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Affinity analysis is a key biotechnique used in the fields of biology and biomedicine. Herein, we advanced the concept of moving affinity boundary (MAB) using metal ion Ni(ii) and histidine (His) as the model inorganic ion and ligand, respectively, developed the simple method of MAB affinity capillary electrophoresis (MAB-ACE), and carried out the relative experiments. The experiments manifested that (a) an MAB could be created with the model metal ion and ligand; (b) the MAB-ACE could specifically capture His rather than other amino acids, or numerous metabolites in human urine; and (c) the capture had the merits of simultaneous focusing and separation to the target metabolite of His. It was further revealed that the specificity of MAB-ACE was originated from the selective affinity interaction and the effective control of affinity conditions. The analyses of His in raw urine by the MAB-ACE are in agreement with those viathe standard amino acid analyzer, indicating the reliability of the developed method. Additionally, the MAB-ACE with UV detector had good sensitivity (LOD = 43 ng mL−1, S/N = 3), 1.0–150 μM linearity and <5% intra-/inter-day variations. The novel method has an evident potential application for capture of a target metabolite in complex biological sample. [ABSTRACT FROM AUTHOR]