Would you accept 97.9% on tirzepatide from ERP without a content assay?

Concretely: 97.9% · tirzepatide · ERP.

I want to decide this in advance so that I am not deciding it under pressure later.

Assume I will follow the plan I write down, so I would like it to be a good one.

What should I decide now, and what should I defer?

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.

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

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

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.

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.

In practice, 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.

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.

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

edited 15 Nov 2024 by petra_hovland — 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.

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.

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.

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

Specifically, area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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.

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.

Stated carefully, read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

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

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

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