Ion Chromatography - Column Dimensions
The subject of column dimensions has already been touched upon but will be now discussed in more detail. Short columns are best used for very fast separations and very simple mixtures (about five moderately resolved components). They need to have a reasonable diameter to provide adequate sample capacity. High-speed columns are usually 3 to 5cm long and 4.6mm in diameter and are packed with the smallest particles that are practical with respect to the packing procedure and available column pressure. If only one component in the mixture is of interest only, then only that component needs to be separated from its neighbours. Consequently, having achieved a separation the flow rate can be increased (resulting in increased peak dispersion as already described) until the base of the peak of interest only just touches its neighbour. This will provide the minimum analysis time for the specific column and phase system.
As an analytical separation becomes more difficult, a longer column must be used to provide more theoretical plates and, thus, better resolution. It may come to a point when there is inadequate pressure to obtain the optimum flow rate or the pressure may start distorting the stationary phase if in the form of polymer beads. Under these circumstances the particle diameter must be increased (to increase column permeability) and the column lengthened. Increasing the particle diameter will reduce the column efficiency and increasing the length will increase the column efficiency (as previously discussed in the HETP equation). In general, if both the particle diameter and the length are doubled then the overall efficiency will be increased by a factor of ca 1.4 (i.e. 21/2).
Wide columns are necessary for preparative ion chromatography but this is a little ambiguous unless the term preparative is defined as it may mean very different column loads from one chromatographer to another. The load that can be placed on a column will also depend on the capacity of the stationary phase In general a 5mm diameter column will separate ion masses of up to 1000mg and a 5cm diameter column will separate ion masses of up to 100mg but this will depend heavily on the ion capacity of the stationary phase, In most instances only one solute in the mixture is of interest in which case it is only necessary to separate it from its nearest neighbour. Consequently, the sample load can be increased with consequent overload until the peaks are broadened to the point where the solute of interest and its nearest neighbour just touch. In this manner, depending on the column resolution, loads significantly greater than those stated above can be realized.