In the basic theory of chromatographic separations, we always hear the familiar name of Van Deemter, who used to draw a check mark on paper to explain the relationship between column efficiency and flow rate of a chromatographic column.
The Van Deemter equation in chromatography is the relationship between the total peak spread per unit column length and the flow rate of the mobile phase, taking into account the physical, kinetic and thermodynamic factors that cause peak spread during the separation process. In general, the factors affecting peak spreading include multi-path effects, diffusion (radial and axial) and mass transfer resistance between the stationary and mobile phases. The most common form of the van der Muente equation is shown in the following equation, which visualises the effect of mobile phase flow rate on separation. where A, B, C are constants and u represents the flow rate of the mobile phase.
In addition to equipment and chromatographic packing, the flow rate plays a crucial role in chromatographic separations. If the flow rate of the eluent is too fast, premature desorption of the target will cause the zone to spread, while if it is too slow, late desorption of the target will cause the peak shape to be too broad. Choosing the right flow rate is the only way to increase the separation efficiency while ensuring a good separation.
Fig. Comparison of separation at different flow rates
If the flow rate is too slow, the last main peak is not separated from the posterior, and if the flow rate is too fast, the components are poorly separated. With a suitable flow rate, the components are well separated.
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