Is 99.1% a realistic purity ceiling for tirzepatide given its chain length?

The case in front of me: 99.1% · 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?

99.1 per cent is at the top of what stepwise synthesis delivers on a chain this long, and it is reachable rather than fictional. Every coupling is high-yielding and none is quantitative, so the deletion and truncation sequences that survive purification are what occupies the remaining 0.9 per cent. Above roughly 98 per cent you are fighting the purification rather than the synthesis, which is why a 99.1 per cent figure on tirzepatide deserves a method question — column, gradient, wavelength — rather than either belief or dismissal.

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.

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.

Mass shifts and what they usually mean

Worth being precise here: 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 Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with 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.

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

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.

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

On the detail: 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.

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.

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

edited 1 May 2025 by sian_llewellyn — corrected a unit error in the worked example

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.

Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.

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

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

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

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.

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

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.

On the detail: 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.

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.

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.

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.