Is the water transpired by plants pure H₂O, or does it contain trace minerals?
It will depend a bit on the specimen of tree / plant that is doing the transpiration. Minerals will typically stay behind during evaporation, but volatile compounds (think the minty smell of Eucalyptus) would evaporate at the same time as the water, and might condense (in some concentration) at the same time as well; this will depend a bit on the temperature of the leaf, the relative humidity of the air (lower will favor evaporation - higher means there is a lot of moisture already in the air, and you don't necessarily know where it came from) and the temperature of the surface used for condensation (lower condensation temperature will trap things besides water).

Note - this is more a physicist's answer than a biologist's... betraying my background here.