How Peptide Purity Is Tested - HPLC Mass Spec and COAs Explained

October 7, 2026

When you buy a research peptide, you are trusting that the vial contains what the label says it does, at the purity claimed, without dangerous contaminants. The only way to verify those claims is through analytical testing, and the only way to evaluate a vendor is by understanding what those tests measure. This article explains the major testing methods used in peptide quality control: what they detect, how they work, and what to look for on a Certificate of Analysis.

HPLC: Measuring Purity

High-Performance Liquid Chromatography (HPLC) is the standard method for determining peptide purity. The technique separates a sample into its individual components based on how they interact with a stationary phase inside a column. Each component exits the column at a different time (called its retention time), and a detector, usually ultraviolet (UV), measures how much of each component is present.

For peptides, reversed-phase HPLC (RP-HPLC) is the most common variant. The peptide sample is dissolved and pushed through a C18 column using a water-acetonitrile gradient. The target peptide produces a main peak on the chromatogram, and impurities appear as smaller peaks. Purity is reported as an "area percent" value: the area under the main peak divided by the total area of all detected peaks.

A research-grade peptide should show purity of 95% or higher. Pharmaceutical-grade standards typically require 98% or above. When a vendor claims "99% purity," this number should appear on a COA alongside a chromatogram that shows a dominant single peak with minimal satellite peaks.

Mass Spectrometry: Confirming Identity

HPLC tells you how pure a sample is. Mass spectrometry (MS) tells you what the sample actually is. The technique ionizes the peptide molecules and measures their mass-to-charge ratio. The resulting spectrum shows whether the molecular weight of the sample matches the expected molecular weight of the target peptide.

For BPC-157, the expected molecular weight is 1419.5 Da (free base). For TB-500, the full-length thymosin beta-4 fragment used commercially weighs approximately 4963 Da. If the mass spectrum shows a different molecular weight, the sample is not what it claims to be, regardless of what the HPLC purity reads.

Liquid chromatography coupled with mass spectrometry (LC-MS) combines both techniques in a single run, providing simultaneous purity and identity data. This is the gold standard for peptide verification.

What a Certificate of Analysis Contains

A COA is a document issued by a testing laboratory that reports the results of quality control testing on a specific batch. For peptides, we expect a COA to include the following elements:

A COA without a chromatogram is incomplete. A COA without mass spectrometry data leaves identity unconfirmed. A COA without a batch number cannot be traced to a specific production run. Any of these omissions should raise questions about vendor transparency.

How to Read Purity Percentages

Purity percentages on COAs are not all comparable. A vendor reporting "99% purity" might be measuring peptide content (how much of the powder is actually peptide versus salt, water, and counterions) or chromatographic purity (how much of the peptide fraction is the target sequence versus related impurities). These are different measurements.

Peptide content is typically 60 to 80% by weight for lyophilized peptides, because the powder includes TFA counterions and absorbed moisture. Chromatographic purity of 99% means that 99% of the peptide material in the sample is the correct sequence. Both numbers matter, but they answer different questions. A reputable COA specifies which measurement is being reported.

Endotoxin Testing: Why It Matters

Endotoxins are fragments of gram-negative bacterial cell walls. They are heat-stable, meaning standard sterilization does not destroy them. A peptide can pass HPLC purity testing at 99% and still contain endotoxin levels that would cause fever, inflammation, or in severe cases, septic shock if injected.

The standard test for endotoxins is the Limulus Amebocyte Lysate (LAL) assay, which uses proteins from horseshoe crab blood cells that clot in the presence of endotoxins. USP chapter 85 establishes the compendial requirements for this test. The USP standard for injectable products sets the limit at 5 EU per kilogram of body weight per dose. Quality manufacturers consistently achieve levels below 0.1 EU/mL.

A newer alternative, the recombinant Factor C (rFC) assay, uses synthetic reagents instead of horseshoe crab blood and achieves comparable sensitivity. USP chapter 86 provides the framework for this non-animal-derived method.

Which Vendors Publish COAs

Among the vendors we have reviewed for this site, transparency varies significantly. Real Peptides publishes batch-specific COAs on their website that include HPLC chromatograms, mass spectrometry data, and endotoxin test results. We verified that COAs were accessible for their BPC-157 and TB-500 products.

PeptideGod does not publish COAs on their website. This does not necessarily mean their products are untested, but it does mean buyers cannot independently verify purity, identity, or endotoxin levels before purchase. We consider published COAs a baseline requirement for vendor credibility. For a detailed comparison, see our vendor comparison page.

Third-Party vs. In-House Testing

Some vendors conduct testing in their own laboratories. Others send samples to independent third-party labs. Third-party testing is more credible because the testing facility has no financial interest in the results. Look for COAs that name the testing laboratory. If a COA lists results without identifying who performed the testing, the data is less verifiable.

The most trustworthy arrangement is when a vendor uses a third-party lab, names that lab on the COA, provides a batch number, and includes raw data (chromatograms, spectra) rather than just summary numbers. This level of transparency is common in pharmaceutical manufacturing and uncommon in the research peptide market, which is itself a signal about the maturity of this industry.