Post-column infusion was done using either the QTRAP built-in syringe pump with a 1.0 mL Hamilton syringe (Sigma-Aldrich, Saint Louis, USA) (for matrix effect exploration and PCIS concentration optimization) or an Agilent 1260 Infinity pump (Agilent Technologies, Santa Clara, USA) with a degasser (for all other experiments). Unless otherwise indicated, the flow rate was set at 50 μL/min. The flows were combined after the divert valve and before entering the MS using a T-piece (Upchurch Scientific, Idex Health & Science LLC, Oak Harbor, WA, USA). A schematic drawing of the instrument setup is included in the SI (Figure ).