Definition
Monoisotopic mass is calculated using the most abundant isotope of each element: carbon-12, hydrogen-1, nitrogen-14, oxygen-16 and sulfur-32. Average mass, by contrast, weights each element by its natural isotope mixture and is the figure usually quoted as molecular weight. For small peptides the two differ by less than 1 Da, but the gap grows with size, exceeding 2 Da for a 4 kDa peptide. High-resolution mass spectrometers resolve individual isotope peaks, so the observed monoisotopic peak is compared with the theoretical monoisotopic mass; lower-resolution instruments report a value closer to average mass. A certificate of analysis should state which convention it uses, because comparing one with the other can create an apparent mismatch.