Follow-up: Comparing two chromatograms from different laboratories, properly posts 61–68

Post #60 and I disagree about the size of the effect, not about the direction.

Before anything else: what was the gradient, and at what wavelength? Area percent at different wavelengths is not the same number even on the same sample because different species absorb differently at different wavelengths. With the method stated, I can tell you something useful. Without it, all I can say is that there is one large peak.

The right answer here may simply be that it has not been measured.

On integration: where the baseline is drawn matters more than most people realise. On a clean chromatogram with well-resolved peaks the choice is inconsequential. On a chromatogram with a trailing shoulder or a rising baseline it matters. Differences of one to two percentage points between defensible integrations are ordinary.

Stating my assumptions rather than smuggling them in.

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.

Narrowing post #63, because the general version has more than one answer.

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 #63 runs together.

This is why a purity figure without the underlying chromatogram is weaker evidence than it appears. It is also why two competent laboratories can report different numbers on the same vial without either being wrong.

This follows post #63 rather than contradicting it.

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.

This settles it for me, at least until somebody posts a reason it should not.

Area percent is a proportion of absorbance, not a proportion of mass. Two species with different extinction coefficients at the detection wavelength contribute unequally to the total, and nothing on the certificate corrects for that.