Abstract: Methods, apparatus, and systems for substrate processing are provided. Apparatus can include a controller; a processing chamber; a substrate supporting a substrate; and an infrared sensor assembly disposed adjacent the substrate support and comprising: a sample chamber one of made from or coated with nickel or nickel alloy and configured to collect chemicals which are present while the substrate is being processed in the processing chamber; an IR light source disposed at one end of the sample chamber and an IR detector disposed at an opposite end of the sample chamber; and a pair of windows positioned in an optical path between the IR light source and the IR detector, wherein the IR light source transmits IR light along the optical path and the IR detector detects the transmitted IR light and transmits a signal to the controller for determining a concentration of the chemicals present in the processing chamber.