How the two halves work together
Liquid chromatography pushes a dissolved sample through a column. Different molecules move at different speeds, so the mixture comes out separated in time. Each separated component then enters the mass spectrometer, which ionizes it and measures its mass‑to‑charge ratio.
A peptide has a known, calculable molecular mass. If the peak coming off the column has that mass, the identity is confirmed. If it does not, something else is in the vial.
Identity versus purity
LC‑MS is strongest at identity. Purity, meaning what percentage of the sample is the target compound, is usually read from the chromatographic trace, often by HPLC with UV detection. Many certificates report both, drawn from the same run.
What it cannot tell you
LC‑MS says nothing about sterility, endotoxin, or how the material was handled after testing. It characterizes a sample drawn at a point in time, not the vial on your shelf today.