shorter column and smaller pore size?

Hi Mark,

shorter columns mean less plates, in your case 100 mm to 50 mm would half the number of plates. In the resolution equation you have the factor √ N_th, that means half the plates = 30 % less resolution. 

Looking at all dependencies of the resolution equation shows this:

And your selectivity is fixed for your choice of phase.

So, what now? Back to the Van Deemter equation (or Knox equation). You need to lower your diffusion factors to get less band broadening and therefore smaller widths peaks, that will increase resolution. Shorter columns is one way, so you might not loose 30% plates after all; another way is using superficially porous materials (and you already do this), smaller particles works also and then there is average linear velocity (flow). For superficially porous materials (and small particles) the results of above mentioned equations show that you can work outside the theoretical optimum flow and go up. Yes, you would loose some plates, but the positive diffusion effects might be a lot bigger. Even with a standard LC system you might want to test higher flows up to the max. pressure of the system.

Also keep in mind all those changes work best if your peaks of interest elute somewhere between retention factors 2 to 5. Maybe adapt your gradient accordingly.

You also see there is no easy answer as everything is connected and the best chance would be a better selectivity (phase).

I still hope this helps a little.

Regards,

Norbert

Dear Norbert

thank you sincerely for the excellent reply and logic/maths behind such variables; this will prove invaluable in furthering my limited knowledge on the subject.

The key, basic thing I would be grateful for reassurance is if I went for a smaller pore size (such as the Poroshell 1.9 um instead of the current 2.6 um) would this be suitable for standard HPLC pressures? I'm very nervous to purchase this only for it not to be suitable due to having to reduce the flowrate to compensate for increased pressures and thus longer runtimes

thank you

Theoretically, if you go from 100 mm x 2.1 mm x 2.6 µm to 50 mm x 2.1 mm x 1.9 µm, the pressure stays the same at the same flow rate and the same phase. Kinetex and Poroshell have a slightly different rho-value, but that would only do a +/-10% change in pressure.

Theoretically, if you go from 100 mm x 2.1 mm x 2.6 µm to 50 mm x 2.1 mm x 1.9 µm, the pressure stays the same at the same flow rate and the same phase. Kinetex and Poroshell have a slightly different rho-value, but that would only do a +/-10% change in pressure.