What does 95.2% purity on a C4 column actually tell me about an orforglipron vial?

The case in front of me: 95.2% · a C4 column · orforglipron.

The figures are clear enough; the question is what they mean and what they do not.

I can supply the numbers if the specifics change the answer.

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.

Mass shifts and what they usually mean

Worth being precise here: sample preparation is almost always under-appreciated — a reconstituted peptide in strong solvent will distort its own peak on the gradient.

I would caution against over-interpreting small differences between runs, because the noise floor of the method is larger than most people assume.

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

The underlying point is that start by understanding what the detector is measuring and what that means about how the molecule needs to be prepared and handled before injection.

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.

Temperature affects both the viscosity of the mobile phase and the dynamics of molecular interactions, and a method developed at 25 degrees and run at 40 degrees will not behave identically.

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.

Ask for the chromatogram and the system suitability data, not just the number.

edited 5 Oct 2024 by low_dead_space — reworded for clarity after a comment

The relevant detail is that high-performance liquid chromatography is a separation technique, not an identification technique, and it answers the question "how much of the detected signal is a single species" not "which species is it."

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.

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.

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

The most important parameter is the one that is almost never specified: the gradient slope during the region where your main peak elutes.

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.

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.

To be exact about it, reverse-phase HPLC is the workhorse for peptide purity work, but it is almost universally run under conditions that are not optimal for a peptide of this chain length.

Proline-rich sequences are particularly problematic because the isomerisation kinetics are in the same timescale as the separation, leading to split or broadened peaks at low temperature.

The resolving power of a separation is quantified by the resolution parameter R, defined from the heights and widths of adjacent peaks, and pharmacopoeial methods typically demand R greater than 1.5 for a method to be considered validated.

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.

Ask for the chromatogram and the system suitability data, not just the number.