This is the sentence which is assumingly wrong:
Solvent remains which could not have been evaporated were removed by [...].
The problem I have here is that the attempted evaporation of the solvent remains took place before the removal while the time at which the sentence takes place is in the past. I simply wanted to put which could not have been evaporated into the past perfect (while the remaining sentence is in the past simple) but since can is an auxiliary verb I do not know how to form it. The paraphrase able to is not really applicable to this problem either.
So how do I express this sentence properly?