Specificity, linearity, accuracy, precision, range, robustness — with real criteria Writing it up because I had to work it out twice and would rather nobody else did.
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.
The opening post is right about the mechanism and I think understates the practical bit.
My position on specificity is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.
If you are new and reading this thread for the answer to specificity: the answer is conditional, the conditions are in the third reply, and the rest of the thread is worth skipping.
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.
If this contradicts something upthread, the upthread version may well be the better one.
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.
The strength of my opinion here exceeds the strength of my evidence.
Coming back to post #5, because the follow-up matters more than the original answer.
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.
Flagging that the sources on this are thinner than the confidence in the thread suggests.
The bit of specificity that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
Clear enough that I do not think I have a follow-up, which is unusual.
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 disagreement above is smaller than it looks once the terms are fixed.
Post #10 answers the question as asked. The question underneath it is different.
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.
Filing this under things that are true until someone shows me otherwise.
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.
It took me longer than it should have to see that.
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.
Adding a note of thanks rather than an opinion. I did not know most of that.
Seconded. It reads as careful rather than confident, which is the right register.
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 kept the units in throughout, for the obvious reason.
I read post #16 twice before replying, because I had assumed the opposite.
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.
Genuinely open to being wrong about this one.
Good question, well framed, and I would like to see it answered properly.
Staff rationale: Hidden by community flags. The claim about a named supplier was not accompanied by a batch, a date, a method or a document, which R6 requires.
Worth separating two things that post #17 runs together.
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.
The corresponding entry is in the public moderation log. Hidden posts are never deleted.
Post #19 is right about the mechanism and I think understates the practical bit.
The most useful reply I ever got about specificity was a request to state my units. It sounds like pedantry and it has saved me twice.
Right — I had this wrong and I am glad to have read it before it mattered.
Noted, and thank you for writing it out rather than summarising it.
Coming back to post #25, because the follow-up matters more than the original answer.
Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.
Scoping that to what I have actually seen rather than what I have read.
I read the earlier replies on specificity twice before writing this, because I had assumed the opposite and wanted to be sure I was disagreeing with what was said rather than what I expected.
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.