Atomic Masses
Atomic masses can be measured very accurately with a mass spectrometer. Atoms in the gas phase are bombarded by a beam of high energy electrons. Electrons are knocked out of the atoms producing positive ions. These ions are then accelerated through an electric field and pass through a magnetic field which deflects the ions. A collector measures the amount of deflection. The amount of deflection is a function of the mass to charge ratio. Of two ions of the same charge the lighter one will be deflected more. The heavier ion can be made to deflect the same amount as the lighter one by decreasing the voltage of the electric field and thus the velocity.
The method can also distinguish between the various isotopes of an element and determine the relative abundance of the isotopes. (Whereas the element depends on the number of protons in the nucleus, a given element can have several isotopes depending on the number of neutrons in the nucleus.) A natural sample of hydrogen consists of 99.985% 1H and 0.015% 2H. (1H has a single proton and no neutrons. 2H consists of a proton and a neutron.) 98.892% of a carbon sample consists of 12C. 1.108% is 13C. 99.76% of oxygen is 16C, 0.037% is 17O and 0.204% is 18O.
Atomic mass is measured in atomic mass units (amu) with the number 12.0000 being assigned to 12C with 6 protons and 6 neutrons. Thus 1 atomic mass unit is 1/12th of the mass of 12C.