Chromatography Forum

I have two questions about eddy diffusion

First, what is the equation for "reduced A term"

Secondly, what impact does inner diameter have on this term in packed,
reverse-phase capillaries?

Eddy diffusion term is used to calculate HETP. In packed column this value is higher in comparison to Capillary column.

Well, I know it impacts "H". It is the "A term" but how is the "A term" different from the "reduced A term" is my first question.

Secondly, let's say you have a series of capillary columns ranging from 30 micron i.d. up to 300 micron i.d.

Eddy diffusion increases at you increase i.d. but why? There are more paths for analyte molecules to diffuse through at higher i.d. On the other hand, analyte molecules are more likely to 'experience' all of the multiple paths with decreasing i.d.

A "reduced A-term" is the A-term measured divided by the particle size.

The A-term is the velocity independent term that one gets from fitting a H/u curve to the van Deemter equation.

I have seen variations in the A-term with column diameter that goes into both directions. Essentially, it is measure of the uniformity of the column packing. The larger the diameter, the more non-uniformity terms can enter into this parameter, and the term can become larger, if one is doing a bad job in column packing. On the other hand, if one is doing a good job suppressing irregularities, the A-term can become small for even rather fat columns. Marcel Golay said at some point that he did not see a good reason for this A-term to exist, and I agree with him.