--- title: "Purity vs net peptide content: two different numbers" description: "HPLC purity and net peptide content measure different things. How each is determined and why a 99% pure vial does not contain 100% peptide by weight.…" canonical: https://www.sablepeptides.co.uk/learn/purity-vs-net-peptide-content dateModified: 2026-10-10 publisher: Sable Peptides Limited (UK company 17488610) note: Research use only. Not for human or veterinary use. --- # Purity vs net peptide content: two different numbers Last reviewed 2026-10-10 · Sable Peptides · Research use only *Sable research method explainer. Updated 2026-10-10. Research use only.* ## Short answer Purity and net peptide content answer different questions. HPLC purity asks: of the peptide-related material, how much is the target sequence? Net peptide content asks: of the total powder weighed, how much is peptide at all? A vial can be 99% pure by HPLC and still have a net peptide content well below 100%, because lyophilised powder also contains counter-ions and bound water. ## What HPLC purity measures Reversed-phase HPLC separates the target peptide from related impurities such as truncated sequences, deletion sequences and incompletely deprotected chains, which arise during solid-phase synthesis. Purity is usually reported as the area of the main peak as a percentage of all integrated peaks at a set UV wavelength, often 214-220 nm, where the peptide bond absorbs. This is a relative measure. Counter-ions such as trifluoroacetate or acetate, and water, do not show up as peptide peaks, so they do not reduce the purity figure. ## What net peptide content measures Net peptide content (NPC) is the fraction of the gross powder weight that is peptide. The rest is mostly counter-ions, which balance the charge on basic residues, and residual water, because lyophilised peptides are hygroscopic. It can be well below 100%, and how far below depends on the sequence, the number of basic residues, the salt form and the drying conditions. It is batch-specific. The classic method is quantitative amino acid analysis. The peptide is hydrolysed to its amino acids, these are quantified against standards, and the result is converted back to the mass of intact peptide. Isotope-dilution mass spectrometry versions of this approach have been developed for accurate quantification of synthetic peptides (Wang et al., 2012). Elemental nitrogen analysis is another approach. ## A worked illustration Suppose a vial is labelled 10 mg and the powder weighs 10 mg. If, for illustration, the batch has 99% HPLC purity and 80% net peptide content, about 8 mg of the powder is peptide, and about 99% of that peptide-related material is the target sequence. The remaining roughly 2 mg is counter-ions and water. The arithmetic is simple; the point is that the two percentages are not interchangeable. ## Why it matters in the lab Concentration. If you prepare a stock solution assuming the whole powder is peptide, the true peptide concentration will be lower by the NPC factor. Batch comparison. Two batches with the same purity can differ in NPC, so equal powder weights are not always equal peptide amounts. Molar calculations. Convert using the free-peptide molecular weight on the /ref page, and apply NPC if it has been measured. ## Reading a certificate A certificate of analysis usually reports HPLC purity and a mass-spectrometry identity check. It may or may not report net peptide content. If NPC is critical to your work, ask whether it was measured for your batch. Sable releases each batch at ≥99% purity by HPLC with identity checked against the expected mass, and the batch certificate, available for every lot at /coa, states what was measured for that lot. Sable Peptides has been operating since 2019 and supplies every compound mentioned here as laboratory research material only. Nothing on this page is advice on use in people or animals, and no administration or medical-use information is provided. ## Key terms Gross weight The total mass of powder in the vial, including peptide, counter-ions and water. Net peptide content (NPC) The percentage of gross weight that is peptide. HPLC purity The main peak's share of the integrated peptide-related peaks at a stated wavelength. Amino acid analysis (AAA) Hydrolysis of a peptide to its amino acids, followed by quantification, used to measure how much peptide is present. Hygroscopic Tending to absorb moisture from the air, as lyophilised peptides do. ## Common questions ### Does 99% purity mean 99% of the vial is peptide? No. Purity is the share of peptide-related material that is the target sequence. Net peptide content is the share of total powder weight that is peptide. ### How is net peptide content measured? Usually by quantitative amino acid analysis after hydrolysis, or by nitrogen analysis. ### Is net peptide content on every COA? Not always. Ask whether it was measured for your batch. ## Related explainers - [Peptide salt forms: TFA vs acetate counter-ions](https://www.sablepeptides.co.uk/learn/counter-ions-tfa-vs-acetate) - [Monoisotopic vs average mass: reading peptide mass data](https://www.sablepeptides.co.uk/learn/monoisotopic-vs-average-mass) ## Related reference pages - [bpc-157](https://www.sablepeptides.co.uk/ref/bpc-157) - [semax](https://www.sablepeptides.co.uk/ref/semax) - [ss-31](https://www.sablepeptides.co.uk/ref/ss-31) - [tb-500](https://www.sablepeptides.co.uk/ref/tb-500) ## Sources - Wang M, et al. Accurate quantification of synthetic peptides by amino acid-stable isotope dilution mass spectrometry. Chinese Journal of Chromatography. 2012;30(3):239-244. [https://doi.org/10.3724/SP.J.1123.2012.01035](https://doi.org/10.3724/SP.J.1123.2012.01035) - Roux S, et al. Elimination and exchange of trifluoroacetate counter-ion from cationic peptides: a critical evaluation of different approaches. J Pept Sci. 2008;14:354-359. [https://doi.org/10.1002/psc.951](https://doi.org/10.1002/psc.951) - Jones J. Amino Acid and Peptide Synthesis. Oxford Chemistry Primers. Oxford University Press; 2002. [https://doi.org/10.1093/hesc/9780199257386.001.0001](https://doi.org/10.1093/hesc/9780199257386.001.0001) ## Questions ### Does 99% purity mean 99% of the vial is peptide? No. Purity is the share of peptide-related material that is the target sequence. Net peptide content is the share of total powder weight that is peptide. ### How is net peptide content measured? Usually by quantitative amino acid analysis after hydrolysis, or by nitrogen analysis. ### Is net peptide content on every COA? Not always. Ask whether it was measured for your batch. ## Sources - [https://doi.org/10.3724/SP.J.1123.2012.01035](https://doi.org/10.3724/SP.J.1123.2012.01035) - [https://doi.org/10.1002/psc.951](https://doi.org/10.1002/psc.951) - [https://doi.org/10.1093/hesc/9780199257386.001.0001](https://doi.org/10.1093/hesc/9780199257386.001.0001) CoA for every lot — [request yours](/coa-request) and we email it within 2 hours. Research use only. Not for human or veterinary use. No health claims. Reviewed by Sable Peptides editorial team · 14 October 2026 · [Editorial policy](/editorial-policy)