strange over pressure problem

[pselley]

Hi

I'm having a strange over pressure problem. I'm using a C-18 column, and reverse phase chromatography. I'm trying to detect catecholamines (dopamine in particular), and my mobile phase is NaH2PO4, EDTA, Octanesulfonic acid, and 10% ACN. Here's my problem: When the OSA is included in the mobile phase, I have overpressure problems. When I "clean" the column by running 20 void volumes of the mobile phase w/o the OSA (ion pairing agent) and then run 20 void volumes of h20, 10% MeOH, the pressure goes down. It stays in a normal range (about 2k psi). When I go back to a mobile phase with the octanesulfonic acid, the pressure goes back up to about 3600+ psi.

Any ideas why this is happening? Should I just buy a new column?

[DR]

I'd prepare that MP by combining your phosphate, solids and water, mixing and filtering that before introducing the ACN.

I suspect you may have some undissolved EDTA or OSA (or common impurities in either) fouling things up.