Recrystallization depends on identical solute molecules colliding so that a crystal can grow. An impurity present in low concentration will generally remain in solution because kinetics will favor crystallization of the high-concentration species, all other factors being equal.
But what about when vapor deposits on a cold finger? I would assume there is no such selectivity, with small crystals of the impurity growing alongside those of the target compound (assuming similar phase diagrams). The issue is never mentioned in any of the sublimation demos I have watched.
Is this correct? Should recrystallization be done first in a case like this?
