Methods to detect, screen, and quantitate biological samples after administration of antibody conjugates, antibody-drug conjugates of Formula I, antibodies, and fragments and metabolites thereof, by affinity separation, chromatography, and mass spectrometry are disclosed. Ab-(L-D).sub.p I wherein Ab is an antibody; D is a drug moiety; L is a linker covalently attached to Ab, and covalently attached to D; and p is 1, 2, 3, 4, 5, 6, 7, or 8.