I am working with some 75 um ID C18 columns right now that are taking an extremely long time to reach running pressure. Pressure begins to climb in a short amount of time but the rate at which it increases is extremely slow (about 45 minutes). I do not understand the reason for this. Does anyone have experience working with these columns and can provide some explanation for what I am seeing?

I have seen this on every batch of columns and they are commericially made. I have not packed them myself.

The issue might be your pumping system, and the compressibility of the solvents. What are you using?

I am pumping with the Gilson 350 micro pumps and I am pumping Gradients with Pump A having 100% water and Pump B having 100% ACN. The system and flow rates have been pumping fine with 300 um columns and the gradient profiles look good at 200 nl/min so I have to assume the pumping system is functioning properly. The compressibilty settings for the solvents being used have been confirmed to be correct. Any other ideas? I am beginning to think this is just the norm with these columns but I don't understand why.

compressibility is a 16-fold higher factor with the smaller i.d. column....

What's the "dwell volume" (gradient delay volume) of your system?

Assuming both columns are the same length and assuming that the flow rate is scaled to the square of the column diameter, then both columns should generate approximately the same back pressure.

For the sake of argument, assume the pressure is 1000 psi. If memory serves, the compressibility of liquids is about 1% per 1000 psi. If your system has a dwell volume of 100 microliters (I couldn't find a spec on Gilson's web site, so I'm guessing here), you will have to pump 1 microliter of solvent just to compensate for the liquid compressibility. At 200 nL/min, this represents 5 minutes' worth of flow all by itself (never mind the fact that some of it is "leaking" out through the column).

-- Tom Jupille / LC Resources Inc.