Algorithm for concentration analysis with laser absorption spectroscopy

To measure the concentration of a fluid under consideration by means of tunable diode laser spectroscopy an algorithm for automatic detection of absorption transitions has been developed. The algorithm consists of a three step process. In the first step, necessary information is taken from the HITRAN database. Measured data are analyzed in the second step in order to eliminate the laser characteristics underlying the absorption spectrum, to localize the absorption lines and to get convenient initial values for fitting the absorption transitions to a Lorentzian line shape profile in step 3. The concentration is calculated from the best approximation among all parameters from the transitions found on the signal. The developed algorithm was tested with a set of simulated curves with different concentrations of carbon dioxide transitions at the 2.004 μm band and it could analyse concentrations with relative error better than 1% for concentrations down to approximately 200 ppb. Computation time including acquisition time and graphical output display is fast enough to enable online monitoring of fluid concentrations.