With little gas flow rub the needle over the small metal cup that is cooled by liq N2. Soon you will have a couple of ml of liquid propane. Do this in a fumehood, which is a good idea when using solvents too. Isopentane becomes very rubbery near liquid nitrogen temperature. The temptation is to warm it a little with a metal rod, reducing the temperature differential some more.
Propane has greater freezing speeds making vitrification possible.
These things where published 20 and more years ago when cryo fixation was developed. I would be surprised if propane penetrates the specimen to any extent because snap freezing is so rapid. Later the specimen is sublimed and the propane would be the first phase to go. Also propane has been used to widely that somebody would have noticed an effect on lipids - if that was greater than isopentane's. I would assume that IF this happens, isopentane being less soluble in water would have less of this kind of effect.