Monoisotopic mass

This article is about the mass spectrometry term. For a list of monoisotopic elements, see stable isotope.

Monoisotopic mass (M_mi), which means single isotopic, is a type of mass spectrometry analytical technique that differs from the nominal mass in which it takes the sum of its most abundant natural isotopic species of all the atoms in the molecule instead of the isotopic average mass (M_avg). For some atom like carbon, oxygen, hydrogen, nitrogen and sulfur the Mmi of these elements is exactly the same as the mass of its natural isotope, with is the lightest one. However this does not hold true for all atoms, Iron must common isotope its fifty six which is its heaviest one. Monoisotopic mass is typically expressed in unified atomic mass units (u), also called daltons (Da).

Nominal mass vs monoisotopic mass

Orbitrap Mass Analyzers

Nominal mass of a molecules can be explain as the calculation by using the most abundant isotope, without regard for the mass defect. For example, when calculating the Nominal mass of a molecule of nitrogen (N2) and ethylene (C2H4) it comes out as.

N₂

(2*14)= 28 u

C₂H₄

(2*12)+(4*1)= 28 u

What this means, is when using mass spectrometer with insufficient source of power "low resolution" like a quadrupole mass analyser or a quadrupolar ion trap, this two molecules won’t be able to be distinguish after ionization, this will be shown by the cross lapping of the m/z peaks. In addition if a high resolution instrument is used, like an orbitrap or an  ion cyclotron resonance, allows us to distinguish this two atoms. When calculating the monoisotopic masses, which includes the must abundant isotopes of the elements including the mass defect, it comes out to be.

N₂

(2*14.007)= 28.014 u

C₂H₄

(2*12.011)+(4*1.008)= 28.054 u

where it will be clear that two different atoms are going through the mass spectrometer.

Another example of why calculating the monoisotopic masses is crucial is when comparing tyrosine which has a molecular structure of C₉H₁₁NO₃ with a monoisotopic mass of 182.081 u and methionine sulphone C₅H₁₁NO₄S which clearly are 2 different compounds but methionine sulphone has a 182.048u.

Isotopic abundance

If piece of iron was put into a mass spretometer to be ananalyse, it would show multiple mass spectral peacks this is because iron accours in the isotopes of Fe-54, Fe-56, Fe-57 and Fe-58. This mass spectral peak represent that 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 molecule contains at least one heavy isotope atom also increases. if there are 100 carbon atoms "C-12" in a molecule, and each carbon has a probability of approximately 1% of being a heavy isotope "C-13" the whole molecule is highly likely to contain at least one heavy isotope atom of carbon 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.

See also

• Mass (mass spectrometry)
• Molecular mass