I would imagine at this scale, It's not so much counting particles, but weighing them. So you need to separate them out to do that. Elements of different mass

I would imagine at this scale, It's not so much counting particles, but weighing them. So you need to separate them out to do that. Elements of different mass can be separated using centrifuges. Not 100% sure for this case, but I know that's how it's done for certain kinds of refinement.