Mass (mass spectrometry)

Mass is a property of physical objects that reveals itself in two ways:

1. as a resistance to change of velocity (inertia)
2. as a force when exposed to a gravitational field

The symbol for the physical quantity mass is m.

The SI unit for mass is kg. In mass spectrometry it is more common to use the unified atomic mass unit u, which is better called Dalton Da.

m = n Da

In spite of their name, mass spectrometers do not measure the mass of ions, but the mass/charge (m/q) of ions.

The mass of a particle (atom or molecule) is determined by the following components:

1. number of nucleons (protons and neutrons)
2. number of electrons
3. mass defect (binding energy of nucleons)

Because (1) protons and neutrons have simillar masses, and (2) the nucleons dominate the total mass, the total mass of particles is dominated by the number of nucleons. This has the effect that all particle masses are close integer numbers when measured in Dalton. Therefore another quantity has been useful in mass spectrometry, the quantity nominal mass. However, the term nominal mass is somewhat confusing since it is not clear if it just means a rounded mass or if it means the number of nucleons in a molecule.

nucleon number N = n := number of nucleons in molecule

nominal mass M = n Da := round(m)

External Links

Wikipedia: Mass