How do I read an impurity table on a survodutide certificate against the 97.9% headline?

The case in front of me: survodutide · 97.9%.

Please show the division. I want to check my own against yours.

I would like the general form as well as the specific number, so I can apply it again.

Is my approach right even if my number is wrong?

Sum the table and see whether it reaches 2.1 per cent. A 97.9 per cent headline leaves 2.1 per cent of the detected area unaccounted for, and a related-substances table exists to say what that 2.1 per cent is. If the listed peaks add to 1.26 per cent, the missing 0.84 is unassigned area — real signal the laboratory saw and did not name — and that is a finding, not an omission. Check three things in order: does the table report each impurity as area per cent on the same basis as the headline; is there a reporting threshold below which peaks were discarded; and does the largest single impurity have a limit against it. A table whose entries do not close on 2.1 per cent is not wrong, but it is incomplete in a way the headline conceals.

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

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.

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 ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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

edited 13 Jan 2025 by b_delacroix — added the method parameters

It helps to be literal here: 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.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.

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.

In practice, the single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.

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

The underlying point is that 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 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.

Specifically, gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

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.

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 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.

Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.

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.