Abstract: A method is provided to optimize the chemical mechanical planarization process of wafers having a target removal layer and a stop layer. A slurry is added to a polishing table which includes a polishing pad and a platen adapted for rotation; a portion of the slurry is allowed to engage an interface between the polishing pad and the wafer. A gaseous sample is continuously extracted from the slurry; the gaseous sample includes a reactant product created when the polishing pad engages the stop layer. The gaseous sample is introduced into a reactant product detector. A first time is determined, corresponding to an initial detection of the reactant product in the slurry, thereby creating a first reference point. A second time is determined, corresponding to the detection of a maximum volume of the reactant in said slurry, thereby creating a second reference point.