What would be the reasonable strategy for washing a reversed-phase (RP) octadecylsilyl (ODS) C18 HPLC columns from lipids and other non-polar residues? The columns are used for biochemical analysis of various matrices using acetonitrile, methanol and water (often with $~0.1\,\%$ formic acid), and it seems that lipids from cell membranes tend to cause clogging and increased pressure over time, despite the derivatized/extracted probes being filtered with 0.20 μm filters prior to injection. The process of washing with an excess (20× column volume) of 2-propanol recommended by manufacturer for removing lipid-soluble substances has only marginal effect, if any.
Usually, in this case it is recommended to try dichloromethane (DCM), dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF) after replacing all plastic tubing and fittings with stainless steel ones. The problem is that the columns have polyether ether ketone (PEEK) lining that obviously cannot be switched, and all three solvents (DCM, DMSO, THF) are known for causing swelling in PEEK parts, possibly resulting in altered column geometry and traces of plasticizers.
Is there a reasonable solution for this issue involving another PEEK- and HPLC-friendly lipophilic solvent, or is there a totally different strategy for PEEK-lined columns?
Please note that I'm asking about the PEEK-compatible solvent in particular, not clogging issue in general:
The clogging occurs at the column, not anywhere else in the system. Replacing the problematic column with a brand new one or installing a resistance tube result in expected pressure values.
The retention times vary within acceptable limits. The columns are relatively new and were used for just several dozens of probes. There are, however, peaks of what appears to be post-eluting components despite a $\pu{40 min}$ washing cycle after each run.
Reversing flow direction and washing at low flow rates of about $\pu{0.1 ml min^-1}$ with either 2-propanol or acetonitrile didn't help at all. Although the pressure was a bit lower for the reversed flow (about $\pu{25 MPa}$ vs $\pu{30 MPa}$).