Why would Janoshik report 96.8% where the supplier certificate says something higher?

Stated plainly: Janoshik · 96.8%.

This is not behaving the way I expected and I want to understand the discrepancy before I act on it.

I have photographed the current state and recorded the conditions, so I can answer follow-up questions precisely.

Should I be treating this as a failure or as noise?

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

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.

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.

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 part that matters: area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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.

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

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

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

Put another way, 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.

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.

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.

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.

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

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.

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 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.

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.