There's an interesting news piece over at news@nature.com that caught my eye - a team led by Helge Riemann at the Institute of Crystal Growth is trying to generate a 'pure' sample of crystalline silicon-28:

The new barbell-shaped crystal, which weighs 5 kilograms and was completed on 23 May by Riemann's team ... is nearly isotopically pure. It was made from Russian source material, whose silicon was 99.994% pure silicon-28.

A standard kilogram is currently calibrated using the "international prototype," which "was manufactured in the 1880s [from] an alloy of 90% platinum-10% iridium" and is housed in the Bureau International des Poids et Mesures in France. Over time, the loss and/or gain of atoms from the international prototype may have altered its weight - the news story suggests that it might be off by 0.1 milligrams/0.01% (but I couldn't find any additional information to verify that statement...)

Making this (two-million euro/2.7-million USD) piece of silicon was no easy task:

The researchers spent six months eliminating contaminating elements by repeatedly melting the silicon in an apparatus that does not touch the material. The resulting crystal is thought to contain one foreign atom to every 10 million atoms of silicon.

Talk about a pure sample...

Joshua

Joshua Finkelstein (Senior Editor, Nature)