The present invention relates to monitoring chemicals in a process chamber using a spectrometer having a plasma generator, based on patterns over time of chemical consumption. The relevant patterns may include a change in consumption, reaching a consumption plateau, absence of consumption, or presence of consumption. In some embodiments, advancing to a next step in forming structures on the workpiece depends on the pattern of consumption meeting a process criteria. In other embodiments, a processing time standard is established, based on analysis of the relevant patterns. Yet other embodiments relate to controlling work on a workpiece, based on analysis of the relevant patterns. The invention may be either a process or a device including logic and resources to carry out a process.