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A new pressure-calibration method for calibrating the reduction of second harmonic (2f) amplitude caused by pressure broadening effect in sealed microbial growth environment is present. The new method combines with linewidth compensation and modulation depth compensation and makes the 2f amplitude accurately retrieve metabolic CO2 in microbial growth. In order to verify the method, a simulation experiment is developed, in which the increasing CO2 concentration leads to the increasing pressure. Comparing with the relation between the traditional 2f amplitude and gas concentration, there is a monotonous relation between the calibrated 2f amplitude and CO2 concentration, particularly, a linear relation is present when the CO2 concentration is replaced with the CO2 particle number. In terms of microbial measurement, the growth of Escherichia coli is measured, and the culture bottle is sealed during the microbial growth process. The experimental results show that, comparing to the microbial growth retrieved by traditional 2f amplitude, the calibrated 2f amplitude can accurately retrieve microbial growth in sealed environment. [ABSTRACT FROM AUTHOR]

N2O is a significant atmospheric greenhouse gas that contributes to global warming and climate change. In this work, the high sensitivity detection of atmospheric N2O is achieved using wavelength modulation spectroscopy (WMS) with an inter-band cascade laser operating around 3.939 µm. A LabVIEW-based software signal generator and software lock-in amplifiers are designed to simplify the system. In order to eliminate the interference from water vapor, the detection was performed at a pressure of 0.1 atm (1 atm = 1.01325×105 Pa) and a drying tube was added to the system. To improve the system performance for long term detection, a novel frequency locking method and 2 f/1 f calibration-free method were employed to lock the laser frequency and calibrate the power fluctuations, respectively. The Allan deviation analysis of the results indicates a detection limit of ∼ 20 ppb (1 ppb = 1.81205 µg/m3) for a 1 s integration time, and the optimal detection limit is ∼ 5 ppb for a 40-s integration time. [ABSTRACT FROM AUTHOR]

Sub-ppmv level detection of hydrogen sulphide (H2S) using a 1.578- distributed feedback tunable diode laser combining with wavelength modulation spectroscopy and second harmonic detection scheme is reported. A home-developed novel compact dense-pattern multipass gas cell with an effective optical path length of 29.37 m is used to improve sensitivity and reduce sample volume. Detection parameters are optimized, including modulation frequency and amplitude. The analysis of Allan variance shows that a minimum detectable concentration 60 ppbv is obtained with a lock-in time constant of 10 ms, and a detection limit of 13 ppbv can be achieved by average in 300 s. The demonstrated H2S sensor has a strong penitential application in natural gas process for regulating and controlling H2S concentration. [ABSTRACT FROM AUTHOR]