The monoisotopic mass is the sum of the masses of the atoms in a molecule using the unbound, ground-state, rest mass of the principal (most abundant) isotope for each element instead of the isotopic average mass. For typical organic compounds, where the monoisotopic mass is most commonly used, this also results in the lightest isotope being selected. For some heavier atoms such as iron and argon the principle isotope is not the lightest isotope. The term is designed for measurements in mass spectrometry primarily with smaller molecules. It is not typically useful as a concept in physics or general chemistry. Monoisotopic mass is typically expressed in unified atomic mass units (u), also called daltons (Da).
The mass spectral peak representing the monoisotopic mass is not always the most abundant isotopic peak in a spectrum despite it containing the most abundant isotope for each atom. This is because as the number of atoms in a molecule increases, the probability that the entire molecule contains at least one heavy isotope atom also increases. For example if there are 100 carbon atoms in a molecule each of which has an approximately 1% chance of being a heavy isotope the whole molecule is highly likely to contain at least one heavy isotope atom and the most abundant isotopic composition will no longer be the same as the monoisotopic peak.
The monoisotopic peak is sometimes not observable for two primary reasons. First the monoisotopic peak may not be resolved from the other isotopic peaks. In this case only the average molecular mass may be observed. In some cases even when the isotopic peaks are resolved, such as with a high resolution mass spectrometer, the monoisotopic peak may be below the noise level and higher isotopes may dominate completely.
Context of usage
The monoisotopic mass is not used frequently in fields outside of mass spectrometry because other fields can not distinguish molecules of differing isotopic composition. For this reason mostly the average molecular mass or even more commonly the molar mass is used. For most purposes such as weighing out bulk chemicals only the molar mass is relevant since what one is weighing is a statistical distribution of varying isotopic compositions.
This concept is most helpful in mass spectrometry because individual molecules (or atoms, as in ICP-MS) are measured, and not their statistical average as a whole. Since mass spectrometry is often used for quantifying trace-level compounds, maximizing the sensitivity of the analysis is usually desired. By choosing to look for the most abundant isotopic version of a molecule, the analysis is likely to be most sensitive, which enables even smaller amounts of the target compounds to be quantified. Therefore, the concept is very useful to analysts looking for trace-level residues of organic molecules, such as pesticide residue in foods and agricultural products.
Isotopic masses can play an important role in physics but physics less often deals with molecules. Molecules differing by an isotope are sometimes distinguished from one another in molecular spectroscopy or related fields, however it is usually a single isotope change on a larger molecule that can be observed rather than the isotopic composition of an entire molecule. The isotopic substitution changes the vibrational frequencies of various bonds in the molecule, which can have observable effects on the chemical reactivity via the kinetic isotope effect, and even by extension the biological activity in some cases.