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A novel capillary partially filled with an affinity ligand-encapsulated hydrogel using sodium alginate(SA)was prepared for affinity capillary electrophoresis [1].An alginate hydrogel can be easily formed by adding Ca2+ into an SA solution,in which an affinity ligand(AL)is encapsulated with maintaining its affinity.An SA solution containing the AL,e.g.,avidin,was partially injected into the capillary filled with a background solution containing calcium dichloride,then an appropriate voltage was applied to introduce Ca2+ into the SA solution zone.The encapsulation of avidin by the formed alginate hydrogel was expected.Consequently,the AL can be encapsulated successfully by the alginate hydrogel with maintaining its affinity.The combination of an on-line sample concentration technique,large-volume sample stacking with an electroosmotic flow pump(LVSEP)[2,3],with mass spectrometry(MS)in capillary electrophoresis(CE)was investigated to improve the detectability.Constructing LVSEP-CE–MS has been difficult due to the lack of the outlet reservoir,since LVSEP requires a BGS from the outlet side of the capillary for pumping out the sample matrix.In this study,the sheath liquid poured into the electrospray ionization(ESI)interface was employed as the BGS,where the sheath liquid was successfully introduced into the capillary from the ESI interface.To evaluate the analytical performance of the system,three aromatic sulfonic acids were analyzed.As a result,the analytes were 75–120-fold concentrated in LVSEP-CE–MS compared to conventional CE–MS.