Highly sensitive electronically modulated photoacoustic spectrometer for ozone detection

An ozone (O₃) gas sensor with a sensitivity of parts per 10 ⁹ (ppb) level and a high level of selectivity based on the resonant photoacoustic effect was developed using an electronically modulated cw CO ₂ laser beam. Quite different from the standard chopper modulation of a laser beam, here the laser source was electronically modulated to overcome the inherent problem of frequency instability associated with chopper modulation. With electronic modulation, in conjunction with the fast Fourier transform (FFT) of transient signals, we were able to improve significantly the sensitivity of the photoacoustic (PA) system for the detection of O₃. In addition to the improved sensitivity, our method proved that the FFT of a laser modulated PA signal could suppress the noise signal generated by spurious window diffused absorption, which in the case of most commonly used lock-in techniques is rather unavoidable. The dependence of the PA signal on various experimental parameters such as buffer gas, laser power, modulation frequency, and trace gas concentration was investigated. In the case of buffer gas, argon proved to be more suitable than nitrogen and helium in terms of enhancing the sensitivity of the system. The limits of detection of O₃ using the 9 P(14) CO₂ laser line in our PA system are 5 parts per 10⁹ by volume (ppbv) and 14 ppbv with electronic and standard chopper modulation, respectively. This detection limit of O₃ is quite applicable for detection of safe levels of O₃, at ground level.