HPLC and Mass Spectrometry, Explained Simply

Two techniques carry most of the weight on a peptide certificate of analysis. They answer different questions, and a supplier reporting only one is telling you half the story.

HPLC — how much of it is the right thing

High-performance liquid chromatography separates the components of a sample as they travel through a column at different speeds. The detector records each component as a peak, and the area under each peak is proportional to the amount present.

Purity is then the target peak's area as a percentage of the total:

purity % = (area of target peak / total peak area) × 100

A figure of 99.1% therefore means that 0.9% of the detected material is something else — closely related peptides, truncated sequences, or residual counter-ions, depending on the method. Two laboratories can report different purities for the same lot if they use different columns, gradients or wavelengths. That is why the method belongs on the certificate alongside the number.

Mass spectrometry — is it the right molecule

Mass spectrometry measures molecular mass. The sample is ionised and the instrument records the mass-to-charge ratio of the resulting ions.

For a peptide, the reported value should match the calculated molecular mass of the expected sequence, within the precision of the instrument. A high HPLC purity with a mass that does not match is a red flag: the dominant peak may be a well-separated but wrong molecule.

Singly charged, doubly charged and sodium or potassium adduct peaks are all normal. What matters is that one of them corresponds to the expected mass, and that the certificate states which.

Reading the two together

| HPLC | Mass spec | Reading | | --- | --- | --- | | High | Matches | Expected material, well separated — the normal case | | High | Does not match | Wrong sequence or a mis-assigned product | | Low | Matches | Correct molecule, but a poorly purified lot | | Low | Does not match | Not usable as a reference material |

What we file for each batch

HPLC purity and peptide content, mass confirmation against the calculated value, appearance, and the fill weight. The certificate carries the method, the laboratory and the test date so a third party can follow the same reasoning you just did.