Platinum has molar mass ~195g, Iridium ~192g. This means the working copies have between ~5.128 and ~5.208 mol atoms in a kg. Let's round it to 5.2 mol, that's

Platinum has molar mass ~195g, Iridium ~192g. This means the working copies have between ~5.128 and ~5.208 mol atoms in a kg. Let's round it to 5.2 mol, that's ~3.13 * 10^24 atoms. Five parts in 100 million is ~1.565 * 10^18 atoms. Sound like a pretty large number to me.