Guides / Purity vs net peptide content
Purity is not content: why a 99 percent peptide is 78 percent peptide
What is the difference between HPLC purity and net peptide content?
HPLC purity is the target peak area divided by total peak area among species the detector can see at 214 nm. Net peptide content is the fraction of total sample mass that is peptide. A vial can be 99 percent pure and 78 percent peptide simultaneously: both are true. The 21 point gap is trifluoroacetate counter-ion at 10 to 25 percent, water at 5 to 15 percent, and residual salts, none of which absorb at 214 nm.
Purity is a ratio inside the peptide-related material. The chromatograph separates what dissolves and elutes, the detector responds to peptide bonds at 214 nm, and the software divides the target peak area by the total. Anything that does not absorb at that wavelength or does not retain on the column is not in the numerator or the denominator; it is simply absent from the calculation.
Content is a ratio against everything in the vial. Combustion elemental analysis burns the sample and quantifies nitrogen and sulphur, from which peptide mass is calculated because nitrogen content is fixed by sequence. Nothing is excluded because nothing has to dissolve or elute.
| HPLC purity | Net peptide content | |
|---|---|---|
| Question | How much of the peptide material is the target | How much of the vial is peptide |
| Technique | RP-HPLC-UV at 214 nm | Combustion analysis or AAA |
| Denominator | Total detected peak area | Total sample mass |
| Sees counter-ion | No | Excluded from the peptide figure |
| Sees water | No | Excluded from the peptide figure |
| Typical result | 98 to 99.5 percent | 70 to 85 percent |
Trifluoroacetate is the largest term. Reversed-phase purification runs in 0.1 percent trifluoroacetic acid, which pairs with lysine, arginine and histidine side chains and the N-terminus. Each trifluoroacetate weighs 114 daltons, so a peptide with three basic sites carries roughly 342 daltons of counter-ion. Against a 1400 dalton peptide that is close to a fifth of the delivered mass before anything else is counted.
Water is next, at 5 to 15 percent. A lyophilised cake is porous and hygroscopic by design, which is what makes it dissolve quickly and also what makes it take up atmospheric moisture within minutes of exposure.
Residual salts from buffer exchange and any bulking agent complete the balance. A theoretical estimate of the counter-ion term is molecular weight divided by molecular weight plus n times 114, where n is the number of basic sites, but that is a sanity check on a measurement rather than a substitute for one.
A vial labelled 10 mg at 78 percent net peptide content delivers 7.8 mg of peptide. Any calculation made from the label is out by more than a fifth before pipetting error, adsorption to plastic or degradation are considered.
For comparative work the effect is worse than a simple offset, because counter-ion load varies with sequence and with how the material was processed. Two lots of the same peptide from different suppliers can differ by ten points of content while both report 99 percent purity, and an experiment comparing them is partly measuring the purification process.
Trifluoroacetate is also not inert. It has documented effects on cell viability and proliferation at the concentrations reached when a trifluoroacetate salt is dissolved for culture work, so the counter-ion is a variable in the experiment rather than merely dead weight.
Purity as area percent with the chromatogram attached, because a number alone cannot show whether the gradient resolved anything. Net peptide content as a percentage and as milligrams per labelled unit. Water content by Karl Fischer. Counter-ion identity and load.
Those four together close the mass balance and should sum to approximately 100 percent. If they do not, something is unaccounted for and that is worth knowing. A certificate carrying only the first of the four has answered the least useful of the four questions.
Can a peptide be 99 percent pure and 75 percent peptide?
Yes, and it commonly is. Purity is a ratio within the material detected at 214 nm; counter-ion, water and salts are invisible at that wavelength while still occupying mass in the vial.
How do I calculate the actual peptide dose?
Multiply the labelled mass by the net peptide content. A 10 mg vial at 78 percent contains 7.8 mg of peptide.
Why does trifluoroacetate end up in the product?
It is the ion-pairing agent used in reversed-phase purification. It binds to basic residues and lyophilisation removes the solvent but not the salt.
Is net peptide content the same as assay?
Not quite. Assay usually means content of active substance determined chromatographically against a standard. Net peptide content is determined by elemental analysis and is independent of solubility, which is why the two can differ on poorly soluble material.
References
- ICH Q6A, Specifications for New Drug Substances and Products
- ICH Q3A(R2), Impurities in New Drug Substances
Sources
- Towards a Consensus for the Analysis and Exchange of TFA as a Counterion in Synthetic Peptides. Pharmaceuticals, 2025. PMID 40872554
- Amino acid analysis of peptides using isobaric-tagged isotope dilution LC-MS/MS. Analytical Chemistry, 2009. PMID 19364092
- Impurity profiling of synthetic cyclic peptides based on orthogonality between separation modes. Journal of Chromatography A, 2025. PMID 39922152
- ICH Q6A, Specifications for New Drug Substances and Products
Updated 2026-08-24