Is 96.8% a realistic purity ceiling for tirzepatide given its chain length?

Concretely: 96.8% · tirzepatide.

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.

Is my approach right even if my number is wrong?

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

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.

It helps to be literal here: the fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

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

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

edited 5 Aug 2026 by ilaria_bertone — added the method parameters

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

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.

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

To be exact about it, reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

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.

It helps to be literal here: retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.

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

The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

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.

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.

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.

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

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