What is the difference between chemical mass and exact mass?
– For most chemical calculations, chemists normally employ the statistically average atomic masses listed in the periodic table. These chemical atomic masses represent a weighted average of all of the naturally occurring isotopes of an element. Because the mass spectrometer will separate molecules with different isotopic compositions, these average atomic masses are not correct for mass spectrometry of small molecules (under 2000 Da) where you have at least unit mass resolution. In this instance, you must use the exact masses of the individual isotopes.
