Monoisotopic mass (Mmi) is one of several types of molecular masses used in mass spectrometry. The theoretical monoisotopic mass of a molecule is computed by taking the sum of the accurate masses (including mass defect) of the most abundant naturally occurring stable isotope of each atom in the molecule. For small molecules made up of low atomic number elements the monoisotopic mass is observable as an isotopically pure peak in a mass spectrum. This differs from the nominal molecular mass, which is the sum of the mass number of the primary isotope of each atom in the molecule and is an integer. It also is different from the molar mass, which is a type of average mass. For some atoms like carbon, oxygen, hydrogen, nitrogen, and sulfur the Mmi of these elements is exactly the same as the mass of its natural isotope, which is the lightest one. However, this does not hold true for all atoms. Iron's most common isotope has a mass number of 56, while the stable isotopes of iron vary in mass number from 54 to 58. Monoisotopic mass is typically expressed in daltons (Da), also called unified atomic mass units (u). Nominal mass vs monoisotopic mass Nominal mass Nominal mass is a term used in high level mass spectrometric discussions, it can be calculated using the mass number of the most abundant isotope of each atom, without regard for the mass defect. For example, when calculating the nominal mass of a molecule of nitrogen (N2) and ethylene (C2H4) it comes out as. N2 (2*14)= 28 Da C2H4 (2*12)+(4*1)= 28 Da What this means, is when using mass spectrometer with insufficient source of power "low resolution" like a quadrupole mass analyser or a quadrupolar ion trap, these two molecules won't be able to be distinguished after ionization, this will be shown by the cross lapping of the m/z peaks. If a high-resolution instrument like an orbitrap or an ion cyclotron resonance is used, these two molecules can be distinguished. Monoisotopic mass When calculating the monoisotopic masses, using the mass of the primary isotope of the elements including the mass defect: N2 (2*14.003)= 28.006 Da C2H4 (2*12.000)+(4*1.008)= 28.032 Da where it will be clear that two different molecules are going through the mass spectrometer. Note that the masses used are neither the integer mass numbers nor the terrestrially averaged standard atomic weights as found in a periodic