A method and an apparatus for determining a usage level of a memory device to notify a running application to perform memory reduction operations selected based on the memory usage level are described. The present invention prepares SnO2 as the detection membrane of an ISFET by sol-gel technology to obtain a SnO2 ISFET. The a separating device further comprises a rotary plate with a curved grooved structure, a plurality of movable valves, a scraper, and a cylinder with a plurality of T-shape grooves, and a plurality of slices.