How do I read an impurity table on an ecnoglutide certificate against the 99.1% headline?

Conditions: ecnoglutide · 99.1%.

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.

Where is my error, and what is the correct working?

Sum the table and see whether it reaches 0.9 per cent. A 99.1 per cent headline leaves 0.9 per cent of the detected area unaccounted for, and a related-substances table exists to say what that 0.9 per cent is. If the listed peaks add to 0.54 per cent, the missing 0.36 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 0.9 per cent is not wrong, but it is incomplete in a way the headline conceals.

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.

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.

Reconciling gross mass to label 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.

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.

Concretely, 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.

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 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.