LC-UV vs Mass Spectrometry: What Each Method Shows on a Peptide Report

LC-UV and mass spectrometry are the two most common analytical methods reported on peptide certificates of analysis, and they measure different things. LC-UV, which stands for liquid chromatography with ultraviolet detection, separates compounds in a mixture by polarity and measures them by absorbance at a chosen wavelength, typically 214 or 220 nm where peptide bonds absorb [1]. Mass spectrometry identifies compounds by molecular mass [2].

What does LC-UV purity mean?

An LC-UV purity of 98 percent means that 98 percent of the total UV absorbance in the chromatogram belongs to the main peak. It says nothing about what the main peak is. If a different peptide happens to elute at the same retention time, it is counted as part of the main peak. UV purity also misses compounds that do not absorb UV light, such as trifluoroacetic acid salts or inorganic ions.

What does mass spectrometry confirm?

Mass spectrometry confirms that a peak with the expected molecular mass is present. It can identify deletion sequences, oxidized forms, and other structural variants by their mass shifts. It can also detect co-eluting impurities that UV alone would miss.

Why do certificates report both?

Because neither method alone gives the full picture. LC-UV gives a quantitative purity number but no structural identity. MS gives structural identity but is less precise for quantitation. A certificate reporting only one of the two leaves a significant gap in the characterization.

Frequently asked questions

Is LC-MS the same as running both methods?

LC-MS is a single instrument that couples liquid chromatography directly to a mass spectrometer. It provides both separation and identification in one run, but the UV detector on an LC-MS system may not match the sensitivity of a dedicated HPLC-UV instrument.

What wavelength should peptide HPLC use?

214 nm is standard for peptides because the amide bond absorbs there. Peptides containing tryptophan or tyrosine also absorb at 280 nm, which can be used as a secondary check.