Primer 05 of 09 · Mass & formula

Two masses, one molecule

Glycine is 75.067 Da on one line of a data sheet and 75.03203 Da on the next. Neither is a typo. Isotopes are the reason, and each number has its own job.

4 min read · Updated 14 Oct 2026

Glycine is 75.067 Da on one line of a data sheet and 75.03203 Da on the next. Neither is a typo. The two numbers answer different questions, and the difference between them is isotopes.

Isotopes

Natural carbon is about 98.9% carbon-12 and 1.1% carbon-13. Hydrogen, nitrogen and oxygen each have a heavier minor isotope too. A sample of any compound is therefore a mixture of molecules with slightly different masses, depending on which isotopes each one happens to contain.

Average mass: the mixture

The mass per molecule of the natural mixture, using each element’s standard atomic weight, the abundance-weighted mean over its isotopes. This is the number a balance agrees with, the one molar mass is built on, and the one to use for weighing out a sample.

Monoisotopic mass: one molecule

The mass of one specific molecule made entirely of each element’s most abundant isotope: every carbon a carbon-12, every hydrogen a hydrogen-1, and so on. It is what a high-resolution mass spectrometer reports as the first, and for a small molecule the tallest, peak. “Exact mass” almost always means this.

Nominal mass: the integer

The whole-number version, using integer isotope masses (C = 12, H = 1, N = 14, O = 16). Glycine is 75. Useful for a quick check, too coarse for anything else.

Glycine, both ways

                 monoisotopic                  average
C  2 ×  12.00000000  = 24.00000     2 × 12.011  = 24.022
H  5 ×   1.00782503  =  5.03913     5 ×  1.008  =  5.040
N  1 ×  14.00307401  = 14.00307     1 × 14.007  = 14.007
O  2 ×  15.99491462  = 31.98983     2 × 15.999  = 31.998
                                 ────────                     ───────
                                 75.03203 Da                  75.067 Da

The monoisotopic column uses exact isotope masses; the average column uses the IUPAC abridged standard atomic weights.

When the gap matters

For a small molecule the two differ by a few hundredths of a dalton. Either rounds to the same integer.
For a peptide of a few thousand daltons the gap is about one dalton, and the monoisotopic peak is no longer the tallest: a molecule with one or two carbon-13 atoms becomes more likely than one with none.
For a protein, the monoisotopic peak is too small to see; only the average mass is practical.

In short

01Average mass is the natural isotope mixture, what a balance measures. Monoisotopic is all-lightest-isotope, what a mass spectrometer measures.
02“Exact mass” means monoisotopic. Nominal mass is the integer.
03The gap grows with size: negligible for small molecules, about a dalton per few thousand daltons.

Figures on this page are computed at render time from one set of constants (the IUPAC abridged standard atomic weights and the exact masses of the most abundant isotopes), the same set the tables and the calculators use. Structures and identifiers after ChEBI, CC BY 4.0.