My experience of Carryover contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
Carryover and the ghost peak from last week's standard — what changed since Writing it up because I had to work it out twice and would rather nobody else did.
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.
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.
Adding a source would improve this post and I do not have one to hand.
The arithmetic in the opening post is right; the assumption feeding it is the part to check.
Nobody has said the unglamorous part of Carryover yet, so: most of the variation is explained by things that are boring to write about and easy to check.
Building on post #4 rather than restating it.
Detection wavelength: 214 nm detects the peptide bond and is relatively insensitive to composition. 280 nm detects aromatic residues and is strongly composition-dependent. Area percent at one wavelength is not area percent at the other.
It is a small point and it changes the answer, which is an awkward combination.
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.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
Carryover is well covered in the tag pages, and the older discussions are better than the recent ones because they were argued out properly. Worth twenty minutes before adding to this one.
Adding the boring version of Carryover, because the interesting version keeps getting posted and the boring one is usually right.
Check the ordinary explanations, in order, and stop when one of them accounts for what you are seeing. Most of the time the second one does.
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.
Useful. I had the fact and not the reason, which turns out to be the important half.
I read post #12 twice before replying, because I had assumed the opposite.
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.
The right answer here may simply be that it has not been measured.
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.
Narrowing post #14, because the general version has more than one answer.
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.
I would want the raw data before agreeing with my own summary of it.
Everything in post #16 holds. The case it does not cover is the one I have.
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.
Worth saying I have only my own numbers here, and n is small.
Two claims get bundled together under Carryover 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.
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.
Worth checking against a second source before it gets quoted onward.
I read post #17 twice before replying, because I had assumed the opposite.
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.
A single observation, in a thread that deserves better than single observations.
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.
A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it.
Worth reading the earlier posts in this thread before acting on mine.
Coming back to post #21, because the follow-up matters more than the original answer.
The useful distinction on Carryover 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 #25 is right about the mechanism and I think understates the practical bit.
What would change my mind on Carryover is a second dataset collected by someone with no stake in the first. Until then I hold it loosely and I would rather say so than pretend to more.
Nothing to add, except that this is the answer I would give if asked.
I disagree with the framing of Carryover 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.
Source for the Carryover figure, since it was asked for. It is in the discussion rather than the abstract, which is why the version circulating is stronger than the paper is.
Reading the surrounding paragraph is worth the two minutes. The authors are more careful than their summarisers.