In the continuous-flow extraction, the flow rate may grad ually decrease as a result of blockage of the membrane fil ter with colloid particles especially in the NaoH extraction step. This causes the operation to be prolonged in some par- ticular samples. Fig. 3 shows the change in the flow rate with subfrac- tion number. It is observed that the flow rate did not change much in the first step of the extraction (NH4Cl). When NaoH was pumped through, flow rates of SRM 2710 and SRM2704 dropped. Changing the extractant from NaoH to HCI slightly increases the flow rate but usually did not re- tum to the original rate