Monoisotopic mass
Encyclopedia
The monoisotopic mass is the sum of the mass
es of the atom
s 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 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.
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.
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
es of the atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s in a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
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
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
the principle isotope is not the lightest isotope. The term is designed for measurements in mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
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
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
(u), also called daltons (Da).
Isotopic abundance
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 massMolar mass
Molar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...
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.