A method for characterizing a target peptide through a detection approach such as mass spectrometry is provided, including: introducing at least one guard molecule to mix with the target peptide; and applying the detection approach for the characterization of the target peptide. Each guard molecule is configured to have similar characteristics as the target peptide, yet is still distinguishable therefrom by the detection approach, such as having a mass spectrometry-distinguishable different M/z value compared with the target peptide. The method can be used to characterize a neoantigen peptide through mass spectrometry, upstream of which the method can further include steps for tissue sample preparation, HLA molecules enrichment, elution, clean-up, and purification. Some or all of these steps can be configured to be executed in a substantially automatic manner with little or no manual intervention. A system for implementing the neoantigen analysis method is further provided.