Details up front: 97.6% · orforglipron · QYB.
This is a planning question. I know what my options are; I do not know how to weigh them.
What I want is the minimum viable version, which I suspect is smaller than what I would design.
What is the minimum version of this that is still defensible?
97.6 per cent purity with no content figure leaves the milligram number unmeasured. Purity says 97.6 of every 100 units of detected area is orforglipron and 2.4 is something else. Content says how many milligrams are in the glass. The two do not constrain each other: a vial can be 97.6 per cent pure and still be under label, because water and counter-ion are part of the gross mass and neither shows up as an impurity peak. If you buy one test, buy the one that changes your arithmetic.
Mechanically, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
| Component | Typical share | Counted in purity? | Counted in content? |
|---|---|---|---|
| Target peptide | 88–94 % | Yes, as main peak | Yes |
| Related impurities | 1–3 % | Yes, as other peaks | No |
| Counter-ion (TFA or acetate) | 2–8 % | No | No |
| Residual water | 2–6 % | No | No |
| Bulking agent, if present | 0–40 % | No | No |
In practice, 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.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsIdentity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
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.
Concretely, temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.
If you are ranking vendors, specify a method and have all samples tested at the same place.
Start from what the detector sees, because that tells you what the number means.
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.
Compare purity within a single laboratory on the same method, never across laboratories.
edited 29 Jan 2025 by helena_vidmar — clarified the distinction between purity and content
Read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
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.
The underlying point is that reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.
Integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.
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.
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.