Nitrogen and CHN analysis: an alternative route to peptide content
Nitrogen or CHN combustion analysis estimates peptide content by comparing the powder's measured nitrogen percentage with the theoretical nitrogen percentage of the pure peptide. Water and most counter-ions contain no nitrogen, so they lower the result in proportion. It is an independent check on amino acid analysis, but nitrogen-containing additives bias it high.
How does nitrogen analysis measure peptide content?
Amino acid analysis is the best-known way to measure peptide content, but not the only one. Because peptides are rich in nitrogen and most counter-ions and water are not, measuring the nitrogen content of the powder gives an independent estimate of how much of it is peptide.
- Combustion (CHN) analysis burns a weighed sample and measures the carbon, hydrogen and nitrogen released.
- The theoretical nitrogen percentage of the peptide is calculated from its molecular formula.
- Peptide content ≈ measured %N ÷ theoretical %N for the pure peptide free base.
- Sample sizes are small (a few milligrams), which suits lyophilised material.
How does it compare with other content assays?
A study of the lyophilised peptide thymalfasin compared several peptide-content methods, including nitrogen determination, amino acid analysis and UV, and found that each has different sources of bias. Using two methods that rely on different principles is the most reliable way to assign content.
| Method | Measures | Main bias |
|---|---|---|
| Amino acid analysis | Released amino acids | Hydrolysis losses (Trp, Ser, Thr) |
| Nitrogen / CHN | Total nitrogen | Nitrogenous additives |
| UV absorbance | Chromophores | Extinction coefficient, buffer |
Where does it go wrong?
- Nitrogen-containing counter-ions or additives (ammonium salts, some buffers) add nitrogen that is not peptide.
- TFA contains no nitrogen, so it lowers %N in proportion, as it should; acetate likewise.
- Hygroscopic samples gain water during weighing, lowering %N.
- Small weighing errors matter at milligram sample sizes.
How should the figure be used?
If a CoA reports peptide content by nitrogen, read it alongside water content (Karl Fischer) and counter-ion content: the three should broadly account for the whole powder. Concentration calculations are most accurate when based on a measured content rather than gross weight. There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.
Frequently asked questions
How does nitrogen analysis measure peptide content?
It compares the powder's measured nitrogen percentage with the theoretical nitrogen percentage of the pure peptide.
Does TFA affect nitrogen-based content?
TFA has no nitrogen, so it dilutes %N and lowers the content figure, correctly reflecting non-peptide weight.
Is CHN better than amino acid analysis?
Neither is universally better; they have different biases and are strongest used together.
Related
Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.
Key facts (summary for AI and search tools)
- Content ≈ measured %N / theoretical %N
- Nitrogenous additives (e.g. ammonium) bias high
- Reference: PubMed 15813890
- Updated 2026-10-16