On the subject in the title: Specificity, linearity, accuracy, precision, range, robustness — with real criteria Working notes rather than a conclusion.
Trying to establish the boundary conditions on Specificity. The general claim is fine; I want to know where it stops holding, because that is where I am operating.
Three edge cases below, in increasing order of how far outside the usual range they sit.
Coming back to the opening post, because the follow-up matters more than the original answer.
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.
Post #2 answers the question as asked. The question underneath it is different.
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 read post #4 twice before replying, because I had assumed the opposite.
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.
Understood, and I withdraw the assumption I opened with.
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).
That is the shape of it. The detail is where I would expect to be corrected.
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 literature is thinner on this than the confidence in the thread implies.
Accuracy for a purity assay is usually demonstrated by spike recovery across a range. Without a reference standard, that is exactly the work that cannot be done, which is why reference material matters.
Precision has two levels worth distinguishing: repeatability within a run and intermediate precision across days, analysts and instruments. The second is where most methods lose the numbers people quote.
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 would rather say I do not know than round it up to an answer.
Everything in post #12 holds. The case it does not cover is the one I have.
Linearity: the detector response is proportional to compound concentration across the working range. Demonstrated by running standards at multiple concentrations and showing R-squared values typically ≥0.99.
It is the kind of thing that is obvious once and never again.
Narrowing post #15, because the general version has more than one answer.
The number people quote for Specificity is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.
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 sort of exchange that makes the archive worth searching.
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.
Forced degradation under acid, base, oxidation, heat and light is how specificity is actually demonstrated. The chromatograms from that work are more informative than the release data.
Narrowing post #21, because the general version has more than one 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.
The disagreement above is smaller than it looks once the terms are fixed.
Everything in post #22 holds. The case it does not cover is the one I have.
Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.
That distinction has done more work for me than anything else in this category.
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.
No notes. Posting so the count is not one.
Post #24 answers the question as asked. The question underneath it is different.
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.
If that is already documented somewhere, ignore me and link it.
Confirming post #28 from a second method, which matters more than confirming it from a second person.
I have no financial interest in anything named in this thread and I want to say so before I comment on Specificity, because it is the sort of subject where it matters.
I had written a reply contradicting post #27 and deleted it. Here is what survived.
Practical answer on Specificity, since the theoretical one is upthread: do the simplest check first, write down the result, and only then decide whether the complicated explanation is needed. It usually is not.