The short answer
HPLC with UV detection separates a sample and reports what share of the detected, peptide-related material is the main component. That is purity. Mass spectrometry measures the mass of the molecules in the sample and compares it with the mass calculated from the expected formula. That is identity evidence. A high purity figure does not say what the main peak is, and a matching mass does not say how much else is present. LC-MS, which places a mass spectrometer after the chromatography column, can answer both questions peak by peak.
| HPLC (UV detection) | Mass spectrometry | |
|---|---|---|
| Question answered | How much of the detected material is the main component? | Does the main component have the expected molecular mass? |
| Typical report line | Purity, area %, with column and wavelength | Observed mass against theoretical mass, with a tolerance |
| Supports identity? | Only by matching a reference standard's retention time | Yes, directly |
| Gives proportions? | Yes, as relative peak area | Not on its own |
| Main blind spots | Co-eluting components; water, salts and solvents | Same-mass variants such as epimers |
How HPLC produces a purity number
In reverse-phase HPLC the dissolved sample is pumped through a column packed with a hydrophobic stationary phase while the mobile phase grows steadily richer in organic solvent. Components leave the column at different times depending on how strongly they interact with it, and a UV detector, usually set between 210 and 220 nm where the peptide bond absorbs, records a peak for each. The chromatogram is that record: detector response plotted against time.[1, 2]
The purity figure is usually area normalization: the main peak's area divided by the summed area of the integrated peaks. Three consequences follow.
- It is relative. It describes how the detected material divides up, not how much peptide is in the vial. That is a separate measurement, net peptide content.
- It is method-bound. Column chemistry, gradient, wavelength and integration settings all move the result, which is why a report should state them, and why two laboratories can differ by a fraction of a percent on the same material.
- It cannot count what the detector does not see. Water, counter-ions such as acetate or trifluoroacetate, and most residual solvents produce no useful peak. Anything that co-elutes with the main peak is counted as main peak.
Some laboratories also cap the figure. The report for lot RT-2502-B (GLP-3 (RT) 20mg) notes that purity values are capped at 99.9% to reflect the instrument's limit of detection. A value printed at the cap means no impurity was detected above that threshold, not that the material is exactly 99.90% pure.
How HPLC can support identity, and where it stops
A chromatogram can also be used for identification. If the sample's main peak elutes at the same retention time as an authentic reference standard run under the same conditions, that is evidence the two are the same substance. Reports often label this "HPLC-RTM" (retention-time match) or simply "identity by HPLC".
The limitation is that a retention time is a single number. Closely related peptides, such as the sequence with one residue missing, or with one amino acid in the opposite (D) configuration, can elute close to the parent peptide and sometimes on top of it.[3] That is why pharmaceutical guidance for synthetic peptides recommends at least two orthogonal identification methods, listing mass, relative retention time, LC-MS, peptide mapping, amino acid analysis and NMR as appropriate choices,[4] and why ICH Q2(R2) says that a procedure lacking discrimination should be supported by one or more others.[5] Research materials are not held to drug standards; the guidance is simply the clearest public statement of what complete identity evidence looks like.
How mass spectrometry produces an identity result
A mass spectrometer turns molecules into gas-phase ions, for peptides most often by electrospray ionization, and measures their mass-to-charge ratio (m/z). Peptides usually carry several extra protons, so one molecule appears as a family of peaks at different charge states, which software combines ("deconvolutes") into a single neutral mass. That observed mass is compared with the theoretical mass calculated from the molecular formula, and where sequence confirmation is needed, tandem MS breaks the peptide into fragments whose masses spell out the sequence.[4]
A worked example with public numbers. BPC-157 has the formula C62H98N16O22. Its monoisotopic mass, built from the most abundant isotope of each element, is 1418.70 Da; its average molecular weight is about 1419.5 g/mol, the figure printed on the BPC-157 5mg listing. In an electrospray spectrum, the ion carrying two extra protons, [M+2H]2+, would appear near m/z 710.36 and the three-proton ion [M+3H]3+ near m/z 473.91. A report that lists one of these, or the deconvoluted mass, next to the theoretical value and a stated tolerance is making a claim you can check with a calculator.
Mass differences are diagnostic too. Common gaps between an observed and an expected mass include:
- +15.995 Da: one oxidation, typically at a methionine or tryptophan residue.
- +0.984 Da: deamidation of an asparagine or glutamine, small enough to need a high-resolution instrument to see clearly.
- −18.011 Da: loss of water, for example from a ring-closing side reaction such as aspartimide formation during synthesis.
- −57.021, −71.037 or −97.053 Da: a missing glycine, alanine or proline residue, the signature of a deletion sequence.
The library applies the same arithmetic to specific materials, for example the 18-dalton question for a cyclic peptide.
What mass spectrometry cannot tell you
- It cannot separate molecules of identical mass. An epimer (one D-amino acid in place of an L) or an isoaspartate variant has the same formula as the parent peptide, so the mass is identical. Chromatography has to do that work.
- It is not a purity measurement on its own. Different molecules ionize with different efficiency, so peak heights in a mass spectrum do not translate directly into proportions.
- It says little about water, counter-ions or most solvents, any more than UV-HPLC does. Those need their own tests.
Which impurities each method can see
The pharmaceutical literature catalogues the peptide-related impurities that synthesis and storage produce: deletion and insertion sequences, diastereomers from racemization, incompletely removed protecting groups, oxidation products and dimers, among others.[3] The table summarises which method is suited to each.
| Impurity type | Mass change | HPLC-UV | Mass spectrometry |
|---|---|---|---|
| Deletion or truncated sequence | Minus one or more residues | Often resolved; can co-elute | Yes, distinct mass |
| Oxidation | +16 Da | Usually resolved | Yes |
| Deamidation | +1 Da | Sometimes resolved | Needs high resolution |
| Epimer (D-amino acid) | None | Method-dependent | No, same mass |
| Counter-ion, water, solvents | Not applicable | Not counted | Not usefully measured |
Counter-ion content, water and residual solvents are listed as separate specification tests in peptide guidance for exactly this reason: neither instrument above measures them.[4]
What Arctic Lab Supply's published reports use
Stated plainly, so you can weigh them correctly: the lot reports currently published in the batch COA library report purity by reverse-phase HPLC with area normalization at 214 nm, and identity by HPLC retention-time match against a reference standard. They do not include a mass-spectrometric identity result. The rest of the panel varies by lot: every current report has an endotoxin line, six of the seven report net peptide content, and four add heavy-metals and sterility sections, so read the report for the exact lot. For example, the report for lot BCP-2502-B of BPC-157 5mg reads 99.59% purity with identity "Confirmed" by HPLC-RTM.
If your laboratory's acceptance criteria call for mass-based or orthogonal identity, read these reports as purity documents with single-method identity, and plan confirmatory analysis accordingly. Contact support with the lot number to find out what documentation exists for a specific vial.
Reading the two lines together
- Match the sample name, strength and lot on the report to the vial label.
- Find the identity method. Is it mass-based, retention-time based, or both?
- If a mass is given, check the theoretical value against the published formula, then the observed value against it within the stated tolerance.
- Read the purity figure with its method: technique, wavelength, and whether a cap or exclusions apply.
- List what neither line covers: content per vial, water, counter-ion and solvents.
- Confirm the certificate number with the issuing laboratory before relying on any of it.
Frequently asked questions
Is HPLC or mass spectrometry better for testing peptides?
Neither replaces the other. HPLC measures purity as a share of the detected material; mass spectrometry confirms molecular mass. The most complete reports use both, often combined as LC-MS.
Can a peptide be 99% pure and still be the wrong compound?
Yes. A purity figure says one component dominates the chromatogram, not what that component is. Identity needs a separate result: a mass measurement, a match against an authentic reference standard, or ideally both.
What does HPLC-RTM mean on a COA?
Identity by retention-time match: the sample's main peak eluted at the same time as a reference standard under identical conditions. It is valid evidence but one-dimensional, so closely related peptides can be hard to rule out.
Why can mass spectrometry not detect a D-amino acid impurity?
Swapping one L-amino acid for its D form does not change the molecular formula, so the mass is identical. Separating the two has to come from chromatography.
Does a mass spectrometry result tell you the purity?
Not on its own. Molecules ionize with different efficiency, so peak heights in a mass spectrum are not proportions. Purity comes from the chromatographic trace.
Sources
- United States Pharmacopeia, General Chapter <621> Chromatography. https://doi.usp.org/USPNF/USPNF_M99380_70101_01.html
- Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods in Molecular Biology 386:3–55 (2007). https://doi.org/10.1007/978-1-59745-430-8_1
- D’Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, De Spiegeleer B. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis 101:2–30 (2014). https://doi.org/10.1016/j.jpba.2014.06.012
- European Medicines Agency. Guideline on the Development and Manufacture of Synthetic Peptides, EMA/CHMP/CVMP/QWP/367182/2025, in effect from 1 June 2026 (characterisation table, identification and specification sections). https://www.ema.europa.eu/en/development-manufacture-synthetic-peptides-scientific-guideline
- ICH Q2(R2) Validation of Analytical Procedures, FDA guidance edition (section 3.1, specificity/selectivity). https://www.fda.gov/regulatory-information/search-fda-guidance-documents/q2r2-validation-analytical-procedures