The instrument turns molecules into ions (for peptides, almost always by electrospray (ESI) or MALDI) and separates them by their mass-to-charge ratio (m/z). A large peptide can appear with several charges at once; the software reconstructs its real mass from those peaks.
The comparison is direct: if the data sheet declares a 1,419.5 Da peptide and the spectrum shows 1,419.5 Da, the identity is consistent. If a mass 16 Da higher appears, it may indicate oxidation; if the mass of one amino acid is missing, a deletion sequence.
The most informative combination is LC-MS: chromatography separates the components and the spectrometer identifies each peak. That shows not only how much of the main peak there is, but what each impurity is.
HPLC and mass answer different questions and both are needed: HPLC says how pure the material is; mass, whether that material really is the declared peptide. A certificate with HPLC only leaves identity unproven.
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Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).