I had written a reply contradicting post #5 and deleted it. Here is what survived. The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it. Scoping that to what I have actually seen rather than what I have read.
Relative retention time as a better inter-laboratory comparator — does this still hold? I have a specific reason for asking rather than idle curiosity, and the context is below.
What changes if the standard account of Relative retention time 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.
Where I part company with the opening post, and it is a narrow parting.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
I would want a second opinion before relying on that.
The opening post is the version of this I will quote in future. One addition.
A request rather than an answer: could whoever has the primary source for Relative retention time post it? I have seen the claim three times this month and each version had lost a qualifier.
Two claims get bundled together under Relative retention time 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.
Useful. I had the fact and not the reason, which turns out to be the important half.
I had written a reply contradicting post #5 and deleted it. Here is what survived.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
Scoping that to what I have actually seen rather than what I have read.
Everything in post #5 holds. The case it does not cover is the one I have.
Relative retention time has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.
Narrowing post #9, because the general version has more than one answer.
The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.
Second-hand, so weight it accordingly.
Everything in post #8 holds. The case it does not cover is the one I have.
Nobody has said the unglamorous part of Relative retention time yet, so: most of the variation is explained by things that are boring to write about and easy to check.
My experience of Relative retention time 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.
Small correction to my own earlier position on Relative retention time. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
Peak purity assessment using a diode-array detector checks whether a peak is spectrally homogeneous. It is a weak test and it is better than not looking.
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