Monoisotopic mass
From Wikipedia, the free encyclopedia
The monoisotopic mass is the sum of the masses of the atoms in a molecule using the principle isotope mass of each atom instead of the isotope averaged atomic mass and is most often used in mass spectrometry. The monoisotopic mass is sometimes defined using the lightest isotope. In most cases the distinction between "principle" and "lightest" isotope is non-existent but is an issue with some elements such as iron and argon. The monoisotopic mass is closely related to the molecular mass. It is usually expressed in atomic mass units, often referred to in mass spectrometry (and other fields) as daltons.
Contents |
[edit] 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 increases. In other words if there are 100 carbon atoms in a molecule each of which has an approximately 1% chance of being a heavy isotope it should be obvious that the whole molecule is highly likely to contain at least one heavy isotope.
The monoisotopic peak is sometimes not observable due to 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 many be below the noise level and higher isotopes may dominante completely due to an extreme version of the isotopic distribution phenomenon above.
[edit] Why isn't this term used more frequently?
The monoisotopic mass is not used frequently in fields outside of mass spectrometry primarily because for the most part other fields can not distinguish molecules of differing isotopic composition. For this reason mostly the average molecular mass is used. For most purposes such as weighing out bulk chemicals only the average molecular 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.
