For in-vitro laboratory research use only. Not for human consumption.
Peptide purity is the single most important attribute of any research-grade compound. A peptide marketed as "99% pure" with no batch-specific documentation is functionally indistinguishable from one labeled "95% pure" or "70% pure" — the number on the label is only as credible as the analytical chemistry behind it.
This guide walks through the four documents that, together, establish purity: the Certificate of Analysis (COA), the HPLC chromatogram, the mass spectrum, and the third-party laboratory attestation. By the end you'll be able to evaluate any vendor's documentation against the same standards used in pharmaceutical-grade peptide chemistry.
Why peptide purity matters in laboratory research
Synthetic peptides are produced by solid-phase peptide synthesis (SPPS). The process is iterative — amino acids are coupled one at a time onto a growing chain — and every coupling step has a yield below 100%. Truncated sequences, deletion sequences, and incompletely deprotected intermediates accumulate as impurities. The final crude product is then purified, typically by reverse-phase HPLC, before being lyophilized and vialed.
The "purity %" on a label reflects how successful that purification was. Common impurity classes include truncated sequences, deletion sequences, oxidation products (particularly on methionine, tryptophan, and cysteine residues), trifluoroacetic acid (TFA) counter-ion residues, acetate or formate counter-ions, and residual solvents such as acetonitrile and water.
For research use, impurity profile matters because impurities can interfere with binding assays, alter apparent receptor activity, and confound experimental results. A 95%-pure peptide with 5% truncated sequence is a fundamentally different experimental input than a 99%-pure peptide.
The Certificate of Analysis (COA): what it must contain
A Certificate of Analysis is the document that summarizes batch-specific analytical testing. The minimum information a credible COA should contain:
- Product name and sequence — the full amino acid sequence in one-letter or three-letter code
- Batch (or lot) number — must match the number printed on the vial
- Manufacture and/or test date
- Molecular formula and molecular weight — theoretical, calculated from sequence
- Appearance — typically "white to off-white lyophilized powder"
- HPLC purity % — with the chromatogram attached or referenced
- Mass spectrometry result — observed mass vs. theoretical mass
- Counter-ion content — TFA, acetate, etc.
- Water content — by Karl Fischer titration
- Testing laboratory — name and location of the lab that performed the analysis
A COA that lists purity as a single number without an attached chromatogram is a summary, not a verification. A COA without a batch number is a catalog document, not a batch-specific test result.
Red flags on a COA
- The batch number does not match the vial
- The COA is identical across multiple production runs (same chromatogram reused)
- The testing laboratory is unnamed or listed as "in-house"
- Only HPLC is reported (no mass spec), or only mass spec (no HPLC)
- The document is a generic image with no analytical data attached
Reading an HPLC chromatogram
High-Performance Liquid Chromatography (HPLC) separates the components of a sample by passing them through a column packed with a stationary phase under high pressure. Different molecules elute (exit the column) at different times depending on their interaction with the stationary phase and mobile phase. The detector records absorbance over time, producing a chromatogram — a plot of detector signal vs. retention time.
For research peptide purity analysis, the standard method is reverse-phase HPLC (RP-HPLC) using a C18 column with a water/acetonitrile gradient containing 0.1% TFA, with UV detection at 214 nm or 220 nm (the peptide-bond absorbance).
What a clean chromatogram looks like
- One dominant peak representing the target peptide
- Baseline noise that is flat and low relative to the main peak
- Small impurity peaks before or after the main peak, each ideally below 1% of total area
- Peak shape — symmetrical, with a sharp leading edge and a sharp trailing edge (no significant tailing)
How purity % is calculated
Purity is calculated as the area of the target peak divided by the total area of all peaks, expressed as a percentage. A 99%-pure peptide shows a single dominant peak occupying essentially the entire chromatogram, with impurity peaks barely distinguishable from baseline. A 95%-pure peptide shows one or two visible impurity peaks summing to approximately 5% of total area.
Common mistakes when reading a chromatogram
- Reading peak height instead of peak area. Tall, narrow peaks can have less area than short, broad peaks. Purity is calculated from area.
- Ignoring the y-axis scale. Impurity peaks that look small may be substantial if the y-axis is heavily zoomed on the main peak.
- Not checking the gradient. A poorly chosen gradient can fail to separate closely related impurities, inflating the apparent purity number.
Mass spectrometry: confirming identity
HPLC tells you how pure the sample is. Mass spectrometry tells you what's in it. The two together are the minimum for peptide characterization.
The most common techniques for research peptide characterization are ESI-MS (electrospray ionization mass spectrometry) and MALDI-TOF (matrix-assisted laser desorption/ionization time-of-flight). Both ionize the peptide and measure the mass-to-charge ratio (m/z) of the resulting ions, allowing the observed molecular mass to be calculated and compared to the theoretical mass derived from the sequence.
Reading a mass spectrum
- Observed mass vs. theoretical mass — should match within ~0.1% (a few Daltons for a typical peptide)
- Charge states — ESI typically produces multiple charge states (e.g., [M+H]⁺, [M+2H]²⁺, [M+3H]³⁺)
- Sodium and potassium adducts — common, expected, not a quality issue (just +22 or +38 Da above the protonated mass)
What a mass spec discrepancy means
- Observed mass exactly 14 Da too low — possible deletion of a single methylene group, suggesting a coupling error
- Observed mass exactly 16 Da too high — oxidation (commonly on methionine)
- Observed mass significantly different — wrong sequence, wrong peptide, or major impurity
If the observed mass on the COA does not match the theoretical mass from the published sequence, the COA fails verification regardless of what the HPLC chromatogram shows.
Third-party testing: what counts and what doesn't
"Third-party tested" is one of the most overused phrases in the research peptide industry. To be meaningful, third-party testing must satisfy three conditions:
- The testing laboratory is named on the COA. Anonymous attestations are not third-party verification.
- The laboratory is independent of the manufacturer. Same parent company, same ownership, same physical address — not third-party.
- Analytical instruments are accessible to the laboratory. HPLC and mass spec results require the equipment that produces them.
A credible COA will list the testing laboratory's name, address, and — often — accreditation. ISO 17025 is the international standard for analytical testing laboratories.
Where to get peptides tested for purity
Researchers who want to verify a vendor's claims independently can submit a sample to a contract analytical laboratory. Several categories of US-based options exist:
- University-affiliated mass spec cores — many research universities operate fee-for-service mass spectrometry facilities
- Independent analytical labs — fee-for-service HPLC and MS, often used for academic and biotech work
- ISO 17025-accredited contract labs — the gold standard, though typically more expensive
A single independent verification run on a sample from a suspect vendor is sometimes worth the cost as a one-time audit.
A worked example: verifying a vial of BPC-157
Suppose a 5 mg vial of BPC-157 arrives with a printed batch number of BPC2026-0517. To verify:
- Locate the vendor's public COA database. A batch-indexed, publicly accessible COA archive is the baseline. If the only option is "email us for a COA," that's catalog-level documentation, not batch-level.
- Pull the COA for batch
BPC2026-0517. The batch number on the document should exactly match the vial. - Check the theoretical molecular weight. BPC-157 (the 15-residue sequence GEPPPGKPADDAGLV) has a theoretical monoisotopic mass of approximately 1419.5 Da. The COA's observed mass should match within a few Daltons.
- Inspect the HPLC chromatogram. Look for one dominant peak, flat baseline, and small impurity peaks. The reported purity should reflect what you see in the chromatogram.
- Confirm the third-party laboratory. The lab name should be present, distinct from the manufacturer, and (ideally) ISO 17025 accredited.
If all five checks pass, the vial is verified to the vendor's published standard. If any one fails, the documentation is incomplete.
Frequently asked questions
What purity is acceptable for research peptides?
The widely accepted minimum for laboratory research is 95%. Most credible US suppliers publish 98%+ as a baseline, with many products routinely exceeding 99%. For binding assays and quantitative work, 99%+ is preferred because impurity contributions to signal become non-negligible below that threshold.
Is 96% purity good for a peptide?
96% is above the 95% research minimum but below the 98–99% standard most reputable suppliers publish. It is acceptable for qualitative or exploratory work; for quantitative receptor or binding assays, the additional 2–3% of impurities can become a confound.
Can I trust a vendor that only publishes purity, with no chromatogram?
No. A purity number without an attached chromatogram is an unverified claim. The chromatogram is the underlying evidence; without it, there is no way to confirm how the purity was calculated, what the impurity profile looks like, or whether the analytical method was appropriate.
How does third-party testing actually ensure peptide purity?
Third-party testing means an independent analytical laboratory — one not owned by or affiliated with the manufacturer — has run HPLC and mass spectrometry on the specific batch shipped, and produced a report identifying the laboratory by name. Independence eliminates the conflict of interest inherent in self-reported purity. The HPLC quantifies the purity percentage; the mass spectrometry confirms the molecule is the one claimed. Together, they verify both quality and identity.
Where can I look up a vendor's COAs?
A credible US research-peptide supplier maintains a public, batch-indexed COA database accessible from the product page or a dedicated COA section. For Excalibur Peptides, every batch is documented in our COA database with the analytical laboratory named on each report.
Where to learn more
For broader vendor-evaluation criteria, see our research peptide supplier comparison and Peptide Sciences alternative guides. For US fulfillment specifics, see the research peptide supplier USA overview.
For research and identification purposes only. Not for human consumption.