When to suspect the method rather than the sample

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.

Confirming post #13 from a second method, which matters more than confirming it from a second person.

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.

No notes. Posting so the count is not one.

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

The mechanism is plausible, which is not the same as established.

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.

I am not the right person to answer the follow-up to this.

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.

That is the version I use. It may not be the version that is correct.

Post #18 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.

Where I part company with post #20, and it is a narrow parting.

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.

The answer changed when I changed how I was measuring, which was informative.

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.

The right answer here may simply be that it has not been measured.

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 short answer was in the first line; everything after is the working.

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.

I had written a reply contradicting post #24 and deleted it. Here is what survived.

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.

Picking up post #26: that is the part I would want checked first.

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.

That matches what I have seen, for whatever a single anecdote is worth.

Adding thanks rather than a view. I do not have a view worth the space.

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.