The chromatographic system consisted of three LC pumps (Shimadzu, Kyoto, Japan): two LC-10AD HPLC pumps, one for analyte trapping, featuring an FCV-10AL solvent selector, and the second for online dilution. The third pump for chromatography was a LC-30AD UHPLC pump equipped with a lowpressure gradient unit. The samples were injected with an HTC PAL autosampler (CTC Analytics, Zwingen, Switzerland) equipped with a 500 µL syringe and a 200 µL injection loop, two switching valves (a Vici 10-port, and a Rheodyne 6-port) (Figure and). Following trapping columns (all from Dr. Maisch Germany) were investigated: ReproSil-Pur 120 C18-AQ trap column (20 mm length, 4 mm I.D., 5 µm particle size), ReproSil 80 SCX, Reprospher SCX/C8, Repromer 100 SCX, Repropart 80 SCX and Reprospher 100 C18/WCX (20 mm length, 4 mm I.D., 10 µm particle size). The mobile phase for trapping was 5 mM NH4HCO2 in H2O at a flow rate of 0.1 ml/min, and for washing 5 mM NH4HCO2 in MeOH (0.4 ml/min). For on-line dilution the mobile phase was 5 mM NH4HCO2 in H2O, and the flow rate of 1 mL/min. The analytes were separated on a reverse-phase Luna C18 column (100 mm length, 2 mm I.D., 2.5 µm particle size, Phenomenex, Switzerland) safeguarded by a C18 guard column (Phenomenex, Switzerland) at 40 o C. Mobile phase A: 5 mM NH4HCO2 in H2O and mobile phase B: 5 mM NH4HCO2 in MeOH. For the investigation of different trapping columns, the following gradient was used: starting at 5% B for 1.30 min and increasing to 95% B in 4.7 min, hold for half a minute and decreasing back to 5% in 0.3 min to end in the re-equilibration step for 4.2 min. For the analysis of wines and juices the gradient started at 5% B for 1.30 min and increase to 95% B in 13.7 min and hold for 1 min to finally decrease to 5% in 0.2 min for re-equilibration for additional 3.8 min. Figures and summarize the column-switching setup and the time sequence.