Minimum Detectable Pollutant Concentrations with Stark Modulated Microwave Spectroscopy

A general theory for the prediction of the minimum detectable concentration using a Stark modulated microwave spectrometer has been developed. This theory relates the characteristics of the molecular rotational resonance of the polar gas and the instrument parameters. The theory shows that the instrument is most sensitive when sufficient microwave power is used to produce saturation and the sample pressure is increased to a value which is limited by the dielectric breakdown of the Stark voltage modulating the gas. Experimental verification using NH3 with N2 as the background gas was performed. A minimum detectable concentration of 8 ppm was observed at a pressure of 1 mm with a spectrometer sensitivity of -135 dBm.