Chromatography Forum

Posted: **Sat Feb 02, 2008 8:35 pm**

Hi, I hope someone can help me.
I thought that eddy diffusion was independent of flow rate.
However, I have recently read some materials that state this is not an absolute, and that sometimes eddy diffusion is dependent.
Can anyone clarify
Thanks
Liv

Posted: **Mon Feb 04, 2008 6:32 pm**

For GC, the A-term of the Van Deemter equation (the "eddy diffusion" or " multipath effect" term is approximately independent of flow rate. For LC, the A-term of the Knox equation (the LC version of the Van Deemter equation) has a weak dependence on flow rate (approximately the 1/3 power). Of course, the actual equations are written in terms of reduced linear velocity, but that is proportional to flow rate.

Since you really seem to be getting seriously involved, you might want to pick up a copy of Giddings' book Dynamics of Chromatography. Amazon.com shows used copies for about $45 (http://tinyurl.com/3yq4mj). It was written in 1966 and laid the path for following 50 years of evolution in chromatography.