detection (2F), the detection/demodulation in this analyser was performed at the laser- modulation frequency (i.e., ‘1F’ detection). Using the 1F-detection scheme enabled the normalisation of the spectra without the need for a separate measurement of the laser power. Specifically, the magnitude of the power envelope of the laser output is contained in the spectra produced by 1F demodulation. After the 1F spectra were normalised, they were differentiated; the resulting derivative spectra approximate the second derivative of the absorption spectrum of the analyte and were referenced as 2F signals in this work. The scan parameters for the laser (such as injection current range, modulation depth) were set to match the desired wavelength range required to cover the width of the ro-vibrational transition. Line width was determined from hydrogen data published in spectral libraries. The wavelength-locking algorithm employed by the instrument was based on two nested levels of temperature control employed to maintain the operation of the laser diode at the proper wavelength. The first level was a simple PID control loop, which maintained the laser at a target temperature. In the second level, the outer control loop, the spectra of the analyte samples in the reference cells were monitored. Minor shifts in the observed peak positions were used as a feedback signal for the temperature set point of the inner control loop. Thus, the outer control loop provided a fine adjustment for the inner control loop. Wavelength selection for hydrogen measurements was dictated by the spectral position and intensity of the hydrogen absorption lines in the near-infrared range, the requirement of minimal spectral interference with other components of the gas stream at a refinery or ammonia production plant (as examples) and, of course, by the availability of laser diodes. From this point of view, the hydrogen ro-vibrational spectral line was the most attractive for measurements. As was demonstrated earlier by others, this line is not only the strongest line in the fundamental vibration band of hydrogen electric- quadrupole transitions, but is also the line with
minimal spectral interference with methane, ammonia, and carbon dioxide. In addition, this region of the spectrum corresponded to high sensitivity for extended InGaAs detectors.
Results and discussion: Hydrogen analyser
The analyser performance was tested with samples of hydrogen in nitrogen. Different concentrations of hydrogen were created by mixing hydrogen with the host gas in a gas mixer. Examples of some measured 2F signals corresponding to different hydrogen concentrations are shown in Figure 3 . In these data, the peak amplitude and area of the 2F hydrogen signal were proportional to the concentration of hydrogen in the sample cell. With increasing hydrogen concentration, a common peak position is observed. The data in Figure 3 also demonstrate the 2F spectrum of nitrogen selected for measurements in the spectral range and represent the optical noise of the system. Utilising an Inverse Least Squares regression, a calibration model was developed to accurately measure the hydrogen concentrations in the presence of the nitrogen sample matrix. The response variables used in the regression were the integrated values observed over three spectral bands in the 2F spectra. Specifically, a band centred at the peak in the water-vapour spectrum and two bands centred at each of the local minima adjacent to the peak were used. Concentration estimates were calculated as:
C j ∑ = 3
j,i R i
Where: C j = concentration estimates for each component a j,i = calibration coefficients R i = integrated band intensities The data shown in Figure 4 are the responses of the instrument to a series of hydrogen challenges over the concentration ranges of interest. The duration for each
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