Why one laboratory's 98.4% and another's 96.1% may not contradict — one year on

Asking directly, because I could not find a straight answer: Why one laboratory's 98.4% and another's 96.1% may not contradict — one year on

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

• Column: C18, 3.0 x 150 mm, 2.6 um
• Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile
• Gradient: 9% to 61% organic over 33 minutes
• Detection: 214 nm
• Injection: 18 uL
• Sample: retatrutide, reconstituted to 2.0 mg/mL, injected within an hour

The main peak integrates at 99.1% 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.

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.

I would treat that as a working assumption and revisit it.

The arithmetic in post #2 is right; the assumption feeding it is the part to check.

The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

One of those cases where knowing the mechanism does not help the decision.

Answering the question the opening post raises rather than the one it answers.

Validation is compound-specific and matrix-specific. A method validated for one peptide is a starting point for another and not a validated method for it.

Two people can read the same figure differently here and both be reasonable.

Sensible. I would want the same detail before I acted on it either.

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.

I would rather be precise about what I do not know than vague about what I do.

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.

Everything in post #4 holds. The case it does not cover is the one I have.

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.

Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.

I had written a reply contradicting post #8 and deleted it. Here is what survived.

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.

Not a strong opinion, just a consistent one.

Post #7 describes the usual case. This is about the unusual one.

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.

Adding the measurement that post #11 says would settle it.

The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

A stability-indicating method is one demonstrated to resolve the analyte from its degradation products, usually through forced degradation. Calling a method stability-indicating without that work is a claim rather than a property.

Appreciated. The plain phrasing does more work here than a longer post would.