All data were acquired using a Waters Xevo QTOF mass spectrometer (Milford, MA, usa). A μ-PrepCell thin-layer electrochemical flow cell (Antec BV, Leyden, The Netherlands) was coupled with the mass spectrometer by liquid sample DESI (Scheme S-1, Supporting Information), which was previously described in details [27 (link)]. The electrochemical flow cell was equipped with a magic diamond electrode (12 mm × 30 mm, Antec BV) as the working electrode (WE), and a Roxy potentiostat (Antec BV) was employed to apply a reduction potential to the WE. A sample syringe was connected with the cell by a piece of PEEK capillary tube. The sample solution was injected at a flow rate of 5 μL/min and flowed through the cell for electroreduction. The reduced species flowed out of the cell via a piece of fused silica capillary and underwent ionization by DESI via the interaction with charged microdroplets from DESI spray. H2O:MeOH (v/v 50:50) containing 1% FA was used as the DESI spray solvent and injected at a flow rate of 5 μL/min. A high voltage of +5 kV was applied to the DESI spray probe with nebulization gas of N2 (pressure 170 psi). Collision induced dissociation (CID) was carried out for ion structural analysis, in which a wide mass selection window was used for simultaneous selection of both DSP-d0 and DSP-d8 labeled cross-link ions or a pair of the reduced cross-link ions.