What a reversed-phase purity number actually is posts 61–90
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.
A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it.
Small point, but it is the one that usually catches people.
Collapsed as off-topic by two members at trust level 3 or above
I read post #59 twice before replying, because I had assumed the opposite.
A definition problem is doing most of the work in this reversed-phase purity number discussion. Once the term is pinned down I suspect the disagreement mostly goes away and what is left is small.
That is a cleaner way of putting what I was circling around.
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.
I have kept the units in throughout, for the obvious reason.
Everything in post #65 holds. The case it does not cover is the one I have.
The version of reversed-phase purity number that I was taught turned out to be a teaching simplification. Useful, and not true in the way I had assumed it was.
Retention time is only comparable within a laboratory on a given method. Quoting a retention time across two reports as evidence of identity is not a valid comparison.
That is a description of practice, not a recommendation of it.
Adding the measurement that post #67 says would settle it.
Reversed-phase separates on hydrophobicity. A peptide is retained on a non-polar stationary phase and eluted by increasing organic solvent. For peptides the mobile phase almost always contains an ion-pairing acid, typically 0.1% TFA, which suppresses secondary interactions and sharpens peaks.
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.
Posted with less confidence than the sentence structure implies.
Post #70 is the version of this I will quote in future. One addition.
Reversed-phase purity number has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.
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.
A partial answer, offered because a partial answer beats none.
Answering the question post #72 raises rather than the one it answers.
On reversed-phase purity number, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.
If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.
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.
The uncertainty is in the assumption, not in the calculation.
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.
I have said this before in a thread nobody could find, so it is worth repeating.
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.
I have deliberately not rounded that, because the rounding is where the argument starts.
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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.
Where I would look next, rather than where I would stop.
Post #81 and I disagree about the size of the effect, not about the direction.
If two laboratories disagree by more than two or three percentage points, work through method, integration, sample handling, whether it was the same lot and the same vial, and whether suitability passed. After all five, a gap needs an explanation.
It is the kind of thing that is obvious once and never again.
Second this, and I would have said it less carefully.
Post #84 answers the question as asked. The question underneath it is different.
System suitability is not paperwork. If the replicate injections failed, the run did not happen — the numbers from it are uninterpretable rather than approximate.
That is the honest state of it as of this week.
Worth separating two things that post #86 runs together.
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.
Genuine question rather than a rhetorical one: has anyone here actually observed reversed-phase purity number, as opposed to read about it? The thread is long and I cannot tell.
Post #86 describes the usual case. This is about the unusual one.
A methods point on reversed-phase purity number rather than a substantive one: if the comparison is not like for like, the difference you are measuring is the difference in method.
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.