Covalent linkage with acetylcholinesterase (AChE) to form ligand-target adduct (L-T) is primarily responsible for pesticide poisoning, notably those carbamates (CARs) and organophosphorus (OPs), which frequently occurs in clinic. Because covalent modification resists proteolysis, L-T measurement can be accomplished by monitoring modified peptide(s) that further act as a quantitatively bidirectional indicator for both ligand and target. Covalent modification locates at hydroxyl group of Ser225 residue. Modified peptide relative response (AM) and the response ratio (RM/U) between modified and unmodified peptides were specifically concerned. When providing adequate AChE, RM/U was correlated linearly with ligand concentration, and similarly, linear relationships occurred between RM/U and AChE content when the given ligand was sufficient. Agreement emerged for the sigmoid trajectories between AM against carbamate concentration and colorimetric Ellman assay. AM against ligand concentration curve uniquely enabled inhibition measurement of the mixed pesticides. Structural annotation of modified peptide facilitated the unknown toxin(s) identity annotation. Together, the measurement of modified peptide that reflects L-T identity and concentration, enabled the qualitative and quantitative analysis of either ligand or target, as well as toxicological performance evaluation of individual ligands and even ligand mixtures.