Which figure should be compared with a mass spectrometry result?

Match convention to instrument: monoisotopic calculations against high-resolution measurements, average mass against centroided low-resolution data. Comparing across conventions produces a discrepancy that is purely arithmetic.

Does molecular weight confirm identity on its own?

Only partly. Mass cannot separate isomers or resolve residue order, and says nothing about purity, so it is normally paired with a chromatographic purity figure before a lot is accepted.

Why does a modified peptide differ by a small fixed amount?

Common modifications shift mass predictably — amidation removes about one Dalton against the free acid, acetylation adds forty-two, oxidation of one sulfur adds sixteen. A consistent offset identifies the modification.

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Chemistry

Molecular Weight

Mass of a molecule, often reported in Daltons (Da) for peptides.

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Research use only. For in vitro research use only. Not for human or animal consumption. Not for human consumption, medical use, or personal application.

Molecular weight is the mass of one molecule, reported for peptides in Daltons. It is calculated by summing residue masses and adding one water, since each bond formed released water and the intact chain keeps its terminal atoms. Two conventions circulate and they are not interchangeable: average mass uses natural isotopic abundance, monoisotopic mass only the lightest isotope of each element. The gap is negligible for short chains and widens with length, so the convention has to be stated.

Average against monoisotopic, and what the instrument reports

A mass spectrometer measures mass-to-charge ratio for ionised species, not mass. High-resolution data resolves individual isotope peaks, so the natural reference is monoisotopic; lower resolution merges the envelope into one signal approximating average mass.

Charge state adds a second layer, since peptides commonly ionise as multiply protonated species. Reading mass spectrometry data means deconvoluting charge before comparing anything with a theoretical figure.

Why a catalogue figure and a weighed vial disagree

Theoretical mass is calculated for the free base. Material isolated as a TFA salt or an acetate salt carries counterions and residual water that add to weighed mass without adding peptide.

That matters whenever concentration is derived from a weighing rather than an assay. Where the Certificate of Analysis reports net peptide content, that value is the correct basis for the calculation.

Frequently asked questions

Which figure should be compared with a mass spectrometry result?: Match convention to instrument: monoisotopic calculations against high-resolution measurements, average mass against centroided low-resolution data. Comparing across conventions produces a discrepancy that is purely arithmetic.
Does molecular weight confirm identity on its own?: Only partly. Mass cannot separate isomers or resolve residue order, and says nothing about purity, so it is normally paired with a chromatographic purity figure before a lot is accepted.
Why does a modified peptide differ by a small fixed amount?: Common modifications shift mass predictably — amidation removes about one Dalton against the free acid, acetylation adds forty-two, oxidation of one sulfur adds sixteen. A consistent offset identifies the modification.

Updated 2026-06-01