Proton NMR Spectrum as Proof of Substance Identity

A chromatogram answers the question “how much,” a nuclear magnetic resonance spectrum answers the question “what.” This distinction is the core of all identity control of a material and, at the same time, the point where misunderstandings most often arise: an excellent chromatogram is sometimes mistaken for proof that the container holds the correct substance, which it is not, even approximately.

This text explains why the proton (1H) NMR spectrum is regarded as strong proof of identity, what exactly follows from it, where the method fails, and how to read the spectrum supplied with a reagent without being a spectroscopist.

Three pieces of information in one spectrum

A proton (1H) NMR spectrum carries three independent kinds of information simultaneously, and it is precisely their agreement that makes it proof rather than circumstantial evidence.

Chemical shift tells you what electronic environment a given proton is in. A proton on an aromatic ring resonates in a different range than a proton of a methyl group on a saturated carbon, and that one in turn differently than a proton on an oxygen atom. The position of a signal is therefore information about the type of molecular fragment.

Integration, i.e., the area under a signal, is proportional to the number of protons giving that signal. The integration ratio reproduces the ratio of the numbers of hydrogen atoms in individual positions — and that is already information about the stoichiometry of the molecule.

Multiplicity, i.e., the splitting of a signal, results from coupling with neighboring protons. It tells you how many neighbors a given proton has, that is, how the fragments are connected to one another.

Only these three things together provide a structural picture. A substance with a different structure would have to reproduce all three at once by chance — positions, proportions, and splittings — which for molecules with a dozen or more protons is practically impossible.

Why this is proof, not a premise

The key difference compared with chromatographic methods is that the spectrum is a positive description of the structure, not merely an indication of retention time. Retention time is a comparative quantity: it says that something behaves like the reference standard under the same conditions. The spectrum says how this molecule is built, regardless of whether anyone has a reference standard.

That is why, in the production of certified reference materials, magnetic resonance regularly appears as a confirmatory technique — for example, in the development of a reference material for a ketoprofen impurity¹ or a D-phenylalanine reference material.² For peptide reference standards, spectral methods are part of the standard set.³

The same technique, used quantitatively, makes it possible to determine purity — the measurement then consists of comparing the integration of the signal of the test substance with the signal of a standard of known purity. The use of proton spectra for determining the purity of pharmaceutical reference materials has been described separately,⁴ and in the certification of a caffeine reference material, the spectrophotometric result was verified precisely by quantitative magnetic resonance.⁵

Where the method fails

An honest description must point out the limitations, because they are real and in several situations decisive.

• Sensitivity. Magnetic resonance is a relatively insensitive method. An impurity at the level of tenths of a percent usually disappears in the noise, whereas chromatography with UV detection will see it without difficulty. NMR is not suitable for detecting traces.
• Signal overlap. In molecules with many similar protons, regions of the spectrum can be crowded and signal assignment becomes ambiguous. In such cases, two-dimensional spectra or nuclei other than hydrogen are used.
• An impurity with a similar spectrum. A close structural analog will give a similar spectrum; the differences can be subtle and require comparison with a reference spectrum, not inspection alone.
• Enantiomers. An ordinary proton spectrum does not distinguish enantiomers — they are spectrally identical. Distinguishing them requires chiral reagents or chromatographic methods with a chiral phase.

The last point can be a source of serious errors for compounds in which optical activity matters. The spectrum will then confirm the skeleton but will say nothing about the configuration.

Complementarity instead of rivalry

The most sensible arrangement is one in which both techniques answer their own questions. Magnetic resonance confirms what it is; chromatography determines how much of it there is and detects traces. Work on impurities and degradation products shows this combination in action — from the identification of major degradation products⁶ to analyses of new drug candidates combining chromatography with diode-array detection and mass spectrometry.⁷ A review of applications in the analysis of impurities and degradation products has been compiled separately.⁸

In areas where identity is sometimes deliberately obscured, combining techniques is actually a prerequisite for the work — this applies both to confirming the authenticity of botanical raw materials⁹ and to identifying new psychoactive substances using molecular networking methods.¹⁰ Similarly, in characterizing complex nitrosamine impurities, a set of methods is used today, not a single one.¹¹

How to read the spectrum supplied with a reagent

You do not need to be a spectroscopist to draw useful conclusions from such an attachment. A few questions are enough.

• Does the number of signals match the number of distinct proton environments in the structure? An extra signal means an admixture or residual solvent.
• Are the integrations in simple ratios? A deviation from the proportions dictated by the formula is a warning sign.
• Is the solvent signal marked? Every deuterated solvent has a residual proton signal at a known position — its absence from the description means the spectrum was not described carefully.
• Are the measurement conditions given? Solvent, frequency, and temperature change signal positions. A spectrum without these data is difficult to compare with the literature.

It is also worth paying attention to the simplest thing: whether a spectrum was attached at all. A certificate stating only purity takes the identity of the material for granted — and that is an assumption, not a measurement. What exactly the declared purity figure means is described in our text on what the number on the certificate really means, and the limitations of chromatography in our text on what this method sees and what it does not.

There are also molecules whose structure itself provides an additional identification tool. If a compound contains fluorine, the way opens to a spectrum on fluorine-19 nuclei: this isotope has a spin of one-half and a sensitivity close to that of protons, and in a typical laboratory sample practically nothing else contains fluorine — the background is therefore nearly zero, and a single signal settles the presence of the element within a few minutes. We devote separate texts in this series to molecules from this group.

Sources

Each item below was retrieved from the PubMed database via the E-utilities interface and is not reproduced from memory. The PMID numbers link to the source records.

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2. Luo S, Liu Y, Wang X et al.. Development of a certified reference material for D-phenylalanine with evaluation of enantiomeric purity. Anal Bioanal Chem. 2024;416:5177-5189. PMID: 39117955. DOI: 10.1007/s00216-024-05456-w.
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