This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
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).
On post #29 — agreed on the reasoning, with one qualification.
Specificity looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
I would keep Specificity and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
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.
Adding the caveat now so it does not have to be extracted later.
Post #33 is right about the mechanism and I think understates the practical bit.
Small methodological point on Specificity: repeating a measurement is cheap and resolves most of what is being argued about here at no cost to anyone.
Reading back through, this was answered upthread and I missed it. My fault.
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 is the version I would want a new member to read first.
Taking post #38 at face value and following it one step further.
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.
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.
That has held every time I have looked, which is not the same as always.
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.
Appreciated. The plain phrasing does more work here than a longer post would.
Worth separating two things that post #44 runs together.
The documentation on Specificity is better than this thread and I say that as someone who has posted in the thread.
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.
If it helps: the failure mode here is usually boring rather than dramatic.
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.
On post #48 — agreed on the reasoning, with one qualification.
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 version of Specificity that circulates here is a simplification of a simplification. It is not wrong, but it has lost the conditions under which it holds, and those conditions are where the interesting cases live.
Post #51 is the version of this I will quote in future. One addition.
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.
Agreed on Specificity, with one qualification that I think matters. The reasoning holds for the case as described. Change the starting assumption and it does not, and the starting assumption is the part nobody states.
Post #51 put the caveat in the right place and I want to underline it.
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.
Building on post #55 rather than restating it.
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.
Post #55 and I disagree about the size of the effect, not about the direction.
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.
Useful. I had the fact and not the reason, which turns out to be the important half.