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.

Posting the method first, because I know what the first three replies will otherwise be.

• Column: C18, 2.1 x 100 mm, 1.7 um
• Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile
• Gradient: 12% to 46% organic over 22 minutes
• Detection: 220 nm
• Injection: 13 uL
• Sample: tirzepatide, reconstituted to 1.0 mg/mL, injected within an hour

The main peak integrates at 98.3% of total area. There is a small feature on the trailing edge that I cannot decide is a shoulder or a baseline artefact, and that is what I am actually asking about.

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).

Coming back to post #42, because the follow-up matters more than the original answer.

Why two laboratories may disagree: after validating the same method, they may still report different purity on the same sample due to integration differences, column age differences, subtle differences in mobile phase pH or temperature. This is normal and not a sign that one is wrong.

This follows post #49 rather than contradicting it.

Why two laboratories may disagree: after validating the same method, they may still report different purity on the same sample due to integration differences, column age differences, subtle differences in mobile phase pH or temperature. This is normal and not a sign that one is wrong.

I disagree with the reply above, and I think the disagreement is substantive rather than terminological.

The distinction being drawn does not survive when you look at the published data for this specific question. I would be glad to be shown wrong on this, because the version I am arguing against is more convenient.

Two things before anyone answers the substance.

First, the context in the first post is clear and specific. Second, the question is framed so that an answer can actually address it. Both are the norm here and both matter more than they sound.

Picking up post #101: that is the part I would want checked first.

For anyone arriving from a search: the marked solution above is the direct answer, and the replies underneath it add the caveats that make it safe to use.

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