[rolph]

use a resonant determination of mass.

https://www.nature.com/articles/nnano.2012.42

https://arstechnica.com/science/2012/04/measuring-yoctogram-...

[rcxdude]

These are very sensitive but they are likely not calibrated to within 0.1%, and certainly it's not easy to transfer this up several orders of magnitude withiut introducing huge errors.