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HPLC purity: what the percentage on a peptide COA means

How HPLC measures peptide purity, what area-percent really reports, why 99% purity can still mean the wrong amount or peptide, and how to read a trace.

Rows of unlabeled glass sample vials in an instrument tray, one with a red cap

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The short answer ·

HPLC (high-performance liquid chromatography) separates the components of a sample and records each as a peak. 'Purity' on a peptide COA is usually the main peak's area as a percentage of all detected peak area at one UV wavelength. It measures how clean the material is relative to other UV-absorbing compounds.

It does not by itself confirm identity, measure how many milligrams are in the vial, or detect water, salts, endotoxin or microbes.

How does HPLC work?

A dissolved sample is pumped at high pressure through a column packed with fine particles. For peptides this is usually reversed-phase, with a C18 or similar stationary phase and a water/acetonitrile gradient. Different molecules interact with the column differently, so they exit, or elute, at different times. A UV detector, usually set near 210-220 nm where peptide bonds absorb, records a signal over time. That signal is the chromatogram: a baseline with peaks.

Each peak has a retention time (when it came out) and an area (roughly, how much UV-absorbing material it contained).

What does “purity” mean on a COA?

Most peptide COAs report area-percent purity:

purity % = area of main peak ÷ total area of all integrated peaks × 100

This number is useful. It tells you how much of the UV-visible material is the main component and how much is related impurities, such as truncated sequences, deletion sequences, oxidized forms or leftover protecting groups. FDA’s 2026 advisory committee briefings flagged “peptide-related impurities” as a core safety concern for compounded peptides (Category 2).

What does HPLC purity not show?

Common assumption Reality
“99% pure = 99% of the vial is peptide” Water, counter-ions (often acetate or TFA) and salts are invisible to UV at these settings. See net peptide content
“99% pure = it’s the right peptide” A wrong peptide can give one clean peak. Identity needs mass spec or a reference standard
“99% pure = safe to inject” Purity says nothing about endotoxin or sterility
“Purity of one vial = purity of the batch” It’s one sample

TitrateLab’s analysis of archived results on one vendor illustrates the gap. Where the labeled peptide was found, purity was high. But in 14 of 125 results the lab did not find the labeled peptide at all, and amounts frequently missed the label (source).

How do you read a chromatogram?

If the report includes the trace, check:

  1. Baseline: flat and quiet, or noisy?
  2. Main peak shape: sharp and symmetrical; shoulders can hide co-eluting impurities.
  3. Small peaks: are they integrated and listed, or ignored?
  4. Method details: wavelength, column and gradient. A method that elutes everything at once will flatter purity.
  5. Integration settings: a high threshold can drop small impurity peaks from the total.

Can HPLC measure amount too?

Labs can also use HPLC quantitatively, comparing peak area to a calibration curve from a reference standard to estimate milligrams. That is a different measurement from area-percent purity, and it depends on having a good standard. Janoshik markets “quantitative HPLC purity analysis” for peptides. Ask which measurement a report gives.

Frequently asked questions

What does 99% purity mean on a peptide?

Usually that 99% of the UV-detected peak area belongs to the main peak. Non-UV-absorbing material such as water, counter-ions and salts is not counted, and identity needs a separate check.

Can HPLC tell if a peptide is the wrong compound?

Not reliably on its own. A different peptide can produce a clean single peak. Mass spectrometry, or matching retention time to a reference standard, is needed to confirm identity.

What wavelength is used?

Peptide methods commonly detect at around 210-220 nm, where the peptide bond absorbs. The wavelength should be stated on the report.

Sources

4 linked records
  1. [1] MZ Biolabs: COA Testing (HPLC-UV with mass spectrometry) MZ Biolabs Primary record
  2. [2] Janoshik: peptide testing and quantitative HPLC purity analysis Janoshik Analytical Primary record
  3. [3] ICH Quality Guidelines (Q2 validation of analytical procedures) ICH Primary record
  4. [4] FDA PCAC briefing (peptide-related impurities and aggregation) U.S. FDA Primary record

This explainer is general information about law and laboratory methods, not legal or medical advice. Amino Wire does not provide dosing information. Spotted an error? See our corrections policy.