Would you accept 98.4% on a GLP-1 receptor agonist from HJ without a content assay?

Setup, so nobody has to ask: 98.4% · a GLP-1 receptor agonist · HJ.

I am trying to build something sustainable rather than something thorough that I will abandon.

I have already decided the broad direction; this is about the specifics.

How would you structure this, and what thresholds would you set in advance?

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

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 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.

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

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

The relevant detail is that 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.

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.

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.

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

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.

The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

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.

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.

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

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.

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.