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Term B, which can be expressed from D_G, the diffusion coefficient of the analyte in the mobile phase, and λ, the above packing factor, is taken into consideration above all when the mobile phase is a gas. The longitudinal diffusion in the column is in effect quite fast.

This term is a consequence of entropy, which states that a system will tend spontaneously towards the greatest degree of disorder, just as a drop of ink diffuses into a glass of water into which it has fallen. Consequently, if the flow rate is too slow, the compounds undergoing separation will mix faster than they will migrate. This is why one must never interrupt a chromatographic analysis, even temporarily, once it is underway, as this could negatively impact efficiency.