Friday, May 22, 2009

Height Equivalent to a Theoretical Plate

Since n depends on the length of the column, another parameter is used to express column efficiency. It is the height (length of column) equivalent to a theoretical plate, HETP, or just H
H=L/n
where L is the length of column in cm or mm. Thus H is the length of column which represents one theoretical plate in units of cm/plate or mm/plate.
The effect of flow on column efficiency is usually shown by plotting H versus flow rate or linear velocity. Such a plot is shown in the figure. Note that the H line goes through a minimum. The minimum occurs at the optimum flow velocity. The simplest equation for the curve in the H versus v figure is the van Deemter equation:
H=A+B/v+Cv
A van Deemter plot for gas chromatography can seen below. A, B and C are constants and v is the linear velocity, the carrier gas flow rate. The A term is independent of velocity and represents "eddy" mixing. It is smallest when the packed column particles are small and uniform.
The B term represents axial diffusion or the natural diffusion tendency of molecules. This effect is diminished at high flow rates and so this term is divided by v.
The C term is due to kinetic resistance to equilibrium in the separation process.
The kinetic resistance is the time lag involved in moving from the gas phase to the packing stationary phase and back again. The greater the flow of gas, the more a molecule on the packing tends to lag behind molecules in the mobile phase. Thus this term is proportional to v.

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