How a peptide's identity is confirmed, and what HPLC misses

Identity is established by measuring the molecule's mass and comparing it against the mass calculated from the intended sequence. That measurement is made by mass spectrometry, using one of the soft ionisation techniques that made intact biomolecules measurable at all. Chromatography does not perform this function. High-performance liquid chromatography separates a mixture and reports how much of the detector signal belonged to each separated component; the output is a retention time and an area, neither of which names a molecule. The two measurements answer different questions, and a certificate carrying only the chromatographic one has established homogeneity without having established what the homogeneous thing is. Identity is also not a single yes-or-no test: intact mass, isotope pattern and fragmentation sequence sit on a ladder, each rung excluding errors the rung below cannot see.

What does chromatography establish about identity?

On its own, very little. Reversed-phase HPLC ranks components by how strongly they partition onto a hydrophobic stationary phase, and the resulting retention time is a property of the method as much as the molecule. Column chemistry, gradient slope, temperature, mobile-phase composition and instrument dead volume all shift it. A retention time carries identity information only when a reference standard of independently confirmed identity is run under identical conditions in the same analytical sequence, and it is weak information even then.

It is weak because the species most likely to share a target peptide's retention behaviour are the ones structurally closest to it. D'Hondt and colleagues catalogued the impurity classes that arise in solid-phase peptide synthesis, and the list is dominated by near-relatives of the target: deletion and insertion sequences from inefficient Fmoc-deprotection or reagent excess, diastereomeric impurities from racemisation during deprotection, side-chain protection adducts from incomplete deprotection, oxidation products, and dimeric-to-oligomeric species. They also recorded contamination of a desired peptide by entirely unrelated peptides, which they attributed to inadequate manufacturing control (J Pharm Biomed Anal, 2014;101:2–30; PMID 25044089, DOI 10.1016/j.jpba.2014.06.012). A deletion sequence missing one residue is a different molecule with a different mass, and chromatography may or may not resolve it. Mass spectrometry does not have that ambiguity.

What does a mass measurement actually establish?

Two developments made peptide mass measurement routine. Karas and Hillenkamp demonstrated laser desorption ionisation of proteins with molecular masses above 10,000 daltons, the technique that became MALDI (Anal Chem, 1988;60(20):2299–301; PMID 3239801, DOI 10.1021/ac00171a028). Fenn and colleagues described electrospray ionisation for mass spectrometry of large biomolecules, in which a molecule acquires multiple charges and therefore appears at a mass-to-charge ratio within the range of conventional analysers (Science, 1989;246(4926):64–71; PMID 2675315, DOI 10.1126/science.2675315). Both place an intact peptide into the gas phase without fragmenting it, which is what allows the whole molecule to be weighed.

The comparison that follows is arithmetic. A sequence implies an elemental composition, an elemental composition implies a monoisotopic and an average mass, and the observed mass either falls within the instrument's accuracy window around that value or does not. A match is positive evidence about composition.

How much evidence depends on how tight the window is, and this is more subtle than it looks. Zubarev and Mann examined the proper use of mass accuracy in proteomics and argued that a quoted accuracy figure is only meaningful when the underlying measurement statistics are handled correctly, and that mass accuracy constrains the space of candidate compositions rather than singling one out (Mol Cell Proteomics, 2007;6(3):377–81; PMID 17164402, DOI 10.1074/mcp.M600380-MCP200). A mass match is a strong exclusion test. It is not a proof of identity.

Which errors survive a matching intact mass?

Three classes, and they are not obscure.

Isomers with identical elemental composition. Leucine and isoleucine differ in connectivity, not composition, so a substitution between them changes nothing about the intact mass. The same holds for aspartic acid and isoaspartic acid, and for D- versus L-amino acid substitutions produced by racemisation during synthesis. Hurtado and O'Connor reviewed the mass spectrometric methods developed specifically to differentiate these residues, covering Asp/isoAsp, Leu/Ile, glutamic acid versus γ-glutamic acid and D/L enantiomers, and characterised isomer differentiation in biological macromolecules as one of the significant standing challenges in the field (Mass Spectrom Rev, 2012;31(6):609–25; PMID 22322410, DOI 10.1002/mas.20357). Ordinary intact-mass confirmation cannot see any of them.

Modifications at or below the accuracy floor. Deamidation of asparagine or glutamine adds slightly less than one dalton. Erckes and colleagues examined exactly this problem and reported that conventional chromatographic methods and standard mass spectrometric analyses often fail to distinguish structurally similar peptides with nearly identical physicochemical properties and masses. Using tandem MS with collision-induced dissociation and electron-transfer dissociation, they could differentiate the resulting species without relying on chromatographic separation, with ETD additionally enabling semi-quantitative detection. Their handling findings are the more striking part: isoaspartate formation was confirmed under mildly basic conditions such as phosphate-buffered saline, while exposure to acidic conditions — particularly trifluoroacetic acid, the additive commonly used during HPLC purification — produced substantial direct deamidation by hydrolysis, with C-terminal amides markedly more susceptible (RSC Med Chem, 2026;17(2):1144–54; PMID 41541711, DOI 10.1039/d5md01025j). The purification step itself can generate the modification the analysis is looking for.

Sequence permutation. Any rearrangement of the same residues yields the same intact mass. Only fragmentation distinguishes order.

How is a sequence actually read?

By breaking the molecule apart inside the instrument and reading the pieces. Steen and Mann's account of peptide sequencing describes how backbone fragmentation generates nested ion series — b and y ions under collision-induced dissociation, c and z ions under electron-transfer methods — whose mass differences correspond to individual residues, so that a complete series reads out as a sequence ladder (Nat Rev Mol Cell Biol, 2004;5(9):699–711; PMID 15340378, DOI 10.1038/nrm1468). This is the difference between confirming that a sample weighs what the intended peptide should weigh and confirming that the residues are in the intended order.

Fragmentation is also where the isomer problem becomes tractable, which is why both the Hurtado and the Erckes work above are tandem-MS work rather than intact-mass work.

What do our certificates report, and what do they not?

Five certificates are currently public, and all five report a chromatographic purity determination by Janoshik Analytical: BPC-157 batch 2026-03, tested 22 January 2026; GHK-Cu batch 2026-03 and Retatrutide batch 2026-03, tested 11 March 2026; and MOTS-c batch 2026-03 and NAD+ batch 2026-03, tested 20 March 2026.

None of them reports a mass spectrometric identity determination. That is a limit of what those documents establish and it should be stated rather than glossed: they are purity measurements, and by the argument above a purity measurement is not an identity measurement. Certificates issued by that laboratory carry a verification key that can be checked with the laboratory directly, so the purity figures are independently confirmable — but confirming a figure is not the same as extending its scope.

Veridian Research supplies these materials strictly for in-vitro laboratory research. They are not drugs and are not approved for human or veterinary use.

The short version

HPLC separates and quantifies; mass spectrometry identifies. An intact mass measurement excludes most gross synthesis errors, including the deletion and insertion sequences that dominate the impurity literature. It does not exclude isomeric substitutions such as Leu/Ile or Asp/isoAsp, sub-dalton modifications such as deamidation, or rearrangements of the same residues — all of which require tandem MS. A certificate that reports purity alone has answered one question well and left the identity question open.