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Why are peptides measured at 214 nm?
The short answer
Because the peptide bond itself absorbs strongly near 214 nm, so every peptide responds regardless of its amino-acid composition. At 280 nm only aromatic residues absorb, so peptides lacking tryptophan, tyrosine or phenylalanine become nearly invisible. Measuring at 214 nm gives a comparable response across the whole class, which is what a general method needs.
In detail
The trade-off is that many solvents and additives also absorb at low wavelengths, which raises baseline noise and constrains which mobile phases can be used.
This is also why the wavelength belongs on the certificate. A purity percentage measured at 280 nm is not comparable to one measured at 214 nm, and an impurity that is invisible at one may be prominent at the other.
When comparing two laboratories' purity figures, confirm the wavelength before treating the numbers as equivalent.
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