A sensitive microwave cavity spectrometer: Direct detection of formaldehyde in automobile exhaust

A newly constructed sensitive microwave cavity spectrometer of the Stark-dc-voltage sweep type has revealed that formaldehyde in concentrations as small as 0.2 ppm can be directly detected by using the 7₂₅ ← 7₂₆ rotational line. The cavity resonator is operated in the rectangular TE_1,0,20 mode. A sample of automobile-engine exhaust showed a strong spectrum indicating a concentration of 24 ppm of formaldehyde.