Difference Between Monoisotopic Mass and Average Mass

The key difference between monoisotopic mass and average mass is that monoisotopic mass is calculated considering a single isotope, whereas average mass is calculated considering all the abundant isotopes of a particular chemical element.

Monoisotopic mass and average mass are important parameters in mass spectrometry. These values deal with the atoms of particular chemical elements.

CONTENTS

1. Overview and Key Difference
2. What is Monoisotopic Mass 
3. What is Average Mass
4. Side by Side Comparison – Monoisotopic Mass vs Average Mass in Tabular Form
5. Summary

What is Monoisotopic Mass?

Monoisotopic mass is the mass of a single atom of a particular isotope. It is one of the several types of molecular masses we use in the mass spectrometric analysis. Usually, this term is used for chemical elements having a single stable isotope which determines the average atomic mass.

Here, the average atomic mass is equal to the monoisotopic mass of the chemical element. For example, the exact mass of a molecule or an ion can be calculated using the masses of the most abundant isotopes that make up the molecule or the ion.

What is Average Mass?

The term average mass is used mainly to indicate the mass of atoms. Therefore, the term actually is “average atomic mass”. It is the mass of an atom of a particular chemical element calculated by considering all the isotopes of that element. Here, the mass value depends on the natural abundance of a chemical element.

We can use two major steps to calculate the average atomic mass of a chemical element. These steps are as follows:

1. Multiply the atomic mass of each isotope from the natural abundance (taking the abundance as a percentage) separately.
2. Add the obtained values together to get the average atomic mass.

For example, the value of the average atomic mass of carbon is 12.02. Carbon has two abundant isotopes: carbon-12 and carbon-13. These isotopes have the abundance percentages 98% and 2, respectively. Using these values, we can determine the average atomic mass of carbon via a calculation. Here, we have to multiply the atomic masses of each isotope with the abundance value. Thereafter, we need to take the abundance as a two decimal placed value, not as the percentage. Next, we can add the obtained values.

For carbon-12: 0.98 x 12 = 11.76

For carbon-13: 0.02 x 13 = 0.26

Average atomic mass of carbon= 11.76+0.26=12.02.

What is the Difference Between Monoisotopic Mass and Average Mass?

Monoisotopic mass and average mass are important parameters in mass spectrometry. These values deal with the atoms of particular chemical elements. The key difference between monoisotopic mass and average mass is that monoisotopic mass is calculated considering a single isotope, whereas average mass is calculated considering all the abundant isotopes of a particular chemical element.

Summary – Monoisotopic Mass vs Average Mass

Monoisotopic mass and average mass are important parameters in mass spectrometry. These values deal with the atoms of particular chemical elements. The key difference between monoisotopic mass and average mass is that monoisotopic mass is calculated considering a single isotope, whereas average mass is calculated considering all the abundant isotopes of a particular chemical element.