Topics - Column Volumes

Column Volumes Although apparently a simple property, the column volume in chromatography (particularly liquid chromatography), is, in fact, very complex and is made up of several individual volumes all of which play important, but different, parts in solute retention. The simple geometric column volume obtained by multiplying the cross-sectional area by the length is made up of three primary volumes, the volume of mobile phase in the column, the volume stationary phase in the column and the volume of the support/adsorbent matrix. The volume of mobile phase is made up of a volume of moving mobile phase (the mobile phase between the particles) and static mobile phase contained in the pores of the particles. The stationary phase (usually taken as the volume of liquid or bonded phase material in the column) is also divided into two parts. Due to the range of pore sizes of the support/adsorbant, for a solute having molecular of a given size, the molecules can only enter those pores that will allow its penetration and, thus, will only come into contact with a portion of the stationary phase. Thus, for a given solute there will be a volume of stationary phase available to the solute and a volume of stationary phase unavailable to the solute. In some cases there may be a third fraction of stationary phase that resides in completely blocked pores. These different column volumes must be divided up even further, but this can not be discussed as a topic. The use of the correct volume in data processing can be extremely important. For example to calculate capacity ratios for retention measurements associated with the Plate theory, the total volume of mobile phase in the column is used as the dead volume. To calculate the capacity ratios and the mobile phase velocity for use in the Rate theory equations, the volume of moving mobile phase must be employed (in conjunction with the dead time for phase velocity calculations). Note, as a result of the different dead volumes, the capacity ratio as defined by the Plate Theory differs significantly from that defined by the Rate theory.