Question

What is the monoisotopic peak?

Kratos PC Airma CFRpis 1163.73 1185.74 1567.87 77 1 1479.90 1481.94 1439.93 1501.90 1399.77

Answer 1

According to the definition

1. "Monoisotopic mass is calculated using the mass of the most abundant natural isotope of each constituent element" and

2. "Some sources suggest that monoisotopic mass is calculated from the lightest natural isotope of each element, but this is wrong. "

With "most abundant natural isotope of each constituting element", they mean the most abundant natural isotope of each individual atom. Not of the resulting molecule! For Carbon, for example, the most abundant isotope is carbon-12 (which also happens to be the lightest). However, for Iron, the most abundant isotope is Iron-56, which is not the lightest.

Now, according to the definition of the monoisotopic mass, it is the total mass of purely the most abundant isotopic species of all the atoms in your molecule. In proteomics, that is mostly Carbon, hydrogen, oxygen and nitrogen. In all of those atoms, the most abundant isotope is also the lightest one.

So, in your spectrum,

Spectrum 1 the peak at 1479.90 and Spectrum 2 the peak at 1567.87 are the monoisotopic peak.

Note:

In larger peptides, the monoisotopic peak is not the most abundant peak in a spectrum, because the probability is higher for that peptide to contain at least 1 naturally occuring isotope.