[2026 update] Detection wavelength and why 214 nm and 280 nm disagree

Detection wavelength and why 214 nm and 280 nm disagree — 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: 8% to 70% organic over 31 minutes
• Detection: 214 nm
• Injection: 10 uL
• Sample: tirzepatide, reconstituted to 1.0 mg/mL, injected within an hour

The main peak integrates at 97.5% 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.

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.

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.

the opening post answers the question as asked. The question underneath it is different.

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.

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

Practical note that does not fit anywhere else. Whatever you conclude from this topic, write down what you did and when. The single most useful thing in your own records is not any individual result; it is that they are dated and consecutive.

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.

Before anything else: what was the gradient, and at what wavelength? Area percent at different wavelengths is not the same number even on the same sample because different species absorb differently at different wavelengths. With the method stated, I can tell you something useful. Without it, all I can say is that there is one large peak.

Coming back to post #7, because the follow-up matters more than the original answer.

Gradient slope is the single biggest driver of apparent purity differences. A shallower gradient over a longer run resolves more impurities and gives a higher purity figure. A steep gradient produces a tidier-looking chromatogram with fewer visible peaks and gives a lower purity figure. Both are legitimate methods and they will not produce the same number.

Picking up post #7: 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.

Picking up post #8: that is the part I would want checked first.

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.

Coming back to post #10, because the follow-up matters more than the original answer.

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.

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.

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.