By using the differential optical absorption technique combined with Fourier transformation, we have achieved a good separation of ammonia from sulfur dioxide in the wavelength range 207 to 217 nm. Measurements show that, with a pathlength of two decimeters and a measuring time of one minute, we can measure less than 1.32 mg/m3 [1.9 ppm (volume)] of ammonia with a standard deviation of 0.46 mg/m3 [0.66 ppm (volume)] with interference from 408.73 mg/m3 [156 ppm (volume)] sulfur dioxide.
L. Axelsson, "Measurement of Ammonia with the Differential Optical Absorption Technique Combined with Fourier Transform," Appl. Spectrosc. 48, 1003-1006 (1994)