Chromatography Forum

it is clear some kind contamination causing this problem.
thinking in detail
thinking things simple
test systematically
I sense nothing complicated here.

peroxide was not the cause.
Polymer out of THF was possible though the chance was slim;
I bet on contamination.
I did not see systematic tests showing good or bad THF.
But there are so many contamination source: containers, utensil, reagents, LC vial/caps; LC tubing; solvent/needle washing of LC; sample decomposition, sample-reagent(THF) inter-reaction; centrifuge tube; filters; pipets; contamination from other projects; a lot other possibility

good luck

Kati:
We are in now in September and maybe you already solved the chalenging problem.
Nevertheless, as I developed a related substances analysis of an steroid with a mobile phase that a remember to be very similar to the one you posted (sorry I am at home, no lab records available), maybe my advice still helps.
The method was a variation from the one published for Norgestimate in the USP.
When we selected the HPLC tetrahidrofuran we finally decided for the HPLC grade Fisher sells (Optima Cat Number T 427-1 in 1 liter amber glass bottles).
The solvent worked well and we have no troubles neither in the validation nor in our QC unit (until now).
I think that trying another THF supplier is worthwhile.

We also use 1-liter bottles for our HPLC THF, and without preservative.