SPECTRALLY RESOLVED DETECTION OF NH3 AT PPB LEVEL

Photoacoustic detection of 30 ppb ammonia in air is demonstrated with a signal to noise ratio of 40, using a 400 MHz tunable CO2 laser as ra diation source. Interference from CO2 is controlled by monitoring near the Doppler limit and by using phase information to separate the two signals. Wall adhesion is studied quantitatively. It is shown that the walls constitute a reservoir of fixed size, which is filled up at a f ate proportional to the NH3 concentration.