Stated plainly: 98.2% · cagrilintide · QST.
I would like to set this up properly once, rather than adjust it repeatedly.
My budget is real but not tight, and my tolerance for uncertainty is low.
What should I decide now, and what should I defer?
In practice, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
| Δ mass (Da) | Most likely cause | Distinguishing feature |
|---|---|---|
| +1 | Deamidation (Asn or Gln) | New peak, slightly earlier retention |
| −17 | Loss of ammonia | Often with deamidation |
| −18 | Dehydration / succinimide | pH-dependent, reversible |
| +16 | Oxidation (Met, Trp) | Earlier retention, light-related |
| −128 | Missing Gln or Lys | Deletion sequence from synthesis |
| 0 | Isomer: racemisation or scrambling | Same mass, shifted retention |
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.
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.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.
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.
On the detail: the fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.
If you are ranking vendors, specify a method and have all samples tested at the same place.
Understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
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.
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.
The limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
Compare purity within a single laboratory on the same method, never across laboratories.
edited 5 Aug 2026 by vialroom — corrected a unit error in the worked example
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.
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.
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.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
edited 12 Aug 2026 by Dr_Hanne_Solberg — expanded the table to cover the lower concentration
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.