Purity, Identity, and Quantity Are Different Measurements

Lab reports for peptides often lead with a single number, such as "99% purity". That figure answers one narrow question. This guide separates three measurements that are often blurred together (identity, purity and quantity) and explains why a report needs all three before it says much about a product.

Identity: is it the right molecule?

Identity testing asks whether the material is the molecule named on the label. For peptides, the usual tool is mass spectrometry, which measures molecular mass and, when the molecule is broken into fragments, can confirm parts of its sequence. In a study of melanotan II vials bought from online shops, researchers confirmed identity from the compound's position in the chromatographic run and the pattern of five fragment ions ^{Source [1]}.

Identity can also reveal that a name means something unexpected. Anti-doping chemists used high-resolution mass spectrometry to show that a product sold as TB-500 contained an acetylated seven-amino-acid fragment of thymosin beta-4 ^{Source [2]}, a different molecule from the full protein its name suggests. Our profile of TB-500 and thymosin beta-4 explains that case.

Identity has to be specific. A peptide's free base and its acetate salt are different ingredients with different molecular weights, and FDA has found supplier listings that apply one form's registry number to the other ^{Source [3]}. A report that gives only a common name, without the sequence, any modifications and the salt form, leaves identity partly open.

Purity: what else is in there?

Purity testing, usually by high-performance liquid chromatography (HPLC), separates the components of a sample and reports how much of the detected signal comes from the main peak. Solid-phase peptide synthesis can leave incompletely built or shortened chains and side-reaction products that are structurally close to the target peptide, and telling them apart and measuring them takes advanced analytical methods ^{Source [4]}. FDA found that publicly available COAs for BPC-157 generally gave a purity figure without identifying impurities or stating limits for them ^{Source [4]}.

A purity result is only as good as the method behind it. In the melanotan II study, the researchers validated their method with reference to guidelines for testing active substances in authorized medicines before measuring impurities, which made up 4.1% to 5.9% of the content of vials from two of the three shops ^{Source [1]}. A report that names the method and shows the chromatogram is easier to interpret than a bare percentage.

Quantity: how much is actually there?

Quantity, or content, asks how much of the peptide is in the container or in a given weight of powder. It is not the same as purity, and the two can point in different directions:

• In the melanotan II study, each shop claimed the same fixed amount per vial, but the measured amounts ranged from roughly 43% to 88% of that claim ^{Source [1]}.
• In a 2024 study of semaglutide vials bought from illegal online pharmacies, the measured semaglutide content was about 29% to 39% above the labeled amount, while measured purity was only 7.7% to 14.4%, against a 99% claim on the labels ^{Source [5]}.
• In a study of growth hormone products bought online, every sample had a significantly lower concentration than labeled ^{Source [6]}.

Part of the gap between purity and content is chemistry. Peptides made by standard solid-phase synthesis are usually isolated as salts. In a 2025 analysis, the trifluoroacetate counterion reached up to 35% of the total weight of peptide salts, and the authors note that counterions can introduce weighing errors and must be accounted for ^{Source [7]}. A powder can therefore show high peptide purity and still contain much less peptide by weight than its label implies.

Sterility and endotoxin are separate again

Microbial tests answer yet another question. FDA treats sterility, bacterial endotoxin and particulate testing as critical safety attributes for injectable products ^{Source [3]}. In the semaglutide study, no viable microorganisms were found, yet endotoxin was detected in every sample ^{Source [5]}. Passing one of these tests does not imply passing the other, and neither is implied by a purity result.

Reading a report with all three in mind

When you look at a lab report, check each measurement separately:

• Identity: the method (usually mass spectrometry), expected and observed mass, and the exact molecule, including modifications and salt form.
• Purity: the method (usually HPLC), the result with an acceptance limit, and ideally the chromatogram and named impurities.
• Quantity: an amount that was measured, per container or as peptide content by weight, not only the amount claimed.
• Batch link: a batch number that matches the product, and a report that lists each test with its limits and numerical results ^{Source [8]}.

For the rest of the document, including who issued it and what accreditation covers, see what a certificate of analysis can and cannot tell you.

Research materials and consumer products belong to different categories. A listing, laboratory report, or research-use label does not establish suitability for personal use. Even a complete set of identity, purity and quantity results describes what is in a sample, not what it does in the body.