Why? Just do it all under vacuum. But in cases where you must (cost/practicality limitations, dealing with liquids, etc), this is done by passing through a thi

Why? Just do it all under vacuum. But in cases where you must (cost/practicality limitations, dealing with liquids, etc), this is done by passing through a thin film of material that has a small cross section of interaction with the particles, typically "low Z" (low atomic number) elements like Beryllium (its rigidity makes withstanding 1 atm pressure difference easy), or simple hydrocarbon plastics like polyethylene/polypropylene. But this depends on what particles you're talking about.