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.
Confirming post #28 from a second method, which matters more than confirming it from a second person.
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.
Someone should write this up properly, and it should probably not be me.
Appreciated. The plain phrasing does more work here than a longer post would.
The arithmetic on Carryover is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.
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.
Speaking for myself and not for anyone else who has posted here.
Worth stating the null on Carryover before we explain it: the observation may be nothing. That possibility deserves a sentence and usually does not get one.
On integration: where the baseline is drawn matters more than most people realise. On a clean chromatogram with well-resolved peaks the choice is inconsequential. On a chromatogram with a trailing shoulder or a rising baseline it matters. Differences of one to two percentage points between defensible integrations are ordinary.
Post #34 describes the usual case. This is about the unusual one.
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.
Checked the Carryover claim against the primary source this morning. It survives, with a narrower scope than the version quoted here. Posting the narrower scope.
This is the answer, and the reason it is the answer is the more useful part.
Reporting rather than recommending, on Carryover. What happened is above. Whether it should have is a different question and not one I am qualified to answer.
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.
Noted, and thank you for writing it out rather than summarising it.
Building on post #42 rather than restating it.
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.
That is a description of practice, not a recommendation of it.
Answering the question post #44 raises rather than the one it answers.
Since Carryover 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.
What I can speak to on Carryover is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know.
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.
Post #47 is the version of this I will quote in future. One addition.
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.
The short version is the first sentence; the rest is why.
Where the Carryover discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.
Adding the measurement that post #47 says would settle it.
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.
Reading it again, the caveat matters more than the finding.
The version of Carryover that I was taught turned out to be a teaching simplification. Useful, and not true in the way I had assumed it was.
The arithmetic in post #50 is right; the assumption feeding it is the part to check.
A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it.
It cost nothing to check and would have cost something not to.
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.
The number people quote for Carryover is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.
Narrowing post #54, because the general version has more than one answer.
Reading this Carryover thread as someone who came in with a fixed view: the third and seventh replies moved me and the confident ones did not.
Quietly grateful for the plain phrasing. Not every thread gets that.
Taking post #56 at face value and following it one step further.
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 would not lead a decision with this, but I would not ignore it either.
Carryover: I have looked for the primary source twice and failed twice. Either it does not exist or it is somewhere I do not know to look, and I would like to know which.