By ChinaPeptides Technical Team · Published May 13, 2026 · 6 min read
Liquid chromatography–mass spectrometry is the identity test that travels with HPLC on a modern peptide CoA. Chromatography separates species; the mass spectrometer reports mass-to-charge ratios that can be assigned to the intended formula, a deletion, an adduct, or an incompletely deprotected chain. LC-MS does not replace a purity percent. A correct mass on a dirty sample is still a dirty sample.
This article is a reading guide for research scientists who receive a spectrum and need to decide whether the lot matches the sequence they ordered through custom peptide synthesis. Instrument brand names and unpublished “in-house methods” are omitted on purpose. What matters is the arithmetic and the common assignments.
What the experiment is actually measuring
Most peptide QC uses electrospray ionization (ESI) in positive mode. The peptide picks up protons on basic sites. A 2 kDa peptide may appear as [M+2H]2+ and [M+3H]3+ rather than a single M+ peak. Software deconvolutes those ions to a molecular mass. You should still glance at the raw charge-state ladder: consistent spacing of 0.5 Da on a 2+ ion is a sanity check that the peak is a peptide, not electronic noise.
MALDI-TOF is still used, especially for crude screening and larger chains. It typically gives singly charged ions and a different adduct pattern (often sodium or potassium). Do not treat a MALDI mass and an ESI deconvoluted mass as different molecules if they agree within the expected accuracy after adduct accounting.
Chromatographic inlet matters. A formic-acid LC-MS method may resolve or merge peaks differently than the TFA UV method used for the purity number. That is why release documentation should present both, not a single merged screenshot without conditions.
Expected mass arithmetic you can do at the bench
Start from the residue monoisotopic masses, add water for a linear peptide (the terminal H and OH), then apply terminal modifications: acetylation adds 42.01 Da, C-terminal amidation subtracts 0.98 Da relative to the free acid, and disulfide formation subtracts 2.02 Da per cystine. Fluorophores, biotin, and PEG have published monoisotopic increments; use those, not a rounded “about 500.”
Average mass versus monoisotopic mass is a frequent source of false alarms. Low-resolution instruments and some CoA tables report average mass. High-resolution ESI often reports monoisotopic. A 4 kDa peptide can differ by a few daltons between those conventions. Check which number the certificate claims before you write a complaint.
Isotope-labeled peptides need a separate calculation. Each 13C or 15N shifts the monoisotopic mass by a known increment. Incomplete labeling appears as a residual unlabeled or partially labeled envelope, which is a specification item for quantification standards, not a surprise impurity.
| Observation | Typical assignment | What to check next |
|---|---|---|
| M+16 or M+32 | Met/Trp oxidation; sometimes sulfoxide plus further oxidation | Storage, cleavage scavengers, handling in air |
| M+22 / M+38 | Sodium / potassium adducts | Desalting; not a different sequence |
| M+56, M+114, etc. | Residual tert-butyl or related protecting groups | Cleavage completeness |
| M−18 | Aspartimide, succinimide, or loss of water | Asp-Gly motifs; HPLC isomers |
| M minus one residue mass | Deletion peptide | Which residue is missing; coupling history |
| M+42 | Acetyl (intended or from capping) | Whether N-acetylation was specified |
Adducts, dimers, and other non-sequence masses
Sodium and potassium adducts are common in ESI and can dominate if the sample is salty. They are not covalent products. A true covalent dimer of a cysteine peptide appears at approximately 2M−2 and should be discussed as an oxidation or oligomer issue. Non-covalent dimers sometimes appear in the gas phase at high concentration; they disappear on dilution and should not be treated as a second product without chromatography.
TFA adducts and cluster ions appear in some spectra. Trifluoroacetate can also suppress signal. If the UV chromatogram is clean and the MS is weak, ionization suppression is a hypothesis before “the peptide is missing.”
When HPLC and MS disagree
Single UV peak, two masses: co-elution. Ask for extracted-ion chromatograms of each mass. If they perfectly overlap, the analytical HPLC method is not orthogonal enough for a ≥98% claim. If they separate slightly, a tighter cut or a different ion-pair reagent may resolve them. This is the most important failure mode when people treat HPLC percent as gospel.
Two UV peaks, one mass: isomers. Cis-trans proline, aspartimide versus parent, and disulfide isomers of a cyclic peptide are classic. Mass cannot distinguish them. You need a method that separates them and a chemical hypothesis. See HPLC method design and, for rings, cyclic peptide methods.
Correct mass, low HPLC purity: the main component is likely the right molecule sitting in a mixture. Purification, not resynthesis, is the first discussion unless the crude is hopeless. Wrong mass, high HPLC purity: the main peak is the wrong molecule. That is a synthesis or sequence-communication error and is more serious than a messy crude of the correct mass.
Fragmentation when order is in doubt
MS/MS (typically b and y ions in CID) can confirm sequence regions, especially when two isobaric possibilities exist (Ile/Leu still fail; those need ECD/ETD or Edman or a labeled analog). Routine research CoAs often stop at intact mass. Request tandem MS when the peptide is a new sequence used as a quantitative standard, when two theoretical sequences share a mass, or when a modification site is ambiguous (for example biotin on one of two lysines).
Do not expect a full sequence ladder on a 40-mer from a single survey scan. Targeted fragmentation of the intact ion is a separate experiment.
How to use LC-MS in a specification
Write “observed mass consistent with calculated monoisotopic (or average) mass within X Da” and name the ionization mode. For modifications, name the expected increment. For disulfide peptides, state whether the mass is for the oxidized or reduced form. Archive the spectrum with the certificate of analysis so a later user can see the charge states, not only a rounded integer.
LC-MS is a research identity tool. It does not make a peptide suitable for diagnostic or clinical use.
Frequently asked questions
Why is my peptide’s mass 1 Da off?
Common causes are monoisotopic versus average mass confusion, a C-terminal amide written as an acid, reduction of a disulfide, or a 13C isotope picked as the monoisotopic peak on a large peptide. Recalculate before assuming a wrong amino acid.
Does a correct mass prove there is no deletion?
A deletion that is fully separated by HPLC will show a different mass on that peak. A deletion co-eluting under the main UV peak can be invisible if its ion is not inspected. Extract the expected deletion masses when the sequence is high risk.
What resolution do I need?
Unit-resolution ESI is enough to confirm many small peptides and to see +16 oxidation. High resolution helps assign formulas on modified or large peptides and to separate overlapping isotope envelopes.
Can LC-MS measure purity?
Ionization efficiency varies by species. Peak heights in a mass spectrum are not HPLC area percents. Use MS for identity and HPLC (or another calibrated method) for purity.
Why do I see M+22 if the peptide is “desalted”?
Trace sodium is enough to form adducts. Desalting reduces salt; it does not guarantee a sodium-free spectrum. Report the protonated deconvoluted mass as the identity result.
Should fluorescent peptides be run on the same LC-MS method as unlabeled ones?
Often yes for identity, but the dye changes hydrophobicity and ionization. Confirm the expected labeled mass and look specifically for free dye and unlabeled peptide ions.
Is amino-acid analysis still needed if LC-MS is correct?
AAA answers composition and can support peptide content. It is not a default on every milligram lot. Request it when the protocol requires composition or content, not as a substitute for mass.