Atoms have isotope distributions
A molecular formula can include elements with naturally occurring isotopes. An average mass incorporates an isotope-weighted convention. A monoisotopic calculation uses a defined isotopic composition appropriate to the molecular formula. The Thermo proteomics introduction illustrates the distinction.
Think of the difference as two defined ways to summarize molecular mass, not two laboratory opinions about which sequence is correct. The convention must remain visible when comparing a calculated reference with an observed result.
Compare like with like before calculating an error
A mass error calculated from an average reference and a monoisotopic observation may be misleading. Ask which expected value the analytical method actually uses. Also check whether the result represents a neutral mass or a charged ion with an attached proton or another species.
In an illustrative review, a catalog provides an average molecular mass while a report names a monoisotopic neutral mass. The first step is to align conventions, not demand a corrected certificate merely because the numbers differ.
| Field | Ask for | Do not mix |
|---|---|---|
| Isotope convention | Average or monoisotopic | Different conventions in one error calculation |
| Ion basis | Neutral mass or specified m/z | M and protonated-ion value |
| Structure | Sequence, termini and modifications | Parent and modified molecule |
| Material amount | Separate content / supplied-form basis | Peptide mass and whole-solid amount |
Structure still matters after the convention is aligned
Termini, labels, covalent modifications and residue composition affect the calculation. A mass convention cannot explain an actually different structural formula. Ask for the exact identity used to generate the expected value, including any intended isotope label.
Keep the supplied salt or constituents distinct from a peptide neutral-mass reference. The headline mass of the peptide molecule is not necessarily the mass basis of the entire isolated material. A buyer should not use one figure to answer every quantity question.
Do not require a universal reporting style
The Waters peptide-isolation application note discusses average and monoisotopic choices in its instrumental context. That context is not a rule prescribing one convention for every peptide report.
Ask the laboratory to state its convention and comparison basis clearly. The responsible reviewer can then assess the result against the method and project. A well-labeled difference is often more informative than forcing every document to display one unqualified molecular-weight number.
Questions buyers ask
Does a different molecular-weight figure always mean a wrong peptide?
No. First align convention, ion basis and structural definition.
Should every report use monoisotopic mass?
Not as a universal demand. The method and purpose determine the appropriate convention, which should be stated explicitly.
Your next step
Request the mass convention and structural basis with the expected and observed values. Recalculate comparisons only after those fields align.
Related reading: mass-to-charge explanation; batch-document review.
AI-assisted educational guide prepared from the cited sources. No testing of a supplied batch is reported here. For research-material purchasing, not medical guidance or instructions for human use. Confirm the actual offered material and available evidence before ordering.