Coming back to: When to suspect the method rather than the sample

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.

Answering the question post #12 raises rather than the one it answers.

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.

If anyone has run this properly I would rather read that than my own guess.

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.

Coming back to post #16, because the follow-up matters more than the original answer.

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.

For what it is worth, the same held on the two occasions I checked.

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.

This is the version I would want a new member to read first.

Nothing to add on the substance. Thank you for taking the question at face value.

I had written a reply contradicting post #21 and deleted it. Here is what survived.

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.

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.

Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.

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.

I would treat the number as indicative rather than as a measurement.

Where I part company with post #25, and it is a narrow parting.

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 have separated what I observed from what I concluded, which does not always happen.

Post #25 is the version of this I will quote in future. One addition.

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.

That is the version I would defend. It is not the version I started with.

Post #25 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.

The distinction between a qualified instrument and a validated method is worth keeping. Both are needed and they fail in different ways.