A sensor, such as a mass spectrometer, capable of detecting the presence of materials in a sampled gas is interconnected with a processing chamber of a vacuum manufacturing tool. The sensor includes a timing circuit which is activated only if certain levels of specific materials are detected. Furthermore, the timer is set to run a predetermined time interval after activation so as to discriminate between known transient processing conditions and the presence of impurities which can greatly influence the manufacturing process. When the timer exceeds the predetermined time duration, an output signal can alert the process operator or automatically shutdown the manufacturing tool.