Effect of Gas Compressibility on Elution Equation for Packed Columns

11a Effect of Gas Compressibility on Elution Equation for Packed Columns 

The effects of gas compressibihty (change of volume as a result of pressure changes) are observed in not only the linear velocity but also the diffusion coefficient of the analyte. For a packed GC column the A-term in the van Deemter equation (multipath effect) is independent of both u and D, so is unaffected. However this is not the case for the B and C terms (see Equations [3.33-3.38]). In fact the van Deemter equation apphes only to each point x along the length of the column if local values of the linear velocity (u ) and diffusion coefficient (D, )) are used. The ideal gas law apphed to the gaseous mobile phase (PVmp = nRT), at constant temperature and constant mass flow rate of mobile phase (dn /dt) leads to the simple relationship for any point x along the length of the column  

Moreover, the kinetic theory of gases shows that the diffusion coefficient of a gaseous solute in a different gas (the mobile phase in our case) is inversely proportional to the pressure, i.e.  

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