This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Post #60 is right about the mechanism and I think understates the practical bit.
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.
Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.
Carryover has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.
I think the carryover question is answerable and has not been answered, which is a more optimistic position than most of this thread.
On post #62 — agreed on the reasoning, with one qualification.
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.
Post #64 answers the question as asked. The question underneath it is different.
On carryover, 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.
Helpful, and easy to find again, which is half of what a good reply is.
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.
Old habit: I write down the expected answer before I calculate it.
I would rather this thread reach "we do not know" about carryover than reach a confident answer that nobody can support when asked.
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.
Where I part company with post #67, and it is a narrow parting.
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.
System suitability testing: injections run before and during the sample run to establish whether the instrument, column and method were performing when the sample was analysed. If suitability did not pass, the sample results from that run are uninterpretable.
The evidence for this is thinner than the way I have phrased it suggests.
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.
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.
I would be cautious about generalising from the carryover example above. It is a good example. It is one example.
On post #73 — agreed on the reasoning, with one qualification.
Where the baseline is drawn is the least documented and most consequential choice in the whole determination. Valley-to-valley, tangent skim and forced-to-zero can span a couple of percentage points on the same chromatogram.
Worth one more sentence than it usually gets.
Where I part company with post #77, and it is a narrow parting.
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.
That holds under the stated conditions and I have stated them.
Taking post #79 at face value and following it one step further.
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.
Post #80 and I disagree about the size of the effect, not about the direction.
A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it.
Take the reasoning and check the arithmetic; I do not always get it right.
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.
Area percent is a proportion of absorbance, not a proportion of mass. Two species with different extinction coefficients at the detection wavelength contribute unequally to the total, and nothing on the certificate corrects for that.
Same conclusion as the reply above, reached differently, which is mildly reassuring.
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.
The bit of carryover that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
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.
A shallow gradient resolves close-eluting species that a steep one merges. Two honest laboratories running different gradients can report genuinely different purities on the same vial.