores is also stationary and thus constitutes part of the stationary phase. Nevertheless, it is such a well established term, it will still be used to denote the total mobile phase in the column, moving and sialic. The term moving phase will be used for liial fraction of the mobile phase that actually moves, whereas the term static phase will be used for that fraction of the mobile phase that is trapped in the pores, or in the interstices of the support particles, and does not move. 

It should be noted that (Vm) is not the same as (Vm) from the plate theory as (Vm) is moving and some of (Vm) will be static. Similarly, (Vs) is not the same as (Vs) from the plate theory as (Vs) refers only to the stationary phase, whereas (Vs)in these circumstances will also include static mobile phase. 

For simplicity, the two phases will be considered 1 iquid/liquid in nature. Whether, in fact, a reverse phase does constitute a liquid-like phase or not is a moot point and even now a subject of some controversy. In any event, the arguments about to be put forward are independent of the exact nature of the interactions of any solute with the stationary phase, so either form may be assumed. The volume of stationary phase, (Vs) can be replaced by (Adf), (A is the surface area of the stationary phase in the column and (df) is the effective film thickness of the stationary phase) if so desired, and the arguments and the conclusions will remain the same. 

The interstitial volume of the column is also made up of two parts that fraction of the interstitial volume that is moving (V (m)), and that fraction that is close to the points of contact between the individual particles and away from the flow-stream that is essentially static (V (S)) 

When mixed solvents are employed as the mobile phase, it has been shown (10) that one solvent can be absorbed preferentially on, or associated with, the surface of the stationary phase. Consequently, some of the solvent contents of the pore, in close proximity with the stationary phase, will not necessarily have the same composition as that of the original mobile phase. 

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