– In Mass Spectroscopy (MS), atomic and molecular weights are generally expessed in terms of atomic mass units (amu).
–The atomic mass unit is based on upon a relative scale in which the reference is the carbon isotope 126C, which is assigned a mass of exactly 12 amu.  Thus the amu is rdefined as 1/12 of the mass of one neutral carbon atom.  Mass spectrometrists also call the amu the Dalton (Da).
Mass Spectroscopy Units