Scale weighs a zeptogram

According to the article they spray the atoms in gas form and let it condense on the blade. How can they be certain the number of atoms and the actual weight of the atoms without disrupting the system. Do they know what it should weigh and are just using it for calibration or is it a "the math works out and is what we expected".

I'm not sure, but I think the key is the blade is oscillating at a particular characteristic frequency. The weight is measured as a change in the osciallation frequency of the blade. The accuracy comes from the minimum number of atoms needed to produce a measurable frequency change in the blade. This particular blade was made of a new material that produced a higher characteristic oscillation which led to more accurate measurements.