We've tried to measure an EPR spectrum of something in THF at -78C and could not tune the cavity on our Bruker EMX500 spectrometer.
THF was clearly liquid in that case.
The odd thing is that we can easily tune the cavity holding the filled dewar alone, and the sample alone (outside the dewar), but not when the sample tube was placed into the dewar.
Small clarification: we used a quartz cold finger dewar filled with hexanes mixture chilled with dry ice. Pre-chilled sample was then placed into the dewar and the whole thing into the EPR cavity.
In the past we were able to record a spectrum this way once - but only at 40 db attenuation, this was the highest power we were able to match the cavity.
Is there something special about THF at lower temperature? We had no problems recording EPR in THF at room temperature before.