What a reversed-phase purity number actually is posts 31–60
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.
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.
Building on post #30 rather than restating it.
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.
The literature is thinner on this than the confidence in the thread implies.
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 would rather be precise about what I do not know than vague about what I do.
Reversed-phase purity number is a question about a distribution, not about a value, and treating it as a value is what produces the confident wrong answers.
Post #34 is the version of this I will quote in future. One addition.
I would call the community position on reversed-phase purity number likely rather than established, and I would be comfortable defending that hedge.
Where I part company with post #36, and it is a narrow parting.
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.
Genuinely open to being wrong about this one.
Picking up post #38: that is the part I would want checked first.
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 strength of my opinion here exceeds the strength of my evidence.
On post #36 — agreed on the reasoning, with one qualification.
Practical answer on reversed-phase purity number, since the theoretical one is upthread: do the simplest check first, write down the result, and only then decide whether the complicated explanation is needed. It usually is not.
Post #37 and I disagree about the size of the effect, not about the direction.
What I would check first on reversed-phase purity number is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.
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.
Reversed-phase purity number was covered in the wiki last year and the page has a review date on it, which is a better starting point than my memory of a thread.
Filing a mild objection to the consensus on reversed-phase purity number. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit.
Post #41 put the caveat in the right place and I want to underline 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.
Not the answer, but possibly the question that gets there.
Whatever the answer on reversed-phase purity number turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, invite the correction.
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.
Post #49 is the version of this I will quote in future. One addition.
Column temperature affects retention and selectivity and is omitted from most certificates. Two runs at twenty-five and forty degrees are not the same method.
Scoping that to what I have actually seen rather than what I have read.
Picking up post #48: that is the part I would want checked first.
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.
On post #50 — agreed on the reasoning, with one qualification.
Since reversed-phase purity number keeps coming up, it should probably be a maintained page rather than a recurring thread. I am happy to draft it if someone with more direct experience will review it.
Particle size and column dimensions determine what resolution is achievable at all. A 5 micrometre 250 millimetre column and a sub-2 micrometre 100 millimetre column are different instruments in practice.
Two sources, same conclusion, and I could not rule out that one copied the other.
Right — I had this wrong and I am glad to have read it before it mattered.
Collapsed as off-topic by two members at trust level 3 or above
A shoulder on the trailing edge is most often a closely related species rather than an artefact. The way to find out is to change the gradient slope, not to argue about the integration.
This is the sort of thing the wiki should carry and currently does not.
Change the wavelength and the proportions change even though the sample has not. That is the reason the wavelength has to be on the document for the number to mean anything.
Reading it back, the second half matters more than the first.
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.
Trifluoroacetic acid at 0.1 per cent is the near-universal ion-pairing additive for this work, and it also raises the baseline at 214 nanometres. That is why the same sample looks noisier at low wavelength.
If the premise is wrong, everything after it is decoration.