Chromatography Forum

We are using a silica gel-based LC, aqueous and slightly acid pH (5.2 +/- .1). We are switching to a newer, spherical media hoping for better performance... I have been asked to calculate the maximum linear flow rate for this media: Particle dia. = ~75 um, Pore diameter 5.4 nm, Pore volume = 0.9 ml/g, SA = 680 m2/g (BET).

The molecules of interest range from monosaccharides to about 600 daltons.

We are using an affinity ligand to achieve a fairly high purity in a single run but would like to cycle though successive runs as fast we can without excessive zone broadening during elution.

I looked up the Van Deemter wiki page and found the relationship: linear flow rate (u) = (B/C)^0.5 where B is the diffusion coefficient, which I believe I can estimate and C is the resistance to mass transfer coefficent, which I don't know how to come up with. In fact I am not sure this is really the equation to use.

If anyone has some thoughts/suggestions to share on a theoretical approach to the problem of establishing a practical "speed limit" I would be very grateful.

For that big a particle and that high a molecular weight, you're going to be waaaay above the optimum linear velocity in any case. You have to make so many assumptions about the mass transfer kinetics in order to estimate the C term that it will be little better than a guess.

My $0.02 worth is that you would be better off measuring the plate height at two different flow rates and just calculate from plate height as a linear function of flow.