Why HPLC Uses UV Detection at 220nm for Peptides
The peptide bond absorbs ultraviolet light strongly near 220nm — which is why peptide purity testing tunes the detector there instead of the more familiar 280nm. Here is the chemistry behind the number on every COA.
If you have ever read a peptide Certificate of Analysis, you have seen the phrase "Detection: 220 nm" tucked into the chromatogram header. It looks like an arbitrary instrument setting. It is not. That single number reflects a specific piece of chemistry — the way the peptide bond itself absorbs ultraviolet light — and understanding it tells you a great deal about what an HPLC purity number can and cannot prove.
This guide explains why 220nm is the default detection wavelength for peptide HPLC, why the more famous 280nm is the wrong choice for most research peptides, and what the wavelength tells you when you are evaluating a supplier's documentation. It builds on the broader method overview in What Is HPLC? — start there if you want the full instrument picture.
For laboratory research use only. Not for human consumption.
The detector needs something to "see"
An HPLC instrument separates a sample in the column, then sends the eluting stream past a detector. For peptide work the detector is almost always a UV absorbance detector: it shines ultraviolet light through the flowing liquid and measures how much is absorbed at a chosen wavelength. Whenever a molecule that absorbs at that wavelength passes through, absorbance rises and the chromatogram shows a peak.
So the detector can only register what absorbs at the wavelength it is set to. Choosing the wavelength is choosing what the instrument is sensitive to. For peptides, the goal is to pick a wavelength where the peptide reliably and strongly absorbs — ideally one that works regardless of the peptide's particular amino acid sequence.
The peptide bond is the universal chromophore
Every peptide, by definition, is a chain of amino acids linked by amide bonds — the peptide bonds. That backbone amide group has an electronic transition that absorbs strongly in the far ultraviolet. The absorbance maximum sits near 190nm, with substantial absorbance still present on the shoulder around 215 to 220nm.
The key property: this absorbance comes from the backbone, not the side chains. A peptide has one amide linkage for every residue it adds, so the 220nm signal scales with the length and amount of peptide present. A short peptide absorbs less per molecule than a long one, but the underlying chromophore is the same for all of them. That universality is exactly what a purity method wants — a signal that shows up for any peptide on the bench, whether it is a five-residue fragment or a forty-residue chain.
220nm detection works because it targets the peptide bond — the one structural feature every peptide shares. The signal is proportional to backbone amount, so it reports peptide material generally rather than depending on any specific residue being present.
Why not 280nm, the wavelength most people know
Biochemists often associate protein detection with 280nm, and many lab spectrophotometers default there. 280nm works by detecting the aromatic side chains of three amino acids: tryptophan, tyrosine, and to a weaker degree phenylalanine. Their ring structures absorb near 280nm, which makes that wavelength convenient for quantifying proteins that contain plenty of those residues.
The problem for research peptides is that many of them contain few aromatic residues — or none at all. A peptide with no tryptophan, tyrosine, or phenylalanine is essentially invisible at 280nm. Detecting there would either miss the compound entirely or report a signal driven by trace aromatic impurities rather than the peptide itself. Because 220nm reads the backbone instead of the side chains, it sees the peptide regardless of sequence. That is the decisive reason peptide HPLC standardizes on the far-UV band rather than 280nm.
Why 220 and not 190, where absorbance peaks
If the amide absorbance maximum is near 190nm, why not detect there for maximum sensitivity? Because the solvents get in the way. The mobile phases used in reversed-phase peptide HPLC — water, acetonitrile, and acidic modifiers such as trifluoroacetic acid — all absorb increasingly as wavelength drops below roughly 210nm. That background raises the baseline and the noise floor, swamping small peaks and making the chromatogram harder to integrate cleanly.
The 215 to 220nm band is the practical compromise. It is low enough that the peptide bond still absorbs strongly, and high enough that solvent and additive background stays manageable. Different labs land on slightly different values inside this window — 214nm, 215nm, 220nm are all common — which is why the exact figure on a COA varies even though the underlying logic is identical.
What this means when you read a COA
Three practical takeaways for evaluating supplier documentation:
- The wavelength must be stated. A legitimate chromatogram header lists the detection wavelength alongside the column and mobile phase. A chromatogram with no stated wavelength is missing information needed to reproduce the result — one of the checks in our visual guide to reading a chromatogram.
- A figure in the 210–220nm band is normal and expected. If a peptide COA claims detection at 280nm, ask why — that wavelength is a poor fit for sequence-agnostic peptide purity work and may indicate a generic template copied from non-peptide analysis.
- 220nm quantifies; it does not identify. Absorbance at 220nm tells you peptide material is present and roughly how much, by integrated peak area. It does not confirm which peptide. That confirmation comes from retention time and, definitively, from mass spectrometry. This is why a strong, clean 220nm peak at the wrong retention time is still a red flag.
Bottom line
220nm is not an arbitrary setting — it is the wavelength where the peptide bond, the one chromophore every peptide shares, absorbs strongly enough to detect and high enough to stay above solvent background. That is what makes it the universal choice for peptide purity work, where 280nm would miss any compound short on aromatic residues.
Knowing this sharpens how you read a COA. The detection wavelength should be present, it should sit in the far-UV band, and the peak it produces is a measure of how much peptide eluted — not proof of what that peptide is. For the compound-level catalog those purity numbers feed into, see our peptides catalog and the broader research library. And before trusting any vendor's chromatogram, walk it through our reading-a-chromatogram guide.
For research use only. Not for human consumption.
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