 The main residual uncertainty in the early measurements was in the measurement of the isotopic composition of the silicon to calculate the atomic weight so , in 2007 , a 4 @.@ 8 @-@ kg single crystal of isotopically @-@ enriched silicon ( 99 @.@ 94 % 28Si ) was grown , and two one @-@ kilogram spheres cut from it . Diameter measurements on the spheres are repeatable to within 0 @.@ 3 nm , and the uncertainty in the mass is 3 µg . Full results from these determinations were expected in late 2010 . Their paper , published in January 2011 , summarized the result of the International Avogadro Coordination and presented a measurement of the Avogadro constant to be 6 @.@ 02214078 ( 18 ) × 1023 mol − 1 . 
