According to the definition by Unimod
- "Monoisotopic mass is calculated using the mass of the most abundant natural isotope of each constituent element" and
- "Some sources suggest that monoisotopic mass is calculated from the lightest natural isotope of each element, but this is wrong. "
Attached is a figure (copied from a reference). For Panel a, it is quite obvious that the monoisotopic peak is the leftmost one.
So for Panel b, is the monoisotopic peak the one indicated by the blue or the red arrow?
Thanks.
http://www.unimod.org/masses.html
This is a good question and is often confused.
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 your 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, the blue peak is the monoisotopic peak. 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.
I hope this is helps.
This will help
https://link.springer.com/content/pdf/10.1007%2Fs13361-017-1647-6.pdf
Actually the link from springer.com does not help. It mentions monoisotopic mass only in passing. It deals mainly with nominal mass.
I'd suggest that the paper linked will only add to anyone's confusion if they do not understand the concept of monoisotopic mass.
Alexander Streng In some cases, the monoisotopic peak shows very small signal and the most abundant signal is near the m/z corresponding the average mass. I was curious and see if that means sometimes an analyte contains more heavy isotopes than what we know the abundance of heavier isotopes(?). For example, we have learned from the isotope distribution table that the 98.92% of carbon is C12 while 1.1% of carbon is C13. For the case of large analytes such as large peptides or proteins, is it possible that there is a greater probability of an analyte containing C13 greater than 1.1% of the carbon in the molecule?
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