This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Two claims get bundled together under Specificity and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
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.
That has been true for the cases I have seen and I have not seen many.
Post #61 answers the question as asked. The question underneath it is different.
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.
I have changed my mind on this once already, so take it as current rather than settled.
Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.
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.
Narrowing post #64, because the general version has more than one 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.
On reflection I would soften that slightly.
Everything in post #66 holds. The case it does not cover is the one I have.
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 evidence for this is thinner than the way I have phrased it suggests.
Post #68 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.
Small point, but it is the one that usually catches people.
Following, with nothing to contribute beyond having asked the same thing elsewhere.
Post #69 is the version of this I will quote in future. One addition.
Whatever the answer on Specificity turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, invite the correction.
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.
I would rather post the uncertainty than round it away.
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 general case is well covered; this is the awkward specific one.
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.
Happy to be the one who is wrong here if it settles the question.
Thank you for taking the time. That was more work than a reply usually is.
Answering the question post #73 raises rather than the one it answers.
On Specificity, I would rather understate and be corrected upward than overstate and be quoted. That is a house style here and it is a good one.
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.
I have written this out at length because the short version keeps being misread.
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.
Written quickly, so the reasoning may be tighter than the wording.
This follows post #78 rather than contradicting 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.
One case, stated as one case.
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).
Stating my assumptions rather than smuggling them in.
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.
I would call the community position on Specificity likely rather than established, and I would be comfortable defending that hedge.
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.
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.
This settles it for me, at least until somebody posts a reason it should not.
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.
Post #89 describes the usual case. This is about the unusual one.
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).
Someone will know this better than I do and I hope they say so.