LC-MS identity confirmation
Confirming that the species eluting at the main peak has the expected mass.
Purpose
Confirming that the species eluting at the main peak has the expected mass.
This page exists so that a discussion about a number can start from a shared understanding of what produced it. It is written for a reader who is technically literate but not a practising analyst.
How it works
Coupling liquid chromatography to a mass spectrometer lets you assign a mass to a chromatographic peak rather than simply noting that a peak exists. For peptides the ionisation source is almost always electrospray, which produces multiply charged ions.
A 4 kDa peptide will typically be observed as a series of charge states rather than as a single intact ion. The observed mass-to-charge ratio for charge state z is (M + z × 1.00728) / z, where M is the neutral monoisotopic mass. Reading a spectrum means recognising the envelope, not looking for one peak.
What determines the result
Mass accuracy is expressed in parts per million and is the difference between observed and theoretical mass divided by the theoretical mass, multiplied by a million. A high-resolution instrument in good calibration will routinely achieve low single-digit ppm on a peptide of this size.
What a mass match establishes is that the composition is consistent. What it cannot establish: purity, sequence order, stereochemistry, or the absence of an isobaric species. Every one of those requires something else — a purity assay, tandem mass spectrometry, chiral analysis, or a separation that resolves the isomer.
Typical criteria
| Parameter | Typical value or criterion |
|---|---|
| Mass accuracy, high-resolution instrument | ≤ 5 ppm typical |
| Mass accuracy, unit-resolution instrument | Within ± 1 Da at best |
| Charge states observed | Usually 2+ through 5+ for a 4 kDa peptide |
| Common adducts to expect | +Na (≈ +22 vs H), +K (≈ +38 vs H) |
These are typical values, not universal standards. The criteria that apply to a specific result are the ones in the validated method that produced it, which is why a report without a method identifier is harder to interpret than one with.
What it cannot tell you
Every analytical method answers one question. The most common error in this community's discussions is treating an answer to one question as an answer to another: a mass match read as a purity result, a purity figure read as a content determination, a sterility test read as a guarantee of safety.
When you read a report, write down which question was asked. The rest follows.
Related pages
Other method notes in this commons cover the neighbouring techniques, and the certificate interpretation page covers how these results get reported. The mass spec subcategory is where the discussion happens.
Revision history
| 2026-03-13 | customs_ledger | Updated a dated claim and added the date explicitly, per the maintenance convention. |
| 2025-10-21 | a.salcedo | Recorded a dissent from one maintainer rather than resolving it silently. |
| 2025-09-07 | PharmNotes_Whitfield | Split an overlong section in two and gave the second a proper heading. |
| 2025-07-10 | g.pemberton_uk | Annual review: checked every figure against its source. Two rounded values tightened. |
| 2025-04-13 | NLoughran | Added the table of acceptance criteria requested in doc review. |
| 2024-12-04 | titration_diary | Initial promotion from the source topic. Structure taken from the marked solution. |
Every edit to a maintained document records its author and a note explaining the change. An edit without a note may be reverted by any wiki editor under R9, and the revert is logged. Disagreements about content belong on the source topic rather than in the revision history (R10).