Research Of The Detection And Analysis Of The Ammonia Concentration Based On The Terahertz Time-domain Spectroscopy

Due to the advantages of high signal-noise ratio (SNR), large dynamic range,wide spectrum, ability of reflecting molecule structure inside materials andpenetrability certain materials, Terahertz time-domain spectroscopy detectiontechnology (THz-TDS) has become focus in the testing area. When the THz-TDS isused in the field of gas detecting, more information of gases will be gotten, moreover,a higher testing sensitivity could be achieved.In this paper, quantitative detection on ammonia using the THz-TDS method atroom temperature and atmospheric pressure is studied. Theoretical relationshipsbetween the parameters in Lambert-Beer’s Law and the density of the demodulationresults are analyzed in detail; Both ends of the gas chamber is made of2mm PTFEplate that is used as the transparent part for Terahertz wave. The influence of thespectral resolution on the maximal intensity of absorption peaks of ammonia is doneand proved by experiments.Absorption spectrum and characteristic absorption peak under different densityare tested, and the amplitude of the absorption peak is obtained via Lorenz fitting.Three methods are carried out in the thesis:1the concentration of ammonia can beobtained by extracting the light intensity from the characteristic absorption peakwithout considering the effect of water;2after influence of the difference of waterdensity is considered, and then the water influence on the ammonia absorption peak,to a certain extent, is eliminated, we also get the concentration of ammonia;3according to the same proportion of intensity between the different absorption peak ofwater, absorption peak of the water is further subtracted from the characteristicabsorption peak of ammonia based on the second processing method. The comparisonresults among the three methods show that the third method has the highest fittinglinearity.Experimental results show that detection sensitivity and relative measurementerror are113ppm and2.64%, respectively. The results further prove the application ofthe THz-TDS in the area of gas detecting.