How do I read an impurity table on a semaglutide certificate against the 97.1% headline?

Conditions: semaglutide · 97.1%.

I want the working, not the result — I need to be able to redo it with different numbers.

I care about the precision as well as the value — I want to know how many figures are real.

How many significant figures are actually justified here?

Sum the table and see whether it reaches 2.9 per cent. A 97.1 per cent headline leaves 2.9 per cent of the detected area unaccounted for, and a related-substances table exists to say what that 2.9 per cent is. If the listed peaks add to 1.74 per cent, the missing 1.16 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 2.9 per cent is not wrong, but it is incomplete in a way the headline conceals.

More usefully, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.

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 underlying point is that 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.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

If you are ranking vendors, specify a method and have all samples tested at the same place.

edited 25 Oct 2025 by claudia_ferrante — added a caveat about sampling

Put another way, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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

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

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

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

The part that matters: 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.

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 underlying point is that 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.

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

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.

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.

The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.

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."

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.

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.

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.