While some columns can be briefly rinsed by reversing the flow, in general, an LC column should only be used with flow in the direction indicated on the column. Some additional information from our instruction sheets: "Flow Direction & Flow Rate The arrows on the column label indicate the recommended flow direction. Begin by connecting the inlet end of the column to the injector or autosampler and allow mobile phase to flow from the outlet end of the column into a beaker for 10-15 minutes. Gradually increase the flow rate to its optimal value (Table I). Then, stop the mobile phase flow and connect the column to your detector. Because every LC system is unique, especially when used in gradient mode, your results may differ from those obtained in our laboratory."