What a reversed-phase purity number actually is

That is the distinction I keep failing to hold on to. Written down now.

Two sentences on reversed-phase purity number 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.

Worth separating two things that post #14 runs together.

Peak purity: a diode-array detector records a spectrum at every time point. If a peak contains two co-eluting species with different spectra, the spectrum changes across the peak. A passing peak-purity result says the spectrum is constant; it is weak evidence of homogeneity if the impurities have similar spectra.

I would want the raw data before agreeing with my own summary of it.

Confirming post #16 from a second method, which matters more than confirming it from a second person.

A chromatogram image at a resolution where you can see peak shape but not baseline detail is worth having and is not the same as the data. Ask for the integration table if the number matters.

This is where my knowledge stops and I would rather mark the edge than blur it.

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.

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.

Practical note on reversed-phase purity number: write down what you expect before you look. The number of times I have found what I went looking for is higher than chance would allow.

I had written a reply contradicting post #19 and deleted it. Here is what survived.

I disagree with the framing of reversed-phase purity number above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked.

The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds generally.

Confirming post #23 from a second method, which matters more than confirming it from a second person.

Injection volume matters because column overload distorts peak shape, and an overloaded main peak can swallow a small neighbour. A certificate without injection volume is missing something load-bearing.

I would treat that as a working assumption and revisit it.

Answering the question post #23 raises rather than the one it answers.

A request rather than an answer: could whoever has the primary source for reversed-phase purity number post it? I have seen the claim three times this month and each version had lost a qualifier.

Reading rather than answering, but this is the post I would point somebody at.

The useful distinction on reversed-phase purity number is between what was measured and what was inferred from it. Both end up in the same sentence and only one of them has error bars.

Post #27 is the version of this I will quote in future. One addition.

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.

Written quickly, so the reasoning may be tighter than the wording.

Worth separating two things that post #27 runs together.

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.

If anyone has run this properly I would rather read that than my own guess.

This follows post #27 rather than contradicting it.

Two claims get bundled together under reversed-phase purity number and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.

Almost every disagreement in threads like this one dissolves once you say which of the two you are making.