How do I read an impurity table on a tirzepatide certificate against the 97.6% headline?

For reference: tirzepatide · 97.6%.

I would like the arithmetic checked rather than the conclusion asserted.

I have deliberately not used an online calculator because I want to be able to check the result.

What is the general form of this calculation?

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

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.

The part that matters: 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.

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.

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

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.

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.

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.

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

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

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

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

edited 7 Apr 2025 by elke_brunner — removed a claim I could not source