While low MS1 resolution mode provides more targeted quantification, open mode may be beneficial to quantify ADC with biotransformation. Common ADC biotransformations include hydrolysis, reduction/oxidation, cleavage by proteases, and linker-payload deconjugation. Biotransformation can occur on antibody, linker, and/or payload, which makes it challenging to quantify the biotransformed species. Multiple species with minor delta masses or of low abundance may impede accurate quantification by intact mode or at peptide level. In contrast, DF-MDMS quantification may overcome this issue by focusing on the payload signature ions only, by following these procedures: 1) set up a steep LC elution gradient for co-elution of parent and biotransformation species 2) apply a slightly wider MS1 isolation window allowing both parent and biotransformed species to enter the collision cell (m/z range between parent and biotransformation species is narrow at high charge state for >25k Da molecules such as payload conjugated to LC or HC) 3) quantify the payload signature product ions. Deconjugation is commonly observed in ADC in vivo study samples. In the case of heterogeneous ADCs where DAR distribution can already be observed in reference material, absolute concentration of different species can be assessed with calibration curve established for each DAR species. In the case that certain species information is missing due to sensitivity limitation, the average DAR information may be obtained by applying a steep LC gradient. Multiple species with various DAR can be merged into a single peak, thus allowing relative quantification of average DAR inherent to LCMS-based quantification methods, the capture affinity and ionization efficiency could be different between different DAR species.