While reading this question: Why do we still not have an exact (constants-based) definition for a kilogram? , I had a crazy thought.
Using PCR, you make a known number of copies of a DNA strand where the length and composition is exactly known for all the copies. The number of copies is two to the power of the number of PCR cycles, and if you start with, say, all C-G DNA, you get a known number of duplicates which are the same molecular weight. If you only provide C and G nucleotides, you don't even have to worry too much about errors in copying, because whichever way they are fixed to make pair-matching DNA, you will get the same atomic weight. You can separate out the DNA and get a known huge macroscopic batch size of macro-molecules where we know the atomic composition exactly.
So we can easily make a batch of atoms where we know the number of atoms with potentially exact precision. Can you use this to make a mass standard? Just weighing the DNA won't work, because the water content will be uncertain. Drying the DNA won't work, because there will always be some completely random stuck protons or hard-to-evaporate water.
But I suspect this can be done, because here is a system with an exactly known macroscopic number of atoms. Perhaps if you centrifuge the DNA and compare the density of water to the density of DNA-water?
Question: Can you define the kg with PCR?