The term “column volume” usually refers to the void volume, which represents the volume of mobile phase that is between the silica particles. This area is referred to as the interstitial space. You can estimate void volume by multiplying the total column volume (pi x radius2 x length) by a factor that estimates the typical packing efficiency for a particular column type. For fully porous columns, the equation for void volume (in mL) is V = (0.68) pi r2 L, where V = column volume in mL; r = column radius in cm; and L = column length in cm. For superficially porous columns, such as our Raptor columns, the factor is different, and the equation is V = (0.50) pi r2 L.
Void volume is more commonly estimated experimentally by injecting a standard containing an analyte that is known to have no, or negligible, retention on that particular column phase. A good example of this for reversed-phase HPLC is uracil. One should be aware that this estimation is also subject to extra column dead volume for the specific instrument that is being used, so it may vary slightly.
