B term longitudinal diffusion

The contribution of the B term to the van Deemter equation is most important at low mobile phase velocities. The B term is more important in GC than LC since diffusion coefficients of component molecules in gases are at least 10 greater than in liquids. 

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The curves represent a plot of log (h ) (reduced plate height) against log (v) (reduced velocity) for two very different columns. The lower the curve, the better the column is packed (the lower the minimum reduced plate height). At low velocities, the (B) term (longitudinal diffusion) dominates, and at high velocities the (C) term (resistance to mass transfer in the stationary phase) dominates, as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, log (h ) is about 0.35. The optimum reduced velocity is in the range of 3 to 5 cm/sec., that is log (v) takes values between 0.3 and 0.5. The Knox... 

Van Deemter plots were shown in Chapter 3, and they illustrate the effect of column flow rate on band broadening, H. There is an optimal flow rate for a minimum of band broadening. With packed columns, and also with thick film megabore columns, nitrogen is the carrier gas of choice since the van Deemter B term (longitudinal diffusion in the gas phase) dominates. Nitrogen being heavier than helium minimizes this B term and produces more efficiency. 

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