An apparatus and method are disclosed for monitoring and/or detecting concentrations of a chemical precursor in a reaction chamber. The apparatus and method have an advantage of operating in a high temperature environment. An optical emissions spectrometer (OES) is coupled to a gas source, such as a solid source vessel, in order to monitor or detect an output of the chemical precursor to the reaction chamber. Alternatively, a small sample of precursor can be periodically monitored flowing into the OES and into a vacuum pump, thus bypassing the reaction chamber.