Exact Mass and Molecular Formulas

Let us briefly repeat i) the isotopic mass is also the exact mass of an isotope it) the isotopic mass is very close but not equal to the nominal mass of that isotope Hi) accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass iv) due to the definition of our mass scale, the isotope represents the only exception from non-integer isotopic mass. As a consequence of these individual non-integer isotopic masses, almost no combination of elements in a molecular or ionic formula has the same calculated exact mass, or simply exact mass as it is often referred to, as any other one [36]. 

Example The molecular ions of nitrogen, N2, carbon monoxide, CO, and ethene, C2H4, have the same nominal mass of 28 u, i.e., they are so-called iso-baric ions. The isotopic masses of the most abundant isotopes of hydrogen, carbon, nitrogen, and oxygen are 1.007825 u, 12.000000 u, 14.003074 u, and 15.994915 u, respectively. Thus, the calculated ionic masses are 28.00559 u for N2, 27.99437 u for CO, and 28.03075 u for C2H4. This means they diffa- by several 10 u, and none of these isobaric ions has precisely 28.00000 u (Chaps. 3.3.4 and 6.9.6). 

Note Historically, 10 u is referred to as 1 millimass unit (mmu). There is still use of mmu in the MS community because of its convenience in dealing with small differences in mass. The mmu is in no way an SI unit. 

---