A method for detecting antibody-drug conjugate compounds comprising: (i) processing a biological sample collected from a biological source selected from a mammal, tissue, cell culture, plasma or serum previously contacted with an antibody-drug conjugate compound having Formula I: Ab-(L-D) p I wherein Ab is an antibody; D is a maytansinoid or monomethylauristatin 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, to form an analysis sample by formulating, immobilizing, centrifuging, isolating, digesting, inducing or preventing blood cell clotting, hydrolyzing, or purifying to form a processed analysis sample; (ii) capturing the processed analysis sample on immunoaffinity beads comprising an antigen specific for the processed analysis sample, wherein the immunoaffinity bead is a streptavidin coated paramagnetic bead; (iii) eluting the processed analysis sample; (iv) applying the eluted analysis sample to a separation media to effect separation of more than one sample constituent by capillary flow liquid chromatography wherein a separated sample constituent comprises an antibody-drug conjugate compound having the Formula I, where p is 0, 1, 2, 3, 4, 5, 6, 7, or 8; and (v) establishing the mass to charge ratio of one or more separated sample constituents which is an antibody-drug conjugate compound having the Formula I, where p is 0, 1, 2, 3, 4, 5, 6, 7, or 8, by electrospray ionization mass spectrometry. (i) processing a biological sample collected from a biological source selected from a mammal, tissue, cell culture, plasma or serum previously contacted with an antibody-drug conjugate compound having Formula I: Ab-(L-D) p I wherein Ab is an antibody; D is a maytansinoid or monomethylauristatin 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, to form an analysis sample by formulating, immobilizing, centrifuging, isolating, digesting, inducing or preventing blood cell clotting, hydrolyzing, or purifying to form a processed analysis sample; Ab is an antibody; D is a maytansinoid or monomethylauristatin 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, to form an analysis sample by formulating, immobilizing, centrifuging, isolating, digesting, inducing or preventing blood cell clotting, hydrolyzing, or purifying to form a processed analysis sample; (ii) capturing the processed analysis sample on immunoaffinity beads comprising an antigen specific for the processed analysis sample, wherein the immunoaffinity bead is a streptavidin coated paramagnetic bead; (iii) eluting the processed analysis sample; (iv) applying the eluted analysis sample to a separation media to effect separation of more than one sample constituent by capillary flow liquid chromatography wherein a separated sample constituent comprises an antibody-drug conjugate compound having the Formula I, where p is 0, 1, 2, 3, 4, 5, 6, 7, or 8; and (v) establishing the mass to charge ratio of one or more separated sample constituents which is an antibody-drug conjugate compound having the Formula I, where p is 0, 1, 2, 3, 4, 5, 6, 7, or 8, by electrospray ionization mass spectrometry.