System suitability injections: if they failed, the run did not happen

On post #2 — agreed on the reasoning, with one qualification.

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.

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

post #13 is right about the mechanism and I think understates the practical bit.

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.

This follows post #18 rather than contradicting it.

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.

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.