What does 99.4% purity on a C4 column actually tell me about an orforglipron vial?

Conditions: 99.4% · a C4 column · orforglipron.

I would like to know the limits of what can be inferred from this.

What I am trying to avoid is over-reading a single result, which I have done before.

What can I legitimately conclude from this figure?

Coupling the chromatography to mass spectrometry adds identity information but trades chromatographic quality for ionisation efficiency.

Wider-pore phases — 300 Angstrom rather than 100 Angstrom — have faster mass transfer and narrower peaks for peptides above three kilodaltons, which is almost every peptide you will see.

Formic acid is the compromise when you need the mass spectrometer on the same run, but the peak shape penalty is real and easily a tenth of a per cent on purity.

Peptide separation by reverse-phase high-performance liquid chromatography is described in the general chapters of the United States Pharmacopeia, European Pharmacopeia and Japanese Pharmacopeia, and the methods converge on essentially the same principles.

Worth noting that the achievable resolution depends on the chemistry of the molecule — some peptide sequences separate easily while others are notoriously difficult regardless of method.

The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.

The limit of this technique for these molecules is reaching a point where small improvements require proportionally large investments in method development.

System suitability checks on replicate injections of a known standard establish whether the method was in control — if the peak area varies by more than two per cent between replicates, something is wrong.

Worth being precise here: the 214 nanometre wavelength is chosen because it corresponds to the amide backbone absorption, making response roughly proportional to the number of peptide bonds.

Inter-laboratory studies using identical methods on identical material show precision well within half a per cent when the method is locked down, pointing to method variability as the primary source of disagreement.

The limitation is that co-elution is invisible — if two species happen to have the same retention time, they will report as a single peak at their combined area.

If two labs give different numbers, the method difference is the first hypothesis, not lab quality.

Gradient design is usually described as though it is a minor technical detail rather than the primary determinant of what the method measures.

Trifluoroacetic acid at 0.1 per cent is the standard ion-pairing agent because it suppresses tailing by neutralising the basic residues, but it also suppresses mass spectrometry signal.

Peptide impurities from solid-phase synthesis include deletion sequences, truncations from premature cleavage, racemised residues from epimerisation and oxidised variants, each of which may have different chromatographic behaviour.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

edited 11 Apr 2025 by eoin_mcgarry — clarified the distinction between purity and content

System suitability is the part of a report that tells you whether the method was under control on the day you were tested, and its absence is concerning.

Acetonitrile is the organic modifier of choice because it has a good UV cutoff, a reasonable viscosity and a refractive index that minimises baseline noise.

One qualification: the limit of detection on a reversed-phase HPLC is set by the noise on the baseline, which for these molecules is usually in the range of a tenth of one per cent or less, and anything smaller is not reproducibly detectable.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.