What a stability-indicating method is and how you demonstrate one — one year on posts 31–60

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 most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

Post #43 put the caveat in the right place and I want to underline it.

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.

Adding it because I spent an afternoon working it out and nobody should have to twice.

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.

The claim is narrower than it sounds, and deliberately so.

Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.

I would put the burden of proof on the interesting explanation, not the dull one.

Post #49 answers the question as asked. The question underneath it is different.

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.

Take it as a starting point and not as a specification.

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.

One more caveat and then I will stop qualifying: the sample selected itself.

Worth separating two things that post #52 runs together.

Validation is compound-specific and matrix-specific. A method validated for one peptide is a starting point for another and not a validated method for it.

Post #54 is right about the mechanism and I think understates the practical bit.

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.

Coming back to post #52, 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.

I would be interested in a counterexample if anyone has one.

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.

The honest answer is that it depends, and here is what it depends on.

The useful distinction on stability-indicating method 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.

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.