Concretely: 96.4% · oral semaglutide · BCH.
I want to decide this in advance so that I am not deciding it under pressure later.
Assume I will follow the plan I write down, so I would like it to be a good one.
What would you do, and what would make you change course?
More usefully, gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.
Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.
Tailing factor measures peak shape, and a badly tailing peak spreads into the region where small impurities live, forcing tangent-skim integration that assigns tail area to the main peak.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.
Compare purity within a single laboratory on the same method, never across laboratories.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsIt helps to be literal here: identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.
Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
More usefully, the short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.
Sample solvent strength affects peak shape — if you inject in strong solvent on a gradient starting in weak solvent, the solvent peak can distort your main peak or create a false shoulder.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.
If you are ranking vendors, specify a method and have all samples tested at the same place.
It helps to be literal here: understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
Detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.
Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
Compare purity within a single laboratory on the same method, never across laboratories.
The single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.
Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
edited 4 Sept 2025 by ayo_fadipe — added a caveat about sampling
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.