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.
The arithmetic in post #28 is right; the assumption feeding it is the part to check.
Sample stability during analysis is part of validation and is routinely ignored. A preparation that degrades in the autosampler over eight hours produces a sequence-dependent result.
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 disagreement above is smaller than it looks once the terms are fixed.
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.
Caveat: everything above assumes the paperwork is what it says it is.
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 step people skip is the one I have spelled out.
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.
The conclusion is tentative; the arithmetic underneath it is not.
Reading rather than contributing, but this is the most useful thread I have found on it.
Post #34 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.
On post #38 — agreed on the reasoning, with one qualification.
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.
Post #39 put the caveat in the right place and I want to underline it.
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 left out the parts I could not verify.
Building on post #39 rather than restating it.
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.
Nothing above should be read as advice about what anyone else should do.
Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.
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.
Reading it again, the caveat matters more than the finding.
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.
Scoping that to what I have actually seen rather than what I have read.
Post #43 is the version of this I will quote in future. One addition.
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.
Happy to expand any of that if it is the useful part.
That reframing is the whole thing. The facts I already had.
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.
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.
Narrowing post #50, because the general version has more than one answer.
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.
I looked this up rather than remembered it, which is the right order.
I had read the opposite somewhere and cannot now find where, which tells me something.
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.
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.
Everything in post #53 holds. The case it does not cover is the one I have.
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 honest answer is that it depends, and here is what it depends on.
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.
Good question, well framed, and I would like to see it answered properly.
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.