By performing SPE on a column, which henceforth will be referred to as trap column due its trapping of the injected sample, the analytes are retained on the stationary phase, while most of the polar sample matrix components are washed away to waste. 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 for further separation.