11 min read · updated 2026-08-01
How to Read a Peptide Certificate of Analysis
What each assay measures, which figures matter, and how to spot a certificate that is not what it appears to be.
Purity is not the same as peptide content
This is the single most misread pair of numbers on any peptide certificate. Purity by HPLC describes what fraction of the peptide material present is the target sequence rather than truncated or deletion sequences. Peptide content describes what fraction of the total net weight in the vial is peptide at all — the remainder being counterion (usually acetate or TFA), residual water, and salts. A vial can be 99% pure and 78% peptide content simultaneously, and both figures are accurate. If you are calculating molar concentration, peptide content is the number you need. Certificates that report purity but omit peptide content are leaving out the figure that matters for quantitative work.
What each assay line actually measures
RP-HPLC with UV detection at 214 nm measures the peptide bond directly and is the standard purity method; detection at 280 nm relies on aromatic residues and will misreport sequences lacking tryptophan or tyrosine. LC-MS confirms identity by measuring molecular mass against theoretical — it answers "is this the right molecule" but says nothing about how much of it there is. Karl Fischer titration measures water content, which matters because water is included in net weight. The LAL assay measures bacterial endotoxin, reported in endotoxin units per milligram. Gas chromatography detects residual synthesis solvents. Each answers a different question, and a certificate reporting only one of them is answering only one question.
Why the batch number has to match
A certificate is a record of measurements performed on a specific batch of material. A certificate from a different batch tells you what a different batch measured, which is not information about the vial in your hand. The lot number printed on the label must match the lot number on the certificate. If a supplier provides a certificate whose lot number does not appear on the product, or provides the same certificate for material purchased months apart, the document is decorative rather than evidential.
Reading the chromatogram, not just the number
A purity figure is a computed area percentage, and area percentage can be manipulated by integration choices. The chromatogram itself shows things the number does not: whether the main peak is symmetrical or tailing, whether impurity peaks are resolved or buried in the baseline, whether the baseline is stable, and whether the run was long enough for late-eluting species to appear. A certificate that reports a purity figure without an attached chromatogram is asking you to accept an integration you cannot inspect.
Specific red flags
Identical purity figures across multiple different compounds suggest values are being written rather than measured — real analytical results vary. A missing or unnamed testing laboratory removes the possibility of verification. Certificates dated before the manufacture date, or with no date at all, cannot be matched to any batch. Purity reported to more decimal places than the method supports indicates false precision. Certificates supplied only as low-resolution images, where the lot number and figures cannot be read clearly, are common in this category and should be treated as absent rather than present.
Independent testing means the lab has no stake in the result
In-house quality control is a legitimate and necessary part of manufacturing, but it is not independent verification. The distinction that matters is whether the laboratory performing the analysis has a commercial interest in the outcome. A certificate should name the testing laboratory. If the manufacturer and the tester are the same entity, the certificate is a self-report — which may still be accurate, but is a different category of evidence, and should be described accurately rather than presented as third-party testing.
Frequently asked
What purity should research peptides meet?
Most published research uses material at 95% or higher by HPLC, with 98%+ typical for quantitative work. The appropriate threshold depends on the assay — work sensitive to truncated sequences requires higher purity than qualitative screening.
Is a COA the same as third-party testing?
No. A certificate documents that testing was performed; it does not by itself establish who performed it. Third-party testing specifically means an independent laboratory with no commercial interest in the result. The certificate should name the laboratory.
Why do peptide vials weigh more than the stated peptide mass?
Net vial weight includes counterion — typically acetate or trifluoroacetate — plus residual water and salts. Peptide content, reported as a percentage, is what allows you to convert net weight into actual peptide mass.
What endotoxin level is appropriate for cell culture work?
Endotoxin is reported in EU/mg. Cell culture applications are generally sensitive to endotoxin, and the acceptable level depends on the cell type and assay. The relevant point is that the figure should be measured and reported rather than omitted.
See the practice, not just the principle
Every lot we release is published in full — assay, method, specification, and measured result — including lots that have sold out.
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