Plate height and gas compressibility

Most of the plate height terms we have discussed are velocity sensitive. For gases, any term containing the mobile (gas) phase diffusion coefficient Dm (Dg for gases) is also pressure dependent, since Dg varies in inverse proportion to pressure p. For an ideal gas 

When we substitute Eq. 12.13 into 12.12, we get Dg in a velocity-dependent form 

Note that this conclusion is independent of how the various mobile phase terms combine thus it is a general conclusion valid for any of the H equations of Section 12.1, whether in a coupled form (Eq. 12.1) or one of the subsequent simplified forms. 

Unlike mobile phase terms, the stationary phase term in both packed and capillary columns, Csv, varies throughout the column, increasing continuously from inlet to outlet as v increases. Its average value is given by 

We can approximate the observed plate height H (which is measured experimentally as Lr2/t2r, Eq. 5.48) as the sum of the last two expressions 

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