On the subject in the title: Detection wavelength and why 214 nm and 280 nm disagree Working notes rather than a conclusion.
I would like to disagree carefully with the settled view on detection wavelength, and I would like to be argued out of it if the disagreement is bad.
The disagreement is about one step, not about the conclusion. If the step holds I withdraw it entirely.
Post #2 answers the question as asked. The question underneath it is different.
My experience of detection wavelength contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
I read the opening post twice before replying, because I had assumed the opposite.
A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it.
The confident version of this sentence would be wrong, so here is the hedged one.
Picking up post #2: that is the part I would want checked first.
Typical suitability criteria are a replicate area relative standard deviation below about two per cent, a tailing factor inside a defined window, a resolution minimum against a specified peak, and a plate-count floor.
That holds for the case as described. Change the assumptions and it may not.
The question underneath detection wavelength is usually "how would I tell?" rather than "what is true?", and that one has a method attached to it.
Write down what you would expect to see under each hypothesis before you collect anything. If they predict the same observation, collecting it will not help.
Coming back to post #4, because the follow-up matters more than the original answer.
On detection wavelength: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.
Adding the measurement that post #6 says would settle it.
Column temperature affects retention and selectivity and is omitted from most certificates. Two runs at twenty-five and forty degrees are not the same method.
The honest answer is that it depends, and here is what it depends on.
Post #8 describes the usual case. This is about the unusual one.
The confident answers on detection wavelength and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
One caution on detection wavelength: everything above assumes the underlying documentation is what it claims to be. That assumption is doing real work and is rarely stated.
Worth separating two things that post #12 runs together.
Change the wavelength and the proportions change even though the sample has not. That is the reason the wavelength has to be on the document for the number to mean anything.
Stating my assumptions rather than smuggling them in.
This follows post #12 rather than contradicting it.
Detection at 214 nanometres sees the peptide bond and therefore sees almost everything peptidic. At 280 it sees aromatic residues, so a peptide without tryptophan or tyrosine will look very different or not appear at all.
Nothing to add, except that this is the answer I would give if asked.
Post #14 is right about the mechanism and I think understates the practical bit.
The number people quote for detection wavelength is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.
On post #16 — agreed on the reasoning, with one qualification.
Detection wavelength was covered in the wiki last year and the page has a review date on it, which is a better starting point than my memory of a thread.
Reversed-phase separates on hydrophobicity. A peptide is retained on a non-polar stationary phase and eluted by increasing organic solvent. For peptides the mobile phase almost always contains an ion-pairing acid, typically 0.1% TFA, which suppresses secondary interactions and sharpens peaks.
Answering the question post #16 raises rather than the one it answers.
Careful with the language on detection wavelength. "Not detected" and "not present" are different findings and the first is a statement about the method.
If two laboratories disagree by more than two or three percentage points, work through method, integration, sample handling, whether it was the same lot and the same vial, and whether suitability passed. After all five, a gap needs an explanation.
Post #20 is the version of this I will quote in future. One addition.
Injection volume matters because column overload distorts peak shape, and an overloaded main peak can swallow a small neighbour. A certificate without injection volume is missing something load-bearing.
Reporting rather than recommending, on detection wavelength. What happened is above. Whether it should have is a different question and not one I am qualified to answer.
Answering the question post #23 raises rather than the one it answers.
Checked the detection wavelength claim against the primary source this morning. It survives, with a narrower scope than the version quoted here. Posting the narrower scope.
Confirming post #23 from a second method, which matters more than confirming it from a second person.
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.
I had written a reply contradicting post #23 and deleted it. Here is what survived.
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.
A note on how detection wavelength gets discussed rather than on detection wavelength itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
Post #26 and I disagree about the size of the effect, not about the direction.
What I can speak to on detection wavelength is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know.