The specifics, since they change the answer: 96.8% · semaglutide · JEEP.
I would like to define my thresholds before I have a result, for obvious reasons.
I want a plan with explicit stopping rules, not just steps.
What does a sensible plan look like, and what are the decision points?
96.8 per cent purity with no content figure leaves the milligram number unmeasured. Purity says 96.8 of every 100 units of detected area is semaglutide and 3.2 is something else. Content says how many milligrams are in the glass. The two do not constrain each other: a vial can be 96.8 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.
Put another way, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
| 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 |
The relevant detail is that 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.
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.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.
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 Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
Compare purity within a single laboratory on the same method, never across laboratories.
Purity is a method-dependent figure, and that is not a limitation of the measurement, it is a property of what the measurement actually answers.
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.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
edited 15 Mar 2026 by lyoph_cake — removed a claim I could not source
Read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
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.
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 13 Feb 2026 by w_okoye — added the citation requested in comments
Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
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.
edited 29 Dec 2025 by bufferline42 — corrected a unit error in the worked example
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.