Can monoisotopic mass and average mass be used interchangeably?

No. They are different calculations. Quote the basis the laboratory printed.

Is monoisotopic mass the labelled milligram size?

No. It is a molecular mass, typically in daltons. The shop milligram figure is a catalogue fill size.

Glossary · 2026-09-24

Monoisotopic mass, laboratory definition

Monoisotopic mass is calculated from the most abundant isotope of each element in the formula. Average molecular weight uses the weighted natural mix of isotopes. The two numbers are close and easy to swap, which is why a peptide report should say which calculation it compared with the observed ion. If it does not say, do not choose one in the margin.

Salt form and adducts move the target again. Free peptide, acetate, and trifluoroacetate are different formulas. Read molecular weight and the COA guide before you file a match. Research use only.

Published lot files for this catalogue are indexed in the certificates library. This definition does not add a report, and it does not assign a task identifier. Research use only.

Frequently asked questions

Can monoisotopic mass and average mass be used interchangeably?: No. They are different calculations. Quote the basis the laboratory printed.
Is monoisotopic mass the labelled milligram size?: No. It is a molecular mass, typically in daltons. The shop milligram figure is a catalogue fill size.

Research use only. BlueNex Labs does not provide medical or veterinary advice. Confirm your institution’s policies before purchase. How a comparison or class note is checked sits on documentation standards. The biochemist of record is Alice Kay, Ph.D.