HPLC purity vs net peptide content
Two vials can both say “99% pure” and contain very different amounts of peptide. That’s because purity and peptide content measure different things. If you’re preparing solutions to a precise concentration, the difference matters.
HPLC purity: how much of the peptide is the right peptide
HPLC purity compares the main peak with all other peptide-related peaks detected at around 214 nm. It answers one question: of the peptide material in this sample, what share is the target sequence rather than truncated, deleted or modified versions?
It does not account for anything that doesn’t absorb at that wavelength, such as water and most counter-ions.
Net peptide content: how much of the powder is peptide at all
A lyophilized peptide is never 100% peptide by mass. It also contains:
- Counter-ions, such as acetate or trifluoroacetate, bound to charged residues. Peptides with many lysines or arginines carry more.
- Residual water, since lyophilized powders absorb moisture.
Net peptide content, measured by amino acid analysis or elemental nitrogen analysis, typically falls between 60% and 90% of the powder’s weight. A 10 mg fill of a peptide with 80% net content contains about 8 mg of peptide.
Putting them together
| Measure | Question it answers | Typical method |
|---|---|---|
| HPLC purity | Is the peptide the right sequence? | Reverse-phase HPLC |
| Identity | Is it the right molecule? | Mass spectrometry |
| Net peptide content | How much peptide is in the powder? | Amino acid or nitrogen analysis |
What this means for your calculations
For most screening work, calculating concentration from the gross fill weight is fine, as long as you’re consistent. For quantitative work, such as dose-response curves in cell assays, correct for net peptide content or determine concentration directly, for example by UV absorbance for peptides containing tryptophan or tyrosine.
Need net peptide content for a specific batch? Ask us and we’ll tell you whether it’s available.