Development of Liquified Air/Fourier Transform Infrared Spectroscopic Method for Simultaneous Measurement of Atmospheric Trace Gases

提出了一种利用液化空气/傅里叶变换红外光谱同时测量大气痕量气体的新方法。该设备由傅里叶变换红外光谱仪、传递光学器件和含光学光管的低温恒温器组成。低温恒温器由氦制冷机和一根光管（直径4毫米，长20厘米，镀金）组成。引入低温恒温器的空气被氦制冷机液化，液化后的空气被引入光管。液化空气的红外光谱由分辨率为1 cm^<-1>的FTIR光谱仪采集。一氧化碳（在合成空气中为1.6 ppm）的吸收波段为2140 cm^<-1>，带宽为20 cm^<-1>。甲烷（在合成空气中为10 ppm）的吸收波段分别为1305和3016 cm^<-1>，带宽分别为10和20 cm^<-1>。这些谱带与空气中CO_2和H_2O的大饱和峰分离较好，可用于CO和CH_4的定量测定。在1305和3016 cm-1波段，利用S/N比值3估计CO的检出限为35 ppb， CH_4的检出限分别为23和40 ppb。与典型的大气CO浓度（100-500 ppb）和CH_4浓度（1.7-2.0 ppm）相比，这些检测限已经足够低了。结果表明，该方法适用于大气中微量气体的检测。

A novel method for simultaneous measurement of atmospheric trace gases using liquified air/Fourier transform infrared spectroscopy has been developed. The equipment consists of Fourier transform infrared spectrometer, transfer optics and a cryostat containing optical light pipe. The cryostat consists of helium refrigerator, and a light pipe (4 mm i. d. and 20 cm long, gold coated). The air introduced into the cryostat was liquified by the helium refrigerator and the liquified air was introduced into the light pipe. Infrared spectrum of the liguified air was taken by the FTIR spectrometer with a resolution of 1 cm^<-1>.Carbon monoxide (1.6 ppm in synthesized air) gave an absorption band at 2140 cm^<-1> with a band-width of 20 cm^<-1>. Methane (10 ppm in synthesized air) gave absorption band at 1305 and 3016 cm^<-1> with band-width of 10 and 20 cm^<-1>, respectively. Those bands were well separated from large saturated peaks of CO_2 and H_2O contained in air and found to be used for quantitative measurement of CO and CH_4. Detection limit estimated from S/N ratio of 3 was 35 ppb for CO,and 23 and 40 ppb for CH_4 using the bands of 1305 and 3016 cm-1, respectively. These detection limit is low enough compared with the typical atmospheric concentration of CO (100-500 ppb) and CH_4 (1.7-2.0 ppm). The proposed method was found to be applicable for the detection of these trace gases in the atmosphere.