Area percent versus weight percent: the confusion that causes most arguments — what changed since

Area percent versus weight percent: the confusion that causes most arguments — what changed since Writing it up because I had to work it out twice and would rather nobody else did.

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

• Column: C18, 4.6 x 250 mm, 5 um
• Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile
• Gradient: 12% to 56% organic over 21 minutes
• Detection: 220 nm
• Injection: 18 uL
• Sample: retatrutide, reconstituted to 1.0 mg/mL, injected within an hour

The main peak integrates at 96% 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.

post #9 is right about the mechanism and I think understates the practical bit.

On integration: where the baseline is drawn matters more than most people realise. On a clean chromatogram with well-resolved peaks the choice is inconsequential. On a chromatogram with a trailing shoulder or a rising baseline it matters. Differences of one to two percentage points between defensible integrations are ordinary.

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.

Picking up post #25: that is the part I would want checked first.

Detection wavelength: 214 nm detects the peptide bond and is relatively insensitive to composition. 280 nm detects aromatic residues and is strongly composition-dependent. Area percent at one wavelength is not area percent at the other.

Column chemistry and particle size: smaller particles (1.7 μm) give better resolution and higher efficiency than larger particles (3.5 μm or 5 μm), at the cost of higher back pressure. Newer methods increasingly use smaller particles.