What I have: 97.1% · dulaglutide · MKM.
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 does a sensible plan look like, and what are the decision points?
97.1 per cent purity with no content figure leaves the milligram number unmeasured. Purity says 97.1 of every 100 units of detected area is dulaglutide and 2.9 is something else. Content says how many milligrams are in the glass. The two do not constrain each other: a vial can be 97.1 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.
The underlying point is that 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.
The part that matters: 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.
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 resultsStart from what the detector sees, because that tells you what the number means.
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.
Mechanically, 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.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
If you are ranking vendors, specify a method and have all samples tested at the same place.
The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.
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.
More usefully, 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.
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.
Reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
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 20 Mar 2026 by kwn_analytical — fixed an arithmetic slip in the third paragraph
The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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 limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."
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.