Concretely: 97.6% · 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 would I need in addition before this supported a decision?
On the detail: start by understanding what the detector is measuring and what that means about how the molecule needs to be prepared and handled before injection.
Reverse-phase stationary phases use C18 or C8 chains bonded to silica, and the pore size of the silica matters more for a peptide of this chain length than the ligand length does.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
Put another way, 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 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.
If two labs give different numbers, the method difference is the first hypothesis, not lab quality.
edited 13 Feb 2026 by mira_sundqvist — fixed an arithmetic slip in the third paragraph
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsPut another way, coupling the chromatography to mass spectrometry adds identity information but trades chromatographic quality for ionisation efficiency.
Detector linearity is why a heavily loaded peak can give false purity by compressing the main peak height while leaving impurity shoulders unchanged.
To be exact about 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.
The selectivity of a reverse-phase separation depends on the hydrophobicity of the side-chain profile, which is why two peptides can co-elute even if they differ by a residue.
The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.
On the detail: 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.
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.
Concretely, 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.
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.
Ask for the chromatogram and the system suitability data, not just the number.
Concretely, method transfer between laboratories is almost impossible without full documentation, which is why two labs reporting the same preparation should expect disagreement.
Mobile phase pH at the point where you inject must match the mobile phase pH at the start of the gradient, or the sample will not be focused at the column head.
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.
If two labs give different numbers, the method difference is the first hypothesis, not lab quality.
Peak shape carries as much information as peak area does, and a badly tailing peak or a shouldered peak is telling you something about the sample or the column that matters.
Sample preparation is almost always under-appreciated — a reconstituted peptide in strong solvent will distort its own peak on the gradient.
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.
I would caution against over-interpreting small differences between runs, because the noise floor of the method is larger than most people assume.
The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.