Does 99% Purity Mean 99% Peptide?
Purity and content are two different measurements, and only one of them is usually printed. HPLC purity describes the proportion of UV-absorbing material that is the target peptide. Net peptide content describes what fraction of the powder's mass is peptide at all. A vial can be 99% pure by the first measure and materially less than that by the second, without either number being wrong.
Reverse-phase HPLC separates a sample into peaks and integrates the area under each one. Purity is reported as the area of the main peak divided by the total area of all detected peaks. The phrase that carries the weight in that sentence is 'all detected peaks' — the calculation is a ratio between things the detector responded to, not an inventory of everything in the vial.
Peptide HPLC detects at roughly 214–220 nm, where the peptide bond itself absorbs. Anything that does not absorb in that window contributes no peak, no area, and therefore nothing to the denominator. It is not counted as an impurity because it is not counted at all.
What the detector cannot see
| Component | Origin | Seen by HPLC at 214–220 nm |
|---|---|---|
| Target peptide | Synthesis | Yes — the main peak |
| Peptide-related impurities | Deletion, truncation, oxidation | Yes — the other peaks |
| Trifluoroacetate counterion | Cleavage and mobile phase | No |
| Residual water | Incomplete drying, absorption | No |
| Residual salts | Purification buffers | No |
| Bulking agent, where used | Formulation | Largely no |
Trifluoroacetic acid is the significant one, because it is not a contaminant so much as a consequence. TFA is used in the cleavage step and in the mobile phase, and peptides come off that process as TFA salts. Each basic site on the molecule can carry a counterion, so a peptide with a net positive charge of three can retain a reported 10–25% of its total mass as trifluoroacetate — while contributing nothing to the chromatogram, because TFA does not absorb in the detection window.
The two questions, stated plainly
| HPLC purity | Net peptide content | |
|---|---|---|
| Question answered | Of the peptide-like material present, how much is the target? | Of the total mass in the vial, how much is peptide? |
| Expressed as | Peak area ratio, % | Mass fraction, % |
| Typical method | Reverse-phase HPLC, UV 214–220 nm | Amino acid analysis, elemental analysis or quantitative UV |
| Counts water and salt | No | Yes |
| Derivable from the other | No | No |
Published figures for the gap vary with sequence, charge and how the material was finished, but suppliers and synthesis literature commonly describe material at 99% HPLC purity sitting somewhere in the region of 70–85% net peptide content. The range is wide because the drivers are wide: a highly basic sequence carries more counterion, and a hygroscopic powder carries more water.
Why the number that gets printed is the purity one
HPLC is the cheaper, faster and more routine assay, and it is the one that produces a chromatogram — a picture that can be shown. Net content requires a separate quantitative determination, and it produces a single number with no visual. Where only one figure is published across a category, it is almost always the purity figure, and it is usually published without the qualifier that would make it interpretable.
That is a documentation gap rather than a deception. Both numbers are real. The failure mode is reading the first as though it were the second, so that a peak-area ratio is taken for a statement about the mass in the vial.
Where this leaves Aurum Bio's own figure
The purity figures published on this catalogue are HPLC purity figures, determined by reverse-phase HPLC, and they carry the same limits described above. They describe the proportion of peptide-related material that is the target compound. They are not statements about net peptide content by mass, and nothing on this site should be read as claiming otherwise.
Stating that plainly costs a marketing number and buys an accurate one. A figure that travels with its method is a record; the same figure with the method stripped off is an adjective.
What neither figure covers
Purity and content are both chemistry. Neither describes the microbiological state of the material, and neither is a substitute for the separate assays that do. Those are different disciplines with different methods, and a chromatogram says nothing about them in either direction.
References
- 01Iris Biotech Net content and purity, two key parameters in peptide synthesis. Technical note.
- 02Roux S, et al. Elimination and exchange of trifluoroacetate counter-ion from cationic peptides. Journal of Peptide Science, 2008.
- 03Sigma-Aldrich Handling and Storage Guidelines for Peptides and Proteins. Technical document.
Every citation links out to the paper on PubMed. Identifiers are omitted deliberately rather than reproduced from memory, so where we do not hold a verified PMID or DOI the link is a PubMed search for that exact title — it resolves to the paper without anything being invented.
FOR RESEARCH USE ONLY · NOT INTENDED FOR HUMAN CONSUMPTION. This article describes compounds and the research literature in which they appear. Nothing here is a recommendation, protocol, or statement of effect.