Purity vs Assay: Two Different Numbers on a Certificate of Analysis

Purity and assay are two different measurements that answer two different questions, and a certificate showing only one of them has told you half the story. Purity is a statement about composition: what proportion of the sample is the target peptide. Assay is a statement about quantity: how much target peptide the laboratory actually measured, against the weight printed on the label. A vial can score highly on one and poorly on the other, in either direction. This article explains why, using published figures from the TXLABS certificate library.

For research use only — not for human or veterinary use.

Purity is a ratio

Purity on a peptide Certificate of Analysis is determined by reversed-phase HPLC with area normalisation. The sample is separated on a C18 column under a solvent gradient and monitored by ultraviolet absorbance near 214–220 nm, the wavelength at which the peptide bond absorbs. The integrated area of the target peak is divided by the total integrated area of all detected peaks, and the result is reported as a percentage.

Two consequences follow directly from that definition:

• The denominator only contains things that absorb UV at the monitored wavelength. Water, inorganic salts and acetate or trifluoroacetate counter-ions are effectively invisible. They contribute mass to the vial and nothing to the purity calculation.
• It is scale-free. A ratio does not know how much material it was calculated from. Two milligrams and twenty milligrams of the same material give the same percentage.

Manufacturers state this explicitly. Bachem's quality-control guidance and the peptide-quality references published by peptide manufacturers describe purity and peptide content as independent measurements, with net peptide content — the proportion of the material that is peptide rather than water and counter-ions — determined by quantitative amino acid analysis rather than by chromatographic purity.

Assay is a mass

Assay answers the other question: how much. The laboratory quantifies the target analyte against reference material and reports a mass, which is compared against the vial's nominal label weight. On a certificate it appears as a figure in milligrams, not a percentage.

The 2025 Journal of Peptide Science review of regulatory guidelines for peptide and protein analysis treats identity, purity and impurity profiling, and content determination as separate analytical requirements with separate validation. That separation is not bureaucratic tidiness — it reflects that no one measurement substitutes for another.

Why assay can exceed the label weight

It surprises people that a measured mass often comes in above the printed label. Across the published TXLABS reports it is the common case rather than the exception:

• Selank — batch CS-sel10-0201, tested 5 February 2026: 99.801% purity, assay 13.00 mg against a 10 mg label.
• DSIP — batch CS-dp101115, tested 21 November 2025: 99.405% purity, assay 12.44 mg against 10 mg.
• Melanotan II — batch CS-mn210-0201, tested 5 February 2026: 99.513% purity, assay 12.32 mg against 10 mg.
• PT-141 — batch CS-pt10-0126, tested 5 February 2026: 99.864% purity, assay 11.86 mg against 10 mg.
• BPC-157 — batch CS-bc10-0309, tested 18 March 2026: 99.789% purity, assay 11.21 mg against 10 mg.
• GHK-Cu — batch CS-gu50-0309, tested 18 March 2026: 99.780% purity, assay 60.04 mg against a 50 mg label.
• NAD+ — batch CS-na500-0228, tested 16 March 2026: assay 576.91 mg against a 500 mg label, reported by mass with no purity percentage.

The straightforward explanation is deliberate overfill: a vial filled to guarantee at least the label weight of target analyte will measure above it. Filling tolerance in a lyophilisation run is not infinitely tight, and the sensible direction to err in is up. A measured mass at or slightly above label indicates a vial filled to specification; a mass materially below label does not.

Not every lot runs high. The same library records a retatrutide lot at 99.762% purity assaying 29.97 mg against a 30 mg label — effectively at label weight. Variation between lots is precisely why certificates are batch-specific.

Why a 99% pure vial can still be underfilled

This is the failure mode purity cannot detect, and it follows from purity being a ratio. Consider a vial labelled 10 mg that in fact contains 6 mg of peptide, of which 99.8% is the target sequence.

• Its purity is 99.8%. Genuinely, correctly, 99.8%.
• Its assay is 6 mg against a 10 mg label — a 40% shortfall.

Nothing about the chromatogram reveals the shortfall, because the missing 4 mg is simply absent from the vial rather than present as an impurity. A supplier publishing purity alone can be telling the exact truth and still be shipping a vial containing well under half of what a researcher would infer from the label.

The same logic explains why residual moisture and counter-ion content matter. A hygroscopic lyophilised cake that has taken up water weighs more than the peptide it contains. Weight in the vial is not analyte in the vial, and only an assay distinguishes them.

Why both figures matter to the work

• Purity governs what else is present. The remainder of the sample is typically synthesis-related impurities, residual solvent or counter-ion. In an assay system, the identity of that remainder can matter.
• Assay governs whether the concentration arithmetic is right. Concentration is mass divided by diluent volume. If the calculation starts from a label weight that the vial does not contain, every derived concentration is wrong by the same proportion, and every result derived from those concentrations inherits the error. The reconstitution calculator does the arithmetic accurately from whatever mass is entered — the certificate is what tells you which mass to enter.
• Identity underwrites both. Purity and assay both describe a peak. Mass spectrometry is what establishes that the peak is the molecule named.

Blends and cofactors report differently

Where a vial contains two peptides, a single combined purity figure is not a meaningful quantity, and the laboratory quantifies each component separately. Those reports show assay masses only — the BPC-157 + TB-500 lot at 5.80 mg and 5.54 mg, and the CJC-1295 (mod GRF 1-29) + ipamorelin lot at 6.61 mg and 6.04 mg. NAD+ is likewise reported by mass. Marking these Assay only is more accurate than inventing a percentage.

What to look for on a certificate

1. A purity percentage with the method named.
2. An assay mass and the label weight it is compared against.
3. An analyte name matching the exact variant on the product page.
4. A batch number and test date.
5. The laboratory's name.

A certificate showing a purity figure and nothing else is not fraudulent — it is incomplete. The batch-specific reports behind every figure quoted above are published in the Certificate of Analysis library, and what to check before ordering from any supplier is covered on the buying research peptides in Australia page.

For research use only. Products supplied by TXLABS are laboratory research chemicals intended for in-vitro and laboratory research by qualified professionals. They are not medicines, supplements, foods or cosmetics, and are not for human or veterinary use.