The 6 cc Oasis MCX cartridges contain the Oasis MCX, which is a novel, mixed-mode polymeric sorbent that has been optimized to achieve higher selectivity and sensitivity for extracting basic compounds with cation-exchange groups. Since the Oasis MCX sorbent is water wettable, it maintains its capability for higher retention and excellent recoveries even if the sorbent runs dry, which means there is no need to take extraordinary steps to keep the sorbent beds from drying out during the critical steps prior to sample loading.