Area percent versus weight percent: the confusion that causes most arguments

Posting this under the heading it deserves: Area percent versus weight percent: the confusion that causes most arguments Everything below is what sits behind that.

Proposing that area percent versus weight percent deserves a maintained page rather than a recurring topic, and drafting the skeleton here so that the argument about scope happens before the writing rather than after.

What I think belongs on it, what does not, and the three claims that would need a citation each.

Worth stating the null on area percent versus weight percent before we explain it: the observation may be nothing. That possibility deserves a sentence and usually does not get one.

Typical suitability criteria are a replicate area relative standard deviation below about two per cent, a tailing factor inside a defined window, a resolution minimum against a specified peak, and a plate-count floor.

I have said this before in a thread nobody could find, so it is worth repeating.

Post #2 put the caveat in the right place and I want to underline it.

Area percent versus weight percent came up in a thread eighteen months ago and was answered well. I cannot find it, which is itself the problem, so here is the reconstruction.

Checked the area percent versus weight percent claim against the primary source this morning. It survives, with a narrower scope than the version quoted here. Posting the narrower scope.

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.

That is the version I would defend. It is not the version I started with.

Taking post #4 at face value and following it one step further.

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.

Helpful, and short, which on this subject is harder than long.

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.

I read post #6 twice before replying, because I had assumed the opposite.

Counterpoint on area percent versus weight percent, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.

Post #9 describes the usual case. This is about the unusual one.

Area percent versus weight percent: I would want to see the raw numbers rather than the summary before agreeing. Summaries lose exactly the information that would settle this.

Two sentences on area percent versus weight percent and then I will stop, because the rest is speculation and the thread is better without mine.

What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.

Confirming post #13 from a second method, which matters more than confirming it from a second person.

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.