Determination of Hydrogen Sulfide by Wavelength-modulated Diode Laser Absorption Spectroscopy
Rapid, accurate, and sensitive determination of hydrogen sulfide was provided by tunable distributed feedback diode laser absorption spectroscopy. The laser wavelength scanning range covered the strongest absorption band of hydrogen sulfide. The absorption line at 1578 nm was used for analytical measurements. A homemade digital lock-in amplifier was used to demodulate the first and the second harmonic signals. Normalization of the second harmonic by the first harmonic signal provided calibration-free measurements so that interferences from light path, circuitry, and source intensity were minimized. A mass flow controller was used to introduce 0-100 ppm hydrogen sulfide for validating the analytical performance. The results demonstrated the linearity of the normalized harmonic signals with concentration with a correlation coefficient of 0.9995. The response time was approximately 3 s. The detection limit by Allan variance was 45 ppb with an integration time of 35 s. The sensitive hydrogen sulfide sensor is suitable for environmental monitoring with real-time detection.