Would you accept 97.9% on dulaglutide from SSA without a content assay?

Details up front: 97.9% · dulaglutide · SSA.

I would rather over-plan the first cycle and simplify later.

I am prepared to do the work if someone can tell me which work matters.

What is the minimum version of this that is still defensible?

97.9 per cent purity with no content figure leaves the milligram number unmeasured. Purity says 97.9 of every 100 units of detected area is dulaglutide and 2.1 is something else. Content says how many milligrams are in the glass. The two do not constrain each other: a vial can be 97.9 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 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.

Reconciling gross mass to label claim

On the detail: 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.

Reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

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.

Compare purity within a single laboratory on the same method, never across laboratories.

The underlying point is that understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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.

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 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.

The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.

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.

Read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

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.

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.

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 23 Mar 2025 by kwn_analytical — added a caveat about sampling