Where the integrator drew the baseline, and how much it moved the number

Where the integrator drew the baseline, and how much it moved the number — setting out what I have, and where I think it stops being reliable.

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

• Column: C8, 4.6 x 150 mm, 3.5 um
• Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile
• Gradient: 13% to 57% organic over 22 minutes
• Detection: 214 nm
• Injection: 20 uL
• Sample: tirzepatide, reconstituted to 0.5 mg/mL, injected within an hour

The main peak integrates at 98% 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.

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.

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.

I read post #3 twice before replying, because I had assumed the opposite.

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.

This follows post #3 rather than contradicting it.

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.

This is why a purity figure without the underlying chromatogram is weaker evidence than it appears. It is also why two competent laboratories can report different numbers on the same vial without either being wrong.

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.

Area percent is not mass percent. It is a proportion of absorbance, weighted by each species' extinction coefficient. For closely related impurities the approximation is usually good. For structurally dissimilar impurities it can be poor.

System suitability testing: injections run before and during the sample run to establish whether the instrument, column and method were performing when the sample was analysed. If suitability did not pass, the sample results from that run are uninterpretable.

Method validation is the demonstration that a method can separate the compound from its degradation products and impurities reliably. A method that cannot resolve an impurity from the parent peak will not detect that impurity.

Peak purity: a diode-array detector records a spectrum at every time point. If a peak contains two co-eluting species with different spectra, the spectrum changes across the peak. A passing peak-purity result says the spectrum is constant; it is weak evidence of homogeneity if the impurities have similar spectra.

On post #10 — agreed on the reasoning, with one qualification.

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.