The disclosed HPLC flow schematic back-flushes the guard column with mobile phase once every operating cycle, the regular cleaning maintaining its effectiveness and useful life. This mobile phase flow reversal is accomplished by imposing a two position multiple port switching valve between the guard and separation columns. In one valve position, the mobile phase is pumped in one flow direction through both the separation and guard columns, but the switched valve maintains the same flow direction only through the separation column and reverses the mobile phase flow direction through the guard column, for effectively cleaning the guard column. Moreover, the mobile phase back flow is directed over the emptied sample drawing needle for cleaning it to reduce possible sample cross contamination. The improved flow circuits use basically only the typical HPLC system components, except for some insignificant modifications from existing injector designs.