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.
I had written a reply contradicting post #27 and deleted it. Here is what survived.
Stability-indicating method: one that can separate a compound from its degradation products. Critical for assay methods that claim to measure actual degradation (as opposed to purity, which is orthogonal).
Confirming post #30 from a second method, which matters more than confirming it from a second person.
A stability-indicating method is one demonstrated to resolve the analyte from its degradation products, usually through forced degradation. Calling a method stability-indicating without that work is a claim rather than a property.
System suitability is the ongoing evidence that a validated method is still performing. Validation is done once; suitability is done every run, and it is the one that appears on a certificate.
I would want to see it done twice before believing it once.
A method transferred between laboratories needs a demonstration that it performs equivalently, not just a document describing it. Transfer is where a great many between-laboratory disagreements originate.
Reading it back, the second half matters more than the first.
Coming back to post #32, because the follow-up matters more than the original answer.
Why two laboratories may disagree: after validating the same method, they may still report different purity on the same sample due to integration differences, column age differences, subtle differences in mobile phase pH or temperature. This is normal and not a sign that one is wrong.
Transfer between laboratories: a method can be transferred from one lab to another, but the receiving lab needs to demonstrate that they can achieve the same performance. This requires comparative testing and sometimes small method refinements.
A partial answer, offered because a partial answer beats none.
A stability-indicating method is one demonstrated to resolve the analyte from its degradation products, usually through forced degradation. Calling a method stability-indicating without that work is a claim rather than a property.
The uncertainty is in the assumption, not in the calculation.
Specificity is the first question: does the method separate the analyte from everything reasonably expected to be present? A method that has not been challenged with its own degradation products has not answered it.
The short version is the first sentence; the rest is why.
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.
Marking that as an opinion rather than a finding.
Sample stability during analysis is part of validation and is routinely ignored. A preparation that degrades in the autosampler over eight hours produces a sequence-dependent result.
I would put the burden of proof on the interesting explanation, not the dull one.
This follows post #39 rather than contradicting it.
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.
Anyone with a larger sample, please post it.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
The reasoning is more useful than the number, which is why I have shown it.
Documented validation is what separates a number from an opinion expressed numerically. That is the whole reason to ask for the procedure identifier rather than the method name.
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.
Post #46 answers the question as asked. The question underneath it is different.
The distinction between a qualified instrument and a validated method is worth keeping. Both are needed and they fail in different ways.
Where I would look next, rather than where I would stop.
I read post #46 twice before replying, because I had assumed the opposite.
A method that reports the same number for every lot is worth a second look. Real processes vary, and a total absence of variation is a statement about the measurement rather than the process.
That much is documented. The rest is how I have interpreted it.
Reading rather than answering, but this is the post I would point somebody at.
Post #46 put the caveat in the right place and I want to underline it.
Reporting a result to more decimal places than the method's precision supports is a small dishonesty that appears everywhere. A method with a two per cent relative standard deviation does not support a figure quoted to a hundredth.
The conclusion is tentative; the arithmetic underneath it is not.
Range and working range are different things and a certificate rarely distinguishes them. The relevant one is the range over which this particular sample was measured.
I have written this out at length because the short version keeps being misread.
Building on post #50 rather than restating it.
System suitability is the ongoing evidence that a validated method is still performing. Validation is done once; suitability is done every run, and it is the one that appears on a certificate.
Written quickly, so the reasoning may be tighter than the wording.
Robustness testing deliberately varies the parameters most likely to drift — organic percentage, pH, temperature, flow — and shows the result does not. It is the part of validation that predicts whether a method will transfer.
Linearity across the working range is a routine demonstration and it constrains how far a result can be extrapolated. A method linear from 80 to 120 per cent of nominal says nothing about a sample at ten per cent.
Two people can read the same figure differently here and both be reasonable.
Post #53 and I disagree about the size of the effect, not about the direction.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
That is the shape of it. The detail is where I would expect to be corrected.
Thank you for taking the time. That was more work than a reply usually is.
An independent laboratory's method being different from the supplier's is not a discrepancy. It becomes one only when the results differ by more than both methods' demonstrated precision.
If anyone has run this properly I would rather read that than my own guess.
Specificity: the method can distinguish the intended compound from related impurities and degradation products. Tested by comparing results on pure compounds, mixtures of compounds, and degraded samples.
Not a conclusion. A place to stand while looking for one.
Stability-indicating method: one that can separate a compound from its degradation products. Critical for assay methods that claim to measure actual degradation (as opposed to purity, which is orthogonal).
Limits of detection and quantitation: LOD is the lowest concentration that produces a signal above background. LOQ is the lowest concentration at which the method meets precision and accuracy acceptance criteria. Both are determined empirically.