Reported proteoform masses is always 9Da less

[pnnl-comp-mass-spec]

Reported proteoform masses is always 9Da less

[FarmGeek4Life]

Note that it is reporting the Most Abundant Isotope m/z, not the monoisotopic m/z; meanwhile the mass refers to the monoisotopic mass. The mass is not directly calculated using the most abundant isotope m/z, instead (if I remember correctly, and somebody may correct me on this) it uses an algorithm to take the whole isotopic distribution and determine what the monoisotopic mass of that distribution is.

I believe that averagine is used for part of this, and since all of those most abundant masses are in a relatively small range, the monoisotopic masses all end up with similar mass differences after adjusting for the atomic relative abundances.

[wingkinlui]

Note that it is reporting the Most Abundant Isotope m/z, not the monoisotopic m/z; meanwhile the mass refers to the monoisotopic mass. The mass is not directly calculated using the most abundant isotope m/z, instead (if I remember correctly, and somebody may correct me on this) it uses an algorithm to take the whole isotopic distribution and determine what the monoisotopic mass of that distribution is.

I believe that averagine is used for part of this, and since all of those most abundant masses are in a relatively small range, the monoisotopic masses all end up with similar mass differences after adjusting for the atomic relative abundances.

Thank you so much for your reply.

But is it true that the monoisotopic species should still be inside of the isotopic envelope?
Using the same PrSM as an example, the calculated m/z from the reported proteoform's monoisotopic mass would be (15171.5+23)/23 = 660.63.
This m/z would be outside of the matched isotopic envelope.
And there is nothing detected in 660.63.