Methoxy-polyethylene glycol amine (mPEG-NH₂), an important pharmaceutical excipient, has been extensively utilized in the field of biomedicine. To advance the development of mPEG-NH₂-related drug delivery systems, it is crucial to understand the safety profile and cellular pharmacokinetic behavior of mPEG-NH₂ polymers. In this study, a straightforward analytical assay based on ultra-high performance liquid chromatography and tandem mass spectrometry (UHPLC-MS/MS) for quantifying mPEG