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HPLC Purity Explainer:
How to Read a Peptide COA
A guide to area%, peak shape and impurity notation on a Certificate of Analysis — what the number says, what it does not say, and how to read the chromatogram behind it.
Research Use Only — Not for Human Use
1. What the Purity Number Actually Measures
The number near the top of almost every research-peptide Certificate of Analysis — Purity ≥ 99% — is a specific, narrowly defined figure. It is the output of reversed-phase high-performance liquid chromatography (RP-HPLC) with ultraviolet detection, typically read at 214–220 nm, where the peptide bond itself absorbs.5
The instrument dissolves the sample and pumps it through a column packed with a non-polar (C18 or similar) stationary phase. Different molecules bind to that phase with different affinities and so emerge — elute — at different times, each registering as a peak on the UV detector. The software integrates the area under every detected peak. The purity figure is then simply:
Purity (%) = (Area of main peak ÷ Total area of all detected peaks) × 100.
This is a relative measurement: the main peak as a fraction of everything the UV detector picked up. It is defined and governed by chromatographic conventions set out in pharmacopoeial general chapters such as USP <621>3 and ICH Q6A.1
The critical implication is what the denominator excludes. Any species that does not absorb UV at 214–220 nm, or that never fully elutes from the column, is simply not counted. The purity percentage is honest about the chromatogram and silent about everything outside it.
2. How to Read a Chromatogram
A chromatogram is the raw output: a time-axis plot with peaks rising from a baseline. Here is a schematic of what you should expect to see on a high-quality peptide COA — and what each feature means:
Schematic only — for illustration of peak-area concepts. Not reproduced from any real COA.
What to look for on a real chromatogram:
- One dominant main peak with a clean, approximately symmetrical shape. The tailing factor (T = W0.05 / 2f) should be close to 1.0; values above 1.5 indicate peak asymmetry that can mask co-eluting impurities.
- Flat, stable baseline before and after all peaks. Baseline drift or noise can inflate or deflate the integrated areas of small peaks.
- Well-resolved impurity peaks, not merged shoulders on the main peak. USP <621> specifies a minimum resolution (Rs) between adjacent peaks as a system-suitability criterion; co-elution hides impurities under the main-peak area.3
- A related-substances table listing every peak above the reporting threshold (typically 0.05–0.1% area), with its retention time and area%. These are deletion sequences, oxidised forms, deprotection fragments, and aggregates — the ordinary by-products of solid-phase synthesis.5
3. What HPLC Cannot Tell You: Identity
Here is the part that is most often omitted from supplier descriptions. A single, tall, symmetrical main peak at 99% area tells you the sample is chromatographically homogeneous — mostly one compound. It does not tell you which compound that is.
Retention time is characteristic but not definitive. A different peptide of similar hydrophobicity can elute in the same region of the chromatogram. HPLC answers the question “how pure?” It cannot, on its own, answer “pure what?”5
The identity question belongs to mass spectrometry. Two methods are standard in peptide COAs:
| Method | What it proves | What it does NOT prove |
|---|---|---|
| RP-HPLC (UV 214–220 nm) |
Relative chromatographic purity — main peak as % of total UV-detected peak area3 | Molecular identity; absolute concentration; anything that doesn’t absorb UV or doesn’t elute5 |
| UPLC / UHPLC | Same purity question at higher resolution — better separation of closely related impurities3 | Identity; non-chromophoric impurities; endotoxin or water content5 |
| ESI-MS | Identity — observed mass vs calculated mass of the intended sequence5 | Quantitative purity %; trace related-substance levels; biological contaminants2 |
| MALDI-TOF | Identity / molecular weight confirmation, robust for intact peptides5 | Fine purity quantitation; counter-ion, water and endotoxin content5 |
Purity (HPLC/UPLC) and identity (ESI-MS/MALDI-TOF) answer different questions. A credible COA reports both, in line with ICH Q6A specification practice1 and the EMA’s synthetic-peptide guideline.4
Mass spectrometry ionises the molecule and measures its mass-to-charge ratio (m/z). The observed mass is then compared against the calculated theoretical mass of the intended amino-acid sequence. A match within instrument tolerance is positive evidence of identity. A mismatch renders the purity figure irrelevant — a very pure sample of the wrong peptide is not a suitable research reagent.5
4. How to Read the Numbers on a COA — Annotated Example
Below is an annotated mock-up of a typical peptide COA purity section. Each field is explained.
≥ 99.0% (area%)
The main peak accounts for ≥99% of the sum of all UV-detected peak areas at 214 nm. Relative, not absolute. A different vial with 97% area% is not necessarily inferior — it depends on which impurities the remaining 3% represents and whether they are qualified per ICH Q3A.2
RP-HPLC, C18 column, UV 214 nm, gradient 5–65% ACN/0.1% TFA, 45 min run
The column chemistry, gradient, wavelength and run time determine which impurities are resolved. Always ask for method details — a 10-minute isocratic run may be insufficient to separate closely related deletion sequences.
Tailing factor ≤ 1.5 · Resolution (Rs) ≥ 2.0 between reference peaks · Plate count ≥ 5000
USP <621> requires these criteria to be met and recorded before the analytical result is valid.3 If this row is absent, the result cannot be assumed to meet pharmacopoeial standards.
See impurity table — all individual impurities < 0.5% area; total impurities < 1.0% area
ICH Q3A sets identification and qualification thresholds for impurities in drug substances.2 For research materials, what matters is whether individual impurity peaks are listed and below meaningful thresholds — not merely that the main peak is large.
Calculated MW: 1419.54 Da · Observed MW: 1419.51 Da · ✓ Confirmed
The observed mass matches the theoretical mass of the intended amino-acid sequence within instrument tolerance (~0.1 Da for small peptides). This is the identity check that HPLC cannot provide. Without this row, identity is unconfirmed.45
≤ 5% (Karl Fischer)
Lyophilised peptides are hygroscopic. Water is real mass in the vial that the HPLC area% figure does not count. A high water content means the vial contains less actual peptide than the nominal fill weight implies.5
TFA salt form / Acetate salt form (ion-exchange conversion)
Synthetic peptides are typically isolated as TFA (trifluoroacetate) salts from the purification step. TFA mass is real mass in the vial not reflected in the area% purity. Ion-exchange to acetate form reduces this. Some COAs report net peptide content (corrected for salt and water); most do not.5
Reported separately — see Endotoxin / Sterility section
Bacterial endotoxins are not chromophores at 214 nm and are completely invisible to the HPLC purity assay. They require dedicated LAL (Limulus Amebocyte Lysate) or recombinant factor C testing. Their presence or absence says nothing about, and is not captured by, the purity percentage.5
5. What a Purity Specification Is — and Is Not
A purity specification on a COA is a batch-tested acceptance criterion, not a per-vial guarantee. ICH Q6A defines it as a list of tests, procedures and acceptance criteria that a drug substance must meet.1 The specification sets a range — “≥ 99%” means the tested batch met or exceeded 99%, not that every vial in that batch was measured individually.
Three structural limits follow from this:
- The percentage is method-dependent. The same compound analysed by two different HPLC methods (different columns, gradients, UV wavelengths) can return different area% values. Without the method, the number is unanchored.
- The denominator is the chromatogram, not the vial. Anything invisible to the method — endotoxins, counter-ions, water — is excluded from the calculation. A “99% pure” vial may be 80% peptide by true weight if water and TFA content are high.
- Impurity control is as important as the headline figure. ICH Q3A requires that impurities above certain thresholds be identified, qualified and reported with limits.2 A COA that lists only the main-peak figure without an impurity profile is not a complete characterisation document.
6. Supplier Due-Diligence Checklist
Use this checklist when evaluating any peptide COA. Every item that is missing represents a question you should put to the supplier before trusting the reagent.
- COA is batch-specific to the vial you are buying — not a generic product sheet reused across batches
- HPLC purity figure is reported as area% with method details (column, gradient, wavelength, run time)
- System-suitability results (tailing factor, resolution, plate count) are recorded and pass criteria per USP <621>3
- Chromatogram (the actual trace) is available on request — not just the integrated percentage
- Related-substances / impurity table lists all peaks above reporting threshold with area% and retention time
- Mass spectrometry identity data (ESI-MS or MALDI-TOF): calculated MW vs observed MW, result confirmed45
- Water content measured and reported (Karl Fischer titration or equivalent)
- Counter-ion form stated (TFA or acetate); net peptide content reported if possible
- Endotoxin level measured separately by LAL or rFC assay and reported5
- Testing performed by an independent, third-party laboratory (not solely in-house)
Frequently Asked Questions
Does “≥99% purity” mean the vial is 99% peptide by weight?
No. It is a relative chromatographic peak-area percentage from RP-HPLC — the main peak as a fraction of all UV-detected peaks at 214–220 nm. A vial can read 99% area% yet contain meaningfully less peptide by weight because adsorbed water and counter-ion (salt) mass are not counted in the area% calculation.35
What does “area%” mean on a chromatogram?
Area% for any given peak = (area of that peak ÷ sum of all integrated peak areas) × 100. The main target-peptide peak area% is the purity figure. All other peaks — impurities, deletion sequences, oxidised forms — appear as smaller area% values in the related-substances table. The values must sum to 100% of the detected area.3
Why does a clean HPLC peak not prove identity?
Retention time is characteristic but not definitive — a different peptide of similar hydrophobicity can elute at the same position. Identity requires orthogonal confirmation by mass spectrometry, which compares the calculated mass of the intended amino-acid sequence against the observed mass. A COA without that identity evidence has not demonstrated identity.45
What is a system-suitability test and why does it matter?
Before reporting a purity result, USP <621> requires the HPLC system to meet criteria such as minimum resolution between adjacent peaks, acceptable peak symmetry (tailing factor ≤ 1.5), and theoretical plate count. A purity result from a system that failed suitability is analytically invalid — it cannot be trusted to resolve closely related impurities from the main peak.3
What does the impurity table in a COA actually list?
The related-substances table lists every peak above the reporting threshold (commonly 0.05–0.1% area) with its retention time and area%. These are typically: deletion sequences (peptides missing one or more residues), oxidised forms of the target peptide, incompletely deprotected fragments, and synthesis by-products. ICH Q3A governs how these must be identified, qualified and assigned limits for drug substances.2
What do HPLC and mass spectrometry together still miss?
Both methods are blind to residual water content, counter-ion (TFA or acetate) content, and biological contaminants such as bacterial endotoxins. Endotoxins in particular carry no informative peptide-bond UV signature and pass through a purity assay entirely unseen — they require dedicated testing (LAL or recombinant factor C assay). A complete COA reports these attributes through separate, dedicated tests.5
References
- International Council for Harmonisation. ICH Q6A: Specifications — Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances. Step 4, 1999. Available at: https://database.ich.org/sites/default/files/Q6A%20Guideline.pdf and https://www.ich.org/page/quality-guidelines
- International Council for Harmonisation. ICH Q3A(R2): Impurities in New Drug Substances. Step 4, 2006. Available at: https://database.ich.org/sites/default/files/Q3A%28R2%29%20Guideline.pdf and https://www.ich.org/page/quality-guidelines
- United States Pharmacopeia. General Chapter <621> Chromatography. USP-NF. Available via the official USP-NF online platform at: https://www.uspnf.com/
- European Medicines Agency. Guideline on the Development and Manufacture of Synthetic Peptides. EMA/CHMP/BWP/704139/2018, 2019. Available at: https://www.ema.europa.eu/en/development-manufacture-synthetic-peptides-scientific-guideline
- Fosgerau K, Hoffmann T. Peptide therapeutics: current status and future directions. Drug Discovery Today. 2015;20(1):122–128. PMID: 25450771. DOI: 10.1016/j.drudis.2014.10.003
- Mateescu DM, Gavrilescu DM, Constantinescu FE, et al. BPC-157 as an Investigational Peptide Therapeutic: Biopharmaceutical Challenges, Formulation Strategies, and Translational Development Barriers. Pharmaceutics. 2026;18(5):625. PMID: 42198317. DOI: 10.3390/pharmaceutics18050625