Why I ask for the gradient before I discuss the number posts 31–40

This follows post #30 rather than contradicting it.

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.

Worth separating two things that post #28 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.

I looked this up rather than remembered it, which is the right order.

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.

I am aware this is the third time this month I have made this point.

A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it.

Picking up post #34: that is the part I would want checked first.

A shallow gradient resolves close-eluting species that a steep one merges. Two honest laboratories running different gradients can report genuinely different purities on the same vial.

That holds under the stated conditions and I have stated them.

Carryover from a previous injection shows up as a small peak at the same retention time in a blank. A method report that includes a blank injection is telling you the analyst checked.

Take the reasoning and check the arithmetic; I do not always get it right.

I will take the caveat as seriously as the claim, which is the point of putting it there.

I had written a reply contradicting post #36 and deleted it. Here is what survived.

Mobile-phase preparation is a genuine source of between-laboratory variation. Acid concentration and organic modifier both shift retention, and neither is usually specified to the precision that would matter.

Method validation is the demonstration that a method can separate the compound from its degradation products and impurities reliably. A method that cannot resolve an impurity from the parent peak will not detect that impurity.

Anyone who has looked at this more carefully, please correct the record.

Noted, and thank you for writing it out rather than summarising it.