Abbreviations: ACN, acetonitrile; BCA, bicinchoninic acid assay; BPC, base peak chromatogram; DTT, dithiothreitol; E/S, enzyme to substrate ratio; EIC, extracted ion chromatogram; FA, formic acid; Fuc, fucose; GalNAc, N-acetylgalactosamine; hCG, human chorionic gonadotropin; hCGα, alpha subunit of the human chorionic gonadotropin; hCGβ, beta subunit of the human chorionic gonadotropin; Hex, hexose; HexNAc, Nacetylhexoseamine; IAM, iodoacetamide; IMER, Immobilized Enzyme Reactor; NeuAc, Nacetylneuraminic acid; NeuGc, N-glycolylneuraminic acid; MS/MS, tandem mass spectrometry; P-IMER, pepsin-based immobilized enzyme reactor; PTM, post-translational modification; r-hCG, recombinant human chorionic gonadotropin; TFA, trifluoroacetic acid; T-IMER, trypsin-based immobilized enzyme reactor; TRIS, Trizma hydrochloride; u-hCG, human chorionic gonadotropin isolated from urine of pregnant women