Parabolic cell for low-background Raman analysis of gas samples.

A compact Raman system constructed by a parabolic sample cell and an imaging spectrograph, which has good capability for enhancing a Raman signal and compressing a continuous background, has been put forward. In the Raman spectra of ambient air acquired by this system, the signal level of N2 was enhanced up to 14 times compared with free space, and the related signal to background ratio was increased nearly to 96. With an integration time of 10 s, the rotational fine structure of O2 and N2 were clearly recognized. Besides, a standard analytic gas mixture consisting of H2, CO2, and CO was also tested, and the 3σ LODs of 68 ppm for H2, 54 ppm for CO2 and 116 ppm for CO were obtained.