Can NMR check a peptide's identity and purity? qNMR explained
NMR spectroscopy can confirm a peptide's structure and detect substances that HPLC-UV and MS miss, such as non-absorbing additives or unexpected counter-ions. Quantitative NMR (qNMR) measures purity or content by comparing signal areas with a weighed internal standard, without needing a reference sample of the peptide itself.
What can NMR see that HPLC and MS miss?
HPLC-UV only sees UV-absorbing species and MS only sees ions. 1H NMR sees almost everything containing hydrogen, so it can reveal residual solvents, additives and unexpected components. A 2020 study used NMR to find an undeclared constituent in custom synthetic peptides that the supplied analyses had not reported.
How does qNMR measure purity?
Signal area in 1H NMR is directly proportional to the number of protons, regardless of structure. Weigh the sample and a certified internal standard (for example maleic acid), dissolve together, and compare a well-resolved peptide signal with the standard's signal; molar ratio and weights give purity or content. qNMR is used to assign purity to reference standards.
Source: Quantitative NMR for purity determination of standard ACE inhibitors, J Pharm Biomed Anal 2015 (PubMed 26070161); Reference standards to support quality of synthetic peptide therapeutics, Pharm Res 2023 (PubMed 36949371)
How does NMR compare with HPLC and MS?
| Technique | Identity | Purity | Sees non-UV species? |
|---|---|---|---|
| HPLC-UV | No (retention only) | Yes, area % | No |
| MS | Yes (mass) | No | Only if ionised |
| 1H NMR | Yes (structure) | qNMR content/purity | Yes, most H-containing |
What are the limits?
- Peptide spectra overlap heavily as length grows; well-resolved signals become scarce.
- Sensitivity is lower than MS; milligram amounts are needed.
- Exchangeable NH protons depend on solvent.
- NMR is rarely on research-grade CoAs.
How does Sable document lots?
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
What is qNMR?
Quantitative NMR: measuring purity or content from the ratio of sample and internal-standard signal areas.
Why can NMR find impurities HPLC misses?
It detects most hydrogen-containing substances, not just those absorbing UV.
Does qNMR need a reference standard of the peptide?
No; it uses a certified internal standard of a different compound.
Related
- Reference materials and purity assignment
- Area % vs weight % vs assay
- Residual solvents
- Orthogonal identity testing
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)
- qNMR: signal area ∝ proton count; internal standard
- NMR detects non-UV-absorbing constituents
- References: PubMed 31671336, 26070161, 36949371
- Updated 2026-10-16