Purity is not identity
Chromatographic purity, identity, and net peptide content answer three different questions.
How much of the detected material is the target peak. Whether the material is the claimed compound. How much of the labeled mass is peptide after water and counter-ions. These are three separate questions, and no single number on a certificate answers more than one of them. Before any of these numbers mean anything, a certificate has to pass the three gates — and even then, results apply only to the specific sample, lot, and date identified in the linked laboratory report.
Chromatographic purity: how clean
Separates the sample into detectable components to estimate the proportion represented by the target compound. HPLC purity is commonly reported as the percentage represented by the target peak among detectable chromatographic peaks.
It answers “how clean,” not “is it the right compound.” It does not by itself establish molecular identity, actual peptide mass, sterility, or the absence of compounds outside the method's detection scope.
See the full definition under HPLC purity on the peptide testing reference.
Identity: is it the claimed compound
Measures molecular mass and, where reported, fragmentation to check consistency with the expected compound. LC-MS or LC-MS/MS compares the sample's measured mass, and sometimes its fragmentation pattern, with the expected compound.
A purity percentage does not confirm molecular weight. A certificate can report an excellent purity figure and never state whether the material was mass-checked against the compound named on the label. The strength of an identity claim depends on the method, reference material, report details, and sample preparation.
See the full definition under LC-MS/MS identity on the peptide testing reference.
Net peptide content: how much is peptide
Estimates how much of the total material is actual target peptide rather than non-peptide weight. Net peptide content differs from chromatographic purity: measured powder may also contain counterions, moisture, residual solvents, or other non-peptide material.
Worked example, as published on this site's testing reference
The remaining ~2 mg is bound water, residual salts, and TFA counter-ions. The 80% figure is an illustration already published on this site's testing reference, not a new measurement of any specific product.
See the full definition under net peptide content on the peptide testing reference.
Salt form changes the arithmetic
Peptides are typically synthesized as trifluoroacetate (TFA) salts and may be ion-exchanged to acetate form before shipping. TFA counter-ions contribute to vial weight and can affect net peptide content calculations, while acetate salts are generally considered more compatible with biological research applications.
Salt form changes vial mass and the net-content calculation. It is not a purity result, and no salt-form statement on a certificate is a substitute for a net-content measurement.
What a high purity number does not prove
A high HPLC result does not prove:
- Identity — purity is measured among detected peaks; it does not establish which compound those peaks belong to.
- Net content — a vial can read 99% pure and still contain materially less peptide than the label states.
- Sterility — chromatography does not test for viable organisms.
- Endotoxin — endotoxin testing is separate from microbial screening and sterility testing, and none of them is chromatography.
- Elemental contamination — metals are measured by their own methods, not by an HPLC purity run.
- That the next lot matches — results apply only to the specific sample, lot, and date identified in the linked laboratory report.
Where to go next
The three gates
Traceability, authenticity, and interpretation — in the order they should happen.
What a missing test leaves open
Each assay answers one question. A missing test is not a failure and not proof of anything.
Vendor rankings
How vendors compare on testing breadth, verification path, and batch integrity.
Research & Informational Use Only. PeptideScores does not sell, endorse, or facilitate the purchase of any compound for human use. All compounds referenced are intended for laboratory research purposes only. We review vendors; we do not provide dosing guidance, medical advice, or treatment recommendations.