Coming back to: When to suspect the method rather than the sample posts 91–120
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.
Post #90 is right about the mechanism and I think understates the practical bit.
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.
Coming back to post #92, because the follow-up matters more than the original answer.
Robustness: the method gives consistent results when minor parameters vary. Tested by deliberately varying pH, temperature, flow rate, and mobile phase composition within reasonable ranges and demonstrating that results stay within acceptance.
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.
I would call that likely rather than established.
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.
Post #94 answers the question as asked. The question underneath it is different.
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.
If anyone can point at the primary source I would be grateful.
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.
None of the above is medical advice and I am not qualified to give any.
Building on post #98 rather than restating it.
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.
Post #96 put the caveat in the right place and I want to underline it.
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.
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.
Noting that the question and the thing people usually mean by it are different.
This is the answer, and the reason it is the answer is the more useful part.
Collapsed as off-topic by two members at trust level 3 or above
Narrowing post #100, because the general version has more than one answer.
Forced degradation studies: deliberately stress the material with acid, base, oxidant, heat, light to generate degradation products and demonstrate that the method can separate them from the parent peak. Acceptance is that the method is stability-indicating.
Everything in post #101 holds. The case it does not cover is the one I have.
Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.
A guess, clearly labelled as one.
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.
Post #104 and I disagree about the size of the effect, not about the direction.
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.
Post #104 answers the question as asked. The question underneath it is different.
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 claim is narrower than it sounds, and deliberately so.
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.
That holds for the case as described. Change the assumptions and it may not.
Right, and stated more narrowly than I would have dared to state it.
Worth separating two things that post #106 runs together.
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).
I am not the right person to answer the follow-up to this.
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.
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.
Two sources, same conclusion, and I could not rule out that one copied the other.
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 confident version of this sentence would be wrong, so here is the hedged one.
This follows post #113 rather than contradicting it.
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 keep a log of this specifically because memory is unreliable about it.
I read post #113 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.
The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.
Worth checking against a second source before it gets quoted onward.
Marking my place. If it changes for me I will come back and say so.
Right — I had this wrong and I am glad to have read it before it mattered.