Reducing the Eddy Diffusion Term

Eddy diffusion is a result of the presence of particles of stationary phase material in a column, and depends on the stationary phase conditions, shape of the column, and the structure of the stationary phase material. The influence of the stationary phase material can be divided into the particle size (dp), the shape of the particles, and the porosity of the particles. The standard deviation for peak broadening due to the particles is described by rF2 = 2XdpL, and therefore from Equation 5.5  

X depends on the irregularity of the particles (particle shape) and on the column material (steel gives more disturbance than glass) the relative effect will always increase as the column diameter decreases. The above-mentioned effects have led to an optimum internal diameter of 2-5 mm for LC columns. In a column with this internal diameter, which is uniformly packed with small spherical particles, the eddy diffusion will be limited. The X value is about 1 for spherical particles. 

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The number of theoretical plates is proportional to the column length and inversely proportional to the particle size. The advantage of using small particles is that they distribute flow more uniformly and, as a result, reduce the eddy diffusion, term A in the Van Deemter equation. However, the smaller particles increase the diffusional resistance of the solvent as well as the pressure drop (for a given flow rate). Choosing the flow rate is a critical parameter in developing an HPLC method. Low flow rates allow the analyte sufficient time to interact with the stationary phase and will affect both the B and C terms of the Van Deemter equation. 

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