Relative retention time as a better inter-laboratory comparator

On the subject in the title: Relative retention time as a better inter-laboratory comparator Working notes rather than a conclusion.

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: 14% to 47% organic over 24 minutes
• Detection: 220 nm
• Injection: 17 uL
• Sample: retatrutide, reconstituted to 1.0 mg/mL, injected within an hour

The main peak integrates at 96.7% 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.

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.

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.

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.

This follows post #6 rather than contradicting it.

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).