When to suspect the method rather than the sample

When to suspect the method rather than the sample — that is the question, and I have not found it answered plainly anywhere I have looked.

Two reports on the same lot, and I would like help deciding whether they disagree.

The first gives 96.6% by reversed-phase at 220 nm. The second gives 98.6% by a method described only as HPLC. Neither states the integration convention.

My instinct is that this gap is method rather than material. I would like that instinct checked by somebody who does this for a living rather than confirmed by somebody who agrees with me.

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.

A partial answer, offered because a partial answer beats none.

Understood. Thank you for being specific about the limits of it.

Building on post #50 rather than restating it.

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.

Written quickly, so the reasoning may be tighter than the wording.

Range: the concentration range over which the method has been validated. Going outside the validated range is going outside the method's demonstrated performance.

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.

Narrowing post #102, because the general version has more than one answer.

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.

That is my reading. Someone else read the same page differently and was reasonable.

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.