A method for combinatorially processing a substrate is provided. The method includes introducing a first etchant into a reactor cell and introducing a fluid into the reactor cell while the first etchant remains in the reactor cell. After initiating the introducing the fluid, contents of the reactor cell are removed through a first removal line and a second removal line, wherein the first removal line extends farther into the reactor cell than the second removal line. A level of the fluid above an inlet to the first removal line is maintained while removing the contents. A second etchant is introduced into the reactor cell while removing the contents through the first removal line and the second removal line. The method includes continuing the introducing of the second etchant until a concentration of the second etchant is at a desired level, wherein the surface of the substrate remains submerged.