qNMR and the Difference Between a Ratio and a Count
A purity percentage has no denominator you can inspect.
That sentence is the whole of the argument, and it is worth sitting with before any instrumentation is discussed. When a certificate reports 98.62% purity by HPLC-UV, the claim is that of the ultraviolet-absorbing material that eluted from the column and was integrated, 98.62% of the peak area belonged to the main peak. Everything in that statement is conditional on the denominator: material that did not absorb at the detection wavelength is not in it, material that did not elute is not in it, and material whose molar absorptivity differs from the main peak’s is in it at the wrong weight.
Quantitative nuclear magnetic resonance answers a structurally different question. It does not compute a ratio of peak areas within one detector’s field of view. It counts nuclei. And because a molecule contains a known number of nuclei in each chemical environment, counting nuclei converts — through molar mass and sample mass — into an absolute quantity of substance.
Ratio and count are not two accuracies of the same measurement. They are two different measurements, and only one of them can answer the question a laboratory usually wants answered: how much peptide is in this vial.
All material discussed here is supplied for research use only and is not intended for human use.
Why the integral is a count
The physical basis is unusually clean. Under fully relaxed acquisition conditions, the integrated area of a proton NMR signal is directly proportional to the number of protons giving rise to that resonance — no response factor, no extinction coefficient, no calibration curve for the specific analyte. A recent review of quantitative proton NMR in pharmaceutical analysis states the practical requirements around that proportionality: an interpulse delay of at least five to seven times the longest longitudinal relaxation time, and a signal-to-noise ratio of at least 150 on the quantitative peak to reach a relative uncertainty of 1% (Tang and colleagues, quantitative proton NMR methodologies and applications, Molecules 2026, 31(12):2010, DOI 10.3390/molecules31122010).
The same review names the property that separates the technique from every chromatographic method in this series: unlike HPLC-UV, LC-MS and GC-MS — which depend respectively on chromophores, ionization efficiency and volatility — NMR offers universal detection for any nucleus with non-zero spin. A contaminant that has no chromophore is invisible to a UV detector by construction and visible to an NMR spectrometer by construction.
An expert panel writing to the United States Pharmacopeia’s revision process put the metrological status in stronger terms still, arguing that properly executed qNMR meets the criteria for a primary ratio method — a method of the highest metrological quality, yielding a result without relation to a standard of the same substance. Their supporting argument is that the theoretical equilibrium magnetization ratio between two nuclear signals carries an error below 10⁻⁷ in the worst case. Their worked example, butyl p-hydroxybenzoate quantified against an internal calibrant, returned a mass fraction of 99.6% with an expanded uncertainty of ±0.6% at k = 2, from a combined standard uncertainty of 0.268%, against an analytical target profile requiring no more than 2.0% expanded uncertainty (Sørensen, Szabo, Nelson and colleagues, quantitative NMR as a metrological method and proposed revisions to USP General Chapters <761> and <1761>, Stimuli to the Revision Process, United States Pharmacopeia).
That “without relation to a standard” clause is what makes the technique structurally interesting for this market rather than merely accurate. There is no certified reference material for most research peptides and there never will be — the catalog is too long, too fast-moving and too commercially marginal to justify one per compound. A method that requires a same-substance reference standard therefore cannot scale to this catalog. A method that requires only a well-characterized internal calibrant of some other substance can.
What it has actually caught
The case for qNMR in peptides is not theoretical, and the most instructive findings are the ones that a purity percentage could not have produced.
Choules and colleagues examined custom synthetic peptides by NMR and found an undeclared constituent — mannitol — present as a substantial fraction of the delivered material, reported at 20% and 43% w/w in the two affected samples (Choules, Bisson, Simmler, McAlpine, Giancaspro, Bzhelyansky, Niemitz and Pauli, NMR detection of an undeclared constituent in custom synthetic peptides, Journal of Pharmaceutical and Biomedical Analysis 2020, 178:112915, PMID 31671336). Mannitol is a common lyophilization bulking agent. It also has no meaningful ultraviolet chromophore. A reversed-phase purity assay is not wrong to omit it; it is structurally incapable of including it. The same research group had earlier demonstrated NMR-based sequencing and quality control of therapeutic peptides by high-fidelity spectral analysis (Choules and colleagues, quality control of therapeutic peptides by proton NMR HiFSA sequencing, Journal of Organic Chemistry 2019, 84(6):3055–3073, PMID 30793905).
The metrology laboratories have shown the same gap at the level of certified materials. In assigning a value to an angiotensin II certified reference material, Melanson and colleagues combined proton qNMR of the intact peptide, LC-MS/MS amino acid analysis after hydrolysis, and fluorine-19 qNMR of the trifluoroacetate counterion, reconciling the three in a Bayesian framework. The assigned purity was 691 ± 9 mg/g at k = 2, with the trifluoroacetate counterion alone accounting for roughly 25% of total mass (Melanson, Thibeault, Stocks, Leek, McRae and Meija, purity assignment for peptide certified reference materials, Analytical and Bioanalytical Chemistry 2018, 410(26):6719–6731, DOI 10.1007/s00216-018-1272-7).
Sit with 691 mg/g. A material good enough to become a certified reference standard was, by mass, roughly 69% angiotensin II. A synthetic oxytocin certified by mass balance came out at 796.5 ± 6.5 mg/g at k = 2, with twenty-one structurally related impurities spanning 0.05 to 15.65 mg/g and the non-peptide fraction resolved by coulometric Karl Fischer, ion chromatography, ICP-MS and headspace GC-MS (Wang and colleagues, mass balance purity assignment of synthetic oxytocin, Journal of Pharmaceutical and Biomedical Analysis 2022, 207:114401, DOI 10.1016/j.jpba.2021.114401).
Neither of those materials was of poor quality. Both would have carried a chromatographic purity figure in the high nineties. The gap between “99% pure” and “69% peptide by mass” is not a contradiction and not a scandal — it is the difference between a ratio and a count, expressed in numbers.
The convention is visible even on national reference certificates. The NIST standard reference material for human angiotensin I reports a certified peptide value alongside a separate reference value for the acetate counterion, with proton integration by NMR used to determine that acetate content. A reference certificate quantifies the counterion as its own line item rather than folding it silently into a percentage.
What qNMR costs, and where it stops
A page that presents qNMR as the answer without stating its limits is selling something. The limits are real and they bear directly on which peptides this is practical for.
The measurement is not free of sample. It needs milligram quantities dissolved in deuterated solvent with a weighed internal calibrant, and that aliquot is consumed. Acquisition is slow by design: the relaxation delay that makes the integral quantitative is the same delay that makes the experiment long, and the signal-to-noise target for low uncertainty compounds the problem. The internal-standard approach, while generally the more accurate one, is vulnerable to peak overlap — and peak overlap is precisely what happens as a peptide gets larger.
That last point is why the published metrology work sits where it does. Angiotensin II is eight residues. Oxytocin is nine. Resolving a clean, isolated, assignable proton signal in a peptide of that size is tractable. The TB-500 10 mg batch certificate describes a synthetic 43-residue sequence with a molecular weight near 4,963 Da; the proton spectrum of a molecule that size is a crowded envelope, and picking a quantifiable resonance out of it is a materially harder problem than the published exemplars represent. The honest generalization is that qNMR scales badly with peptide length, which is one reason the metrology laboratories pair it with amino acid analysis after hydrolysis rather than relying on it alone. How that hydrolysis-based route closes the mass balance is the subject of a separate article in this series.
Nor does qNMR replace chromatography. It will not resolve a deamidated analog from its parent, or an epimer, or an insertion variant, because those species have nearly the same proton count in nearly the same environments. Chromatography separates species; NMR counts nuclei. The published certifications use both because neither is sufficient.
What our certificates report
Both certificates behind this article were fetched and read in full this week, and the position is straightforward.
The TB-500 certificate — report DBAV-TB500-10-051226, tested 23 May 2026 by Testides Analytical — reports an expected content of 10.00 mg, an actual mass of 9.84 mg, a fill accuracy of 98.4% and a purity of 98.62% by HPLC-UV at 214 nm. The BPC-157 certificate, DBAV-BPC-10-051226, reports 10.29 mg against 10.00 mg expected at 99.41% purity by the same method. Neither certificate carries a qNMR section, an amino acid analysis, a counterion determination, a water determination or a net peptide content.
So the accurate description of what those documents establish is this: a chromatographic purity ratio and a gravimetric content mass. The 9.84 mg is a weight of the material in the vial. It is not a count of TB-500 molecules, and no HPLC-UV certificate in this market produces one. A laboratory that needs absolute peptide content — for a molar concentration, for a stoichiometry, for anything where milligrams of powder and milligrams of peptide must not be confused — has to determine it, and should design the work on the assumption that the certificate has not.
That is a scope statement rather than a defect. HPLC-UV purity plus gravimetric fill is the prevailing standard of documentation for research-grade peptides, and our certificates meet it. What we will not do is let a percentage stand in for a quantity it structurally cannot express. If a supplier’s page implies that a purity figure tells a buyer how much peptide is in the vial, that page is wrong in a way this literature settles.
All material referenced on this site is supplied for research purposes only. It is not for human use, not for diagnostic use, and not for therapeutic use. The analytical work cited above was performed on reference and laboratory materials and is described here as published methodology, not as a claim about any product.
References
- Sørensen, Szabo, Nelson and colleagues — quantitative NMR as a metrological method, and proposed revisions to USP General Chapters <761> and <1761>, Stimuli to the Revision Process, United States Pharmacopeia.
- Tang, An, Zheng, Tang, Shen, Wang, Hua and Zhao — quantitative proton NMR in pharmaceutical and biomedical analyses, methodologies and applications, Molecules 2026, 31(12):2010, DOI 10.3390/molecules31122010.
- Melanson, Thibeault, Stocks, Leek, McRae and Meija — purity assignment for peptide certified reference materials combining qNMR and LC-MS/MS amino acid analysis, applied to angiotensin II, Analytical and Bioanalytical Chemistry 2018, 410(26):6719–6731, DOI 10.1007/s00216-018-1272-7.
- Wang, Wu, Li, Huang, Shi, Feng and Li — mass balance approach to SI-traceable purity assignment of synthetic oxytocin, Journal of Pharmaceutical and Biomedical Analysis 2022, 207:114401, DOI 10.1016/j.jpba.2021.114401.
- Choules, Bisson, Simmler, McAlpine, Giancaspro, Bzhelyansky, Niemitz and Pauli — NMR detection of an undeclared constituent in custom synthetic peptides, Journal of Pharmaceutical and Biomedical Analysis 2020, 178:112915, PMID 31671336.
- Choules and colleagues — quality control of therapeutic peptides by proton NMR HiFSA sequencing, Journal of Organic Chemistry 2019, 84(6):3055–3073, PMID 30793905.
- McCarthy, Han, Carrick, Schmidt, Workman, Matejtschuk, Duru and Atouf — reference standards supporting quality of synthetic peptide therapeutics, Pharmaceutical Research 2023, 40(6):1317–1328, DOI 10.1007/s11095-023-03493-1.
- National Institute of Standards and Technology — Standard Reference Material 998, Angiotensin I (Human), certificate of analysis.