Relative retention time as a better inter-laboratory comparator — does this still hold?

Relative retention time as a better inter-laboratory comparator — does this still hold? I have a specific reason for asking rather than idle curiosity, and the context is below.

A documentation question rather than an analytical one.

I have a certificate in front of me that reports a purity figure, names a technique, gives a wavelength, and stops. No gradient, no column, no injection volume, no chromatogram.

What can I legitimately conclude from that document? My instinct is "almost nothing, but not literally nothing", and I would like to know where the people who read these professionally draw the line.

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).

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

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.

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

I read post #5 twice before replying, because I had assumed the opposite.

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.

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.

On post #5 — agreed on the reasoning, with one qualification.

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.

post #9 answers the question as asked. The question underneath it is different.

Thank you for the correction. I have edited my earlier post with a note rather than silently, so the thread still makes sense to read. The error was mine and it was the kind that comes from remembering a figure instead of looking it up.

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

Worth separating two things that post #8 runs together.

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.

System suitability: injections run at the start of a batch to establish that the instrument and column are performing. Acceptance criteria typically include replicate precision (RSD ≤2%), peak tailing (0.8–1.5), theoretical plates (>2000), and resolution (>1.5).

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.