A zeptogram (zg) is an SI unit of mass representing 10^-21 grams, or 10^-24 kilograms. It is used in particle physics and molecular biology to quantify the mass of individual proteins, large molecules, or nanoscale structures. Highly sensitive nanomechanical mass sensors are required to detect changes at the zeptogram level.
Heard on the job
The mass spectrometer can resolve differences down to a single zeptogram.