Monoisotopic mass

The monoisotopic mass is the sum of the masses of the atoms in a molecule using the unbound, ground-state, rest mass of the principle (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).

Monoisotopic mass, abundance & mass spectrometry
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 due to the fact that as the number of atoms in a molecule increases the probability of the entire molecule containing at least one heavy isotope atom 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.

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. In such cases, again, the isotopic mass can be important since the change in mass changes the vibrational frequencies for example but the monoisotopic mass is generally not relevant or at least uninteresting.