Optoacoustic trace-gas monitoring with near-infrared diode lasers

An inexpensive resonant optoacoustic monitoring system using near-infrared laser diodes was developed. It was demonstrated that wavelength modulation at the resonance frequency of the cell provides a superior signal-to-noise ratio compared with amplitude modulation and eliminates background drifts and fluctuations. The system was tested out on ammonia. Its sensitivity is 8 parts in 10(9) (S/N = 1) at atmospheric pressure, which corresponds to a minimum detectable absorption coefficient of approximately 3.5 × 10(-11) cm(-1) W(-1). The pressure dependence of the optoacoustic resonance was also investigated. The monitor can be used as a continuous flow-through system up to a flow rate of approximately 3.5 L/min.