mass defect

The term mass defect has an “official” meaning that is quite different from one of its meanings in mass spectrometry. Officially, the mass defect is the difference in the mass of an atom and the sum of the masses of all of the particles (electrons, protons, and neutrons) of which it is composed.  This mass defect occurs because matter is converted into energy according to the Einstein equation; this energy binds the nucleus together and overcomes the mutual repulsion between  protons. In mass spectrometry, the mass defect is the term also used for the difference (whether positive or negative) between the exact mass of an ion, and the nearest integer mass.