On the subject in the title: Carryover and the ghost peak from last week's standard Working notes rather than a conclusion.
What changes if the standard account of carryover is wrong? I ask because I have been treating it as settled and I noticed this week that I could not say why.
Working through the consequences rather than the evidence, since others here are better placed on the evidence.
The opening post and I disagree about the size of the effect, not about the direction.
Method validation is the demonstration that a method can separate the compound from its degradation products and impurities reliably. A method that cannot resolve an impurity from the parent peak will not detect that impurity.
On balance I think that is right, and I would not bet much on it.
The version of carryover that circulates here is a simplification of a simplification. It is not wrong, but it has lost the conditions under which it holds, and those conditions are where the interesting cases live.
I have no financial interest in anything named in this thread and I want to say so before I comment on carryover, because it is the sort of subject where it matters.
Building on post #5 rather than restating it.
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.
Two people can read the same figure differently here and both be reasonable.
Post #4 put the caveat in the right place and I want to underline it.
Carryover looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
I had written a reply contradicting post #8 and deleted it. Here is what survived.
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.
I have written this out at length because the short version keeps being misread.
Two things can be true about carryover at once: the mechanism is plausible and the evidence for the size of the effect is thin. Most of the argument here is people defending the first against attacks on the second.
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.
I have seen it go both ways, which is why I hedge.
Noted, and I have changed what I was going to do on the strength of 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.
One case, stated as one case.
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.
The short answer was in the first line; everything after is the working.
Post #16 is the version of this I will quote in future. One addition.
Before the thread moves on from carryover — what is the sample size behind the claim? I am not being difficult; I have seen the same figure quoted from an n of four and from an n of four hundred.
The practical version of carryover is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
Coming back to post #18, because the follow-up matters more than the original answer.
I would keep carryover and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
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.
On carryover the community has more anecdote than the confidence in this thread implies, and I include my own contribution in that.
I had written a reply contradicting post #22 and deleted it. Here is what survived.
Careful with the language on carryover. "Not detected" and "not present" are different findings and the first is a statement about the method.
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.
Caveat: everything above assumes the paperwork is what it says it is.
Post #27 is the version of this I will quote in future. One addition.
Trying to state the carryover position in a way that someone who disagrees would recognise as fair, because I do not think the version in this thread passes that test.
Carryover sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.