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.
This follows post #89 rather than contradicting 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.
One of those cases where knowing the mechanism does not help the decision.
My experience of stability-indicating method 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.
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 would treat that as a working assumption and revisit it.
Taking post #94 at face value and following it one step further.
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.
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.
The literature is thinner on this than the confidence in the thread implies.
I read post #97 twice before replying, because I had assumed the opposite.
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.
I would rather be precise about what I do not know than vague about what I do.
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.
Adding the measurement that post #100 says would settle it.
Precision and repeatability: within-run and between-run variability of the method. Acceptance criterion is typically a relative standard deviation of ≤2% for area measurements.
The practical version of stability-indicating method is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
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.
That is where I would start, not where I would stop.
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).
Happy to be the one who is wrong here if it settles the question.
Accuracy: the method measures what you intend to measure. For purity methods, this is tested by spike-and-recover experiments: add a known amount of impurity to a sample and measure whether you recover the added amount.
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 is what the documentation says. What happens in practice is usually close.
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.
If that is already documented somewhere, ignore me and link it.
Post #106 put the caveat in the right place and I want to underline it.
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.
The strength of my opinion here exceeds the strength of my evidence.
Useful. I had the fact and not the reason, which turns out to be the important half.
On post #106 — agreed on the reasoning, with one qualification.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
I have deliberately not rounded that, because the rounding is where the argument starts.
Post #110 describes the usual case. This is about the unusual one.
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.
Adding the measurement that post #110 says would settle it.
Offering a way to settle stability-indicating method rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
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.
Anyone with a larger sample, please post it.
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