What does a 1 Da mass shift on an ESI-MS spectrum indicate?

The method section is present, which is unusual enough that I want to make use of it.

I have read the primary source rather than the summary, which has left me with more questions.

I understand the headline. I do not understand the footnotes, and the footnotes look important.

How should I read this, and where are the traps?

The limit of detection in electrospray is orders of magnitude better than in HPLC, which means a mass spectrometer can see species that the chromatograph cannot.

A mass shift of minus one hundred and twenty-eight usually means a missing Gln or Lys residue from a synthesis deletion sequence.

On the detail: a mass shift of minus eighteen usually means dehydration or a succinimide intermediate, which is pH-dependent and can be reversible.

The limitation is that mass spectrometry tells you the mass and almost nothing else, so it needs to be paired with chromatography or other identity information.

The practical summary: use mass spectrometry for identity, not for purity.

A D-amino-acid substitution has the same molecular weight as the L-form, so mass spectrometry cannot distinguish them without fragmenting the peptide.

A mass shift of plus sixteen usually means oxidation at methionine or tryptophan, which is common in peptides and often comes from sample handling rather than synthesis failure.

A mass shift of plus one usually means deamidation at asparagine or glutamine, which creates a secondary amine instead of an amide and changes the mass by exactly one.

False positives from contamination are common in mass spectrometry work, and running a blank between every sample and a solvent background are standard practice.

The caveat is that a correct mass does not mean the peak is correct — isomers and co-eluting species can have the same m/z.

Always run a blank between samples and check for carry-over.

A mass shift of exactly zero with a shifted retention time points to an isomer — a scrambled disulfide or a racemised residue — which mass spectrometry alone cannot identify.

For a large peptide with multiple peaks in the mass spectrum, comparing the observed isotope pattern to the calculated pattern is a quick check that the formula matches.

Worth noting that source contamination is common and silent, so a result that looks too good to be true often is.

A correct mass is necessary for identity but not sufficient — you also need the chromatography to confirm it.

edited 14 Jul 2026 by lane_transit — added the placebo-arm figures

More usefully, the single most important fact about mass spectrometry for peptides is that it measures only the molecular weight and tells you almost nothing about whether the peak is actually your target.

High-resolution mass spectrometry can distinguish a Lys-containing peptide from an Arg-containing peptide of similar mass because of the isotope difference.

Electrospray ionisation soft-ionisation behaviour is well-characterised and standards exist for m/z calibration and mass accuracy assessment.

The practical summary: use mass spectrometry for identity, not for purity.