Peptide testing, explained.
Purity, content, identity, endotoxin, sterility — the laboratory methods behind a trustworthy Certificate of Analysis, in plain language. Start with the COA basics, then go method by method.
Start here.
Batch-specific COAs: why they matter.
A batch-specific COA reports the actual laboratory results for one production lot — the same lot number printed on the vial you received. It is distinct from a "typical" or example COA, which describes what a product usually looks like. Real batches vary; only batch-specific documents let you verify what you actually have.
Read the guideHow to read a peptide COA.
A Certificate of Analysis (COA) is a lab report for a specific production batch of peptide. A trustworthy COA names the batch, dates the test, states the methods used, and reports purity, content, and identity results. If a COA has no batch number, no date, or no method names, treat it as evidence of nothing.
Read the guideWhat peptide purity really means.
Purity is the share of the peptide-containing material in your sample that is the correct peptide, usually measured as percent peak area on an HPLC chromatogram. It is not the share of the vial's mass that is peptide — water, salts, and counterions sit outside the purity number. That's why 99% purity and peptide content are different figures.
Read the guidePeptide content: the number purity hides.
Peptide content is the percentage of the vial's total mass that is actually peptide — the rest being residual water, counterions (TFA, acetate), and salts. A peptide can be 99% pure and only ~70% content. Content is the number that makes reconstitution math accurate; purity alone can mislead it.
Read the guideTesting methods.
Endotoxin testing: the LAL assay.
Bacterial endotoxins are heat-stable fragments of Gram-negative bacterial cell walls. Unlike live bacteria, they survive sterilization and can cause fever and inflammatory reactions at tiny doses. The LAL (limulus amebocyte lysate) assay detects them, and results are reported in endotoxin units per milliliter (EU/mL) on the COA.
Read the guideSterility testing: USP <71> in plain language.
Sterility testing checks whether a product contains any live, replicating microorganisms. The standard method (USP <71>) filters the sample, transfers the filter to growth media, and incubates it for about 14 days — if nothing grows, the batch passes. It answers a different question than endotoxin testing, which detects dead bacterial debris that sterilization cannot remove.
Read the guideHeavy metals testing: lead, arsenic, cadmium, mercury.
Heavy metals testing measures toxic elemental impurities — lead, arsenic, cadmium, and mercury above all — that can enter peptides through reagents, catalysts, and equipment. The standard method is ICP-MS (inductively coupled plasma mass spectrometry), and limits follow the USP <232>/<233> elemental impurities framework. Results should appear on the COA for serious research material.
Read the guideWhy peptide labs screen for fentanyl.
Some peptide testing laboratories now screen products for fentanyl and related opioids. The reason is supply-chain risk: in unregulated markets, cross-contamination, substitution, and adulteration have been documented in adjacent product categories, and fentanyl is active at doses too small to see. LC-MS/MS screening detects it; a negative result on your batch's COA is the assurance that matters.
Read the guideHPLC, explained without the chemistry degree.
HPLC (high-performance liquid chromatography) separates a peptide sample into its components and measures how much of the mixture is the target peptide. The output is a chromatogram; purity is read as the percentage of total peak area belonging to the main peak. It is the standard purity method reported on peptide COAs.
Read the guideLC-MS: what mass spectrometry confirms.
LC-MS combines liquid chromatography separation with mass spectrometry detection: the sample is separated as in HPLC, then the molecules are weighed. For peptide testing its core job is identity — the measured molecular weight should match the theoretical mass of the labeled sequence within a tight tolerance. It confirms what the peptide is, not just how pure it is.
Read the guideIdentity testing: proving what the peptide is.
Identity testing confirms that the substance in the vial is the peptide named on the label. The main tools are molecular weight by mass spectrometry (LC-MS), sequence confirmation by MS/MS or amino acid analysis, and chromatographic retention time against a reference standard. Identity is separate from purity: a sample can be 99% pure and still be the wrong peptide.
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