Revisiting: Why I ask for the gradient before I discuss the number

Posting this under the heading it deserves: Revisiting: Why I ask for the gradient before I discuss the number Everything below is what sits behind that.

I would like to understand what this number means before I repeat it anywhere.

A Medutest report on a tirzepatide lot gives 98.4% purity. The supplier certificate for the same lot states 98.2%. Both documents name a reversed-phase method; neither states the same gradient.

My question is not "who is right". It is: given that those two figures were produced by different methods, what is the largest difference I should expect from method alone, and at what point does a gap stop being explainable that way?

Column chemistry and particle size: smaller particles (1.7 μm) give better resolution and higher efficiency than larger particles (3.5 μm or 5 μm), at the cost of higher back pressure. Newer methods increasingly use smaller particles.

Picking up the opening post: that is the part I would want checked first.

Gradient delay volume differs between instruments and shifts the whole chromatogram. It is why a transferred method rarely reproduces retention times exactly on a different system.

Clear enough that I do not think I have a follow-up, which is unusual.

Purity by chromatography answers "what proportion of what I detected is the intended species". It does not answer how much is in the vial, which is a separate assay.

Marking that as an opinion rather than a finding.

Where the baseline is drawn is the least documented and most consequential choice in the whole determination. Valley-to-valley, tangent skim and forced-to-zero can span a couple of percentage points on the same chromatogram.

That is what the documentation says. What happens in practice is usually close.

System suitability testing: injections run before and during the sample run to establish whether the instrument, column and method were performing when the sample was analysed. If suitability did not pass, the sample results from that run are uninterpretable.

A single observation, in a thread that deserves better than single observations.

Narrowing post #7, because the general version has more than one answer.

Area percent is not mass percent. It is a proportion of absorbance, weighted by each species' extinction coefficient. For closely related impurities the approximation is usually good. For structurally dissimilar impurities it can be poor.

Everything in post #6 holds. The case it does not cover is the one I have.

Change the wavelength and the proportions change even though the sample has not. That is the reason the wavelength has to be on the document for the number to mean anything.

I would be glad to be shown a cleaner way of putting this.

Worth separating two things that post #9 runs together.

Gradient slope is the single biggest driver of apparent purity differences. A shallower gradient over a longer run resolves more impurities and gives a higher purity figure. A steep gradient produces a tidier-looking chromatogram with fewer visible peaks and gives a lower purity figure. Both are legitimate methods and they will not produce the same number.

That much is documented. The rest is how I have interpreted it.

Fair, and the limits you put on it are the part I will remember.

Trifluoroacetic acid at 0.1 per cent is the near-universal ion-pairing additive for this work, and it also raises the baseline at 214 nanometres. That is why the same sample looks noisier at low wavelength.

It is the kind of thing that is obvious once and never again.

Post #13 answers the question as asked. The question underneath it is different.

Peaks that do not elute do not appear in the area percentage. Aggregates and strongly retained species can be entirely invisible to a standard method, which is a ceiling on what any purity figure can claim.

I would put a moderate confidence on that and no more.