A method for detecting antibody-drug conjugate compounds comprising: (i) processing by immunoaffinity membrane selection a biological sample which has been collected from a biological source previously contacted with an antibody-drug conjugate compound having Formula I: Ab-(L-D) p I wherein Ab is an antibody which binds to one or more tumor-associated antigens or cell-surface receptors; D is a drug moiety selected from a maytansinoid or an auristatin; L is a linker covalently attached to Ab, and covalently attached to D, wherein L is formed from a linker reagent selected from N-succinimidyl-4(2-pyridylthio)propanoate (SPDP), succinimidyl-4-(N-maleimidomethyl) cyclohexane-1-carboxylate (SMCC) and N-succinirnidyl-4-(2-pyridylthio)pentanoate, or L is selected from maleimidocaproyl, maleimidopropanoyl, and maleimidocaproyl-valine-citrulline-para-aminobenzyloxycarbonyl (MC-vc-PAB); and pis 1,2,3,4, 5, 6, 7, or 8; wherein an analysis sample is formed by contacting the biological sample with the immunoaffinity membrane comprising an immobilized antigen and elution of the analysis sample; (ii) applying the analysis sample to a separation media to effect separation of more than one sample constituent wherein a separated sample constituent comprises an antibody fragment of an antibody-drug conjugate compound having the Formula I, or metabolite thereof, and where p is 0, 1, 2, 3, 4, 5, 6, 7, or 8; and (iii) establishing the mass or mass to charge ratio, and quantitating one or more separated sample constituents which is an antibody fragment of an antibody-drug conjugate compound having the Formula I or metabolite thereof, where p is 0, 1, 2, 3, 4, 5, 6, 7 or 8, by single ion monitoring mass spectrometry. (i) processing by immunoaffinity membrane selection a biological sample which has been collected from a biological source previously contacted with an antibody-drug conjugate compound having Formula I: Ab-(L-D) p I wherein Ab is an antibody which binds to one or more tumor-associated antigens or cell-surface receptors; D is a drug moiety selected from a maytansinoid or an auristatin; L is a linker covalently attached to Ab, and covalently attached to D, wherein L is formed from a linker reagent selected from N-succinimidyl-4(2-pyridylthio)propanoate (SPDP), succinimidyl-4-(N-maleimidomethyl) cyclohexane-1-carboxylate (SMCC) and N-succinirnidyl-4-(2-pyridylthio)pentanoate, or L is selected from maleimidocaproyl, maleimidopropanoyl, and maleimidocaproyl-valine-citrulline-para-aminobenzyloxycarbonyl (MC-vc-PAB); and pis 1,2,3,4, 5, 6, 7, or 8; wherein an analysis sample is formed by contacting the biological sample with the immunoaffinity membrane comprising an immobilized antigen and elution of the analysis sample; Ab is an antibody which binds to one or more tumor-associated antigens or cell-surface receptors; D is a drug moiety selected from a maytansinoid or an auristatin; L is a linker covalently attached to Ab, and covalently attached to D, wherein L is formed from a linker reagent selected from N-succinimidyl-4(2-pyridylthio)propanoate (SPDP), succinimidyl-4-(N-maleimidomethyl) cyclohexane-1-carboxylate (SMCC) and N-succinirnidyl-4-(2-pyridylthio)pentanoate, or L is selected from maleimidocaproyl, maleimidopropanoyl, and maleimidocaproyl-valine-citrulline-para-aminobenzyloxycarbonyl (MC-vc-PAB); and pis 1,2,3,4, 5, 6, 7, or 8; (ii) applying the analysis sample to a separation media to effect separation of more than one sample constituent wherein a separated sample constituent comprises an antibody fragment of an antibody-drug conjugate compound having the Formula I, or metabolite thereof, and where p is 0, 1, 2, 3, 4, 5, 6, 7, or 8; and (iii) establishing the mass or mass to charge ratio, and quantitating one or more separated sample constituents which is an antibody fragment of an antibody-drug conjugate compound having the Formula I or metabolite thereof, where p is 0, 1, 2, 3, 4, 5, 6, 7 or 8, by single ion monitoring mass spectrometry.