Column switching is the means to change the direction of the solvent flow by the use of valves, and thereby back flush the trap column, eluting the analytes onto the analytical column during a defined timeframe [9], for further separation. The combination of the two columns should be designed in such a fashion that elution of the analytes from the trap column is performed with a solvent composition that allows an on-column concentration, or stacking effect, onto the analytical column [10]. Otherwise, the gained selectivity and sensitivity will be lost due to increased band broadening [11]. With the use of a flow system being able to deliver a mix of different solvents, gradient elution can be used to achieve a continuous on-column stacking, improving the sensitivity and resolution of the analytes.