Preparative Chromatography - The Loading Capacity of a Column > Page 4

Equation (1), although, apparently simple in form, has some very significant implications on preparative column design. It is clear that increasing radius and length of the column increases both the maximum sample volume and the maximum sample mass. It is also seen that increasing the column length will also increase the column efficiency (unless it is accompanied by an corresponding increase in the particle diameter).

However, increasing the column efficiency will have the opposite effect, as seen by equation (1), it will reduce the maximum sample load. Consequently, if the necessary efficiency to achieve the required separation has been obtained, then if the column is lengthened to increase the loading capacity for optimum performance, either the flow rate will need to be increased to reduce the efficiency and thus maintain the maximum loading, or the particle size will need to be increased to reduce the efficiency to its required value. However, an increased flow rate will also reduce separation time and thus increase sample throughput. Conversely, the alternative use of larger particles will increase column permeability and thus the column can be operated at a lower pressure and be constructed of lighter materials. Again, a an alternative, if the sample is merely a two component mixtures such as a chiral pair, using automated sampling on a long permeable column will permit multiple injections, so that there can be a number of solute pairs in the column being separated at one time and be eluted sequentially, each solute being eluted alternately with the other. Such a system will also help increase throughput and thus, improve productivity.

One of the limiting factors that controls the throughput of the preparative chromatograph will be the maximum permissible sample volume. The maximum sample volume is that volume that will limit any loss of resolution to an acceptable and pre-defined level. To extend the basic examination of those factors that control the efficient operation of preparative columns, the factors that determine the maximum sample volume will now be considered in detail. To do this, an equation will be derived that allows the maximum sample volume to be calculated.