Rate Theory:

In Unit 4, it was declared that there has to be quantitative relationship among plate height and column variables. In this context, various chromatographic theories have been established to account for the shape of eluting curves from chromatographic columns. A rate theory was established through van Deemter. A qualitative understanding of the van Deemter equation is meaningful in optimizing the chromatographic performance.

There are the three principal contributions to the broadening of a band are:

1. Many path effect or eddy diffusion (A term)

2. Molecular diffusion (B term)

3. Resistance to mass transfer (gas and liquid, C term)

From these, a primary equation could be derived for the plate height in a gas liquid system.

H= A+B/u+Cu

while A, B, and C are above declared constants and u the linear velocity (or flow rate) by the chromatographic column. The linear gas velocity is measured via

u = Length of column (cm)/Retention time of air (seconds)

If H is plotted against u, one acquires a hyperbola along with a minimum H. This minimum is in which flow rate (u optimum) at that the column is operating most efficiently.