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.
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Analytics · HPLC & UHPLC · continued
[2026 update] Detection wavelength and why 214 nm and 280 nm disagree posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Useful. I had the fact and not the reason, which turns out to be the important half.
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cannula_drift, post #4: The arithmetic in post #2 is right; the assumption feeding it is the part to check. The honest answer on Detection wavelength is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter. Most people get the first two right and then argue about the fourth. Go to post
Before the thread moves on from Detection wavelength — what is the sample size behind the claim? I am not being difficult; I have seen the same figure quoted from an n of four and from an n of four hundred.
This follows post #34 rather than contradicting it.
Gradient delay volume differs between instruments and shifts the whole chromatogram. It is why a transferred method rarely reproduces retention times exactly on a different system.
Anyone with a larger sample, please post it.
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Worth separating two things that post #32 runs together.
Particle size and column dimensions determine what resolution is achievable at all. A 5 micrometre 250 millimetre column and a sub-2 micrometre 100 millimetre column are different instruments in practice.
This is where my knowledge stops and I would rather mark the edge than blur it.
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Where I have landed on Detection wavelength, having got it wrong once in public: the direction is clear, the magnitude is not, and anyone quoting a precise magnitude has borrowed it from somewhere that did not measure it.
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m.coelho, post #8: A relative retention time against a known peak travels much better than an absolute one, and almost nobody reports it. I have written this out at length because the short version keeps being misread. Go to post
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.
PSundberg, post #25: Detection wavelength looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour. Go to post
Carryover from a previous injection shows up as a small peak at the same retention time in a blank. A method report that includes a blank injection is telling you the analyst checked.
I am reporting what happened, not recommending it.
That reframing is the whole thing. The facts I already had.
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p.lindqvist, post #16: The arithmetic on Detection wavelength is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it. Go to post
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.
I read post #42 twice before replying, because I had assumed the opposite.
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.
Not a conclusion. A place to stand while looking for one.
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Collapsed as off-topic by two members at trust level 3 or above
Post #42 answers the question as asked. The question underneath it is different.
Detection wavelength came up in a thread eighteen months ago and was answered well. I cannot find it, which is itself the problem, so here is the reconstruction.
Counterpoint on Detection wavelength, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.
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a.nyberg, post #22: Careful with the language on Detection wavelength. "Not detected" and "not present" are different findings and the first is a statement about the method. Go to post
This follows post #45 rather than contradicting it.
Purity by chromatography answers "what proportion of what I detected is the intended species". It does not answer how much is in the vial, which is a separate assay.
Noting that the question and the thing people usually mean by it are different.
Coming back to post #45, because the follow-up matters more than the original answer.
Trifluoroacetic acid at 0.1 per cent is the near-universal ion-pairing additive for this work, and it also raises the baseline at 214 nanometres. That is why the same sample looks noisier at low wavelength.
Two people can read the same figure differently here and both be reasonable.
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On Detection wavelength I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
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Collapsed as off-topic by two members at trust level 3 or above
On post #45 — agreed on the reasoning, with one qualification.
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.
I would treat that as a working assumption and revisit it.
p.fontaine, post #45: Counterpoint on Detection wavelength, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out. Go to post
Picking up post #48: that is the part I would want checked first.
I would call the community position on Detection wavelength likely rather than established, and I would be comfortable defending that hedge.
a.mwangi, post #3: This is the answer, and the reason it is the answer is the more useful part. Go to post
Marking my place. If it changes for me I will come back and say so.
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.
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Coming back to post #50, because the follow-up matters more than the original answer.
Area percent is a proportion of absorbance, not a proportion of mass. Two species with different extinction coefficients at the detection wavelength contribute unequally to the total, and nothing on the certificate corrects for that.
That is my reading. Someone else read the same page differently and was reasonable.
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h.varga, post #34: Before the thread moves on from Detection wavelength — what is the sample size behind the claim? I am not being difficult; I have seen the same figure quoted from an n of four and from an n of four hundred. Go to post
Adding the measurement that post #52 says would settle it.
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.
Worth checking against a second source before it gets quoted onward.
Collapsed as off-topic by two members at trust level 3 or above
A chromatogram image at a resolution where you can see peak shape but not baseline detail is worth having and is not the same as the data. Ask for the integration table if the number matters.
Happy to expand any of that if it is the useful part.
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.
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quiet_lurker, post #23: 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. Where I would look next, rather than where I would stop. Go to post
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.
The arithmetic in post #57 is right; the assumption feeding it is the part to check.
Detection wavelength is one of those subjects where the general answer and the answer for a specific case diverge, and the thread will go in circles until someone says which one is being asked for.
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