Optimization of parabolic cell for gas Raman analysis.

Raman spectroscopy is a widely practicable technique in gas analysis, but the application in simultaneous detection of multiple‐trace gas is still a challenge for its weak signal level. To extend the application of Raman gas analysis to detection of trace gas constituents, a closed parabolic sample cell proposed in our previous work is optimized by optical analysis and experimental verification. The optical analysis shows that for a parabolic reflector with rim diameter 6p (p, semi‐latus rectum), the collecting solid angle can be as large as 90% of full space (4π) and the power‐collecting efficiency can be as high as 80%. The variation of the Raman signal level with the position of collecting aperture and the parameter p of parabolic reflector was verified by the experimental results. At the optimized structure parameters, we get a signal‐to‐background ratio of 122 and a signal‐to‐noise ratio of 305 in the Raman spectra of ambient air with exposure time of 1 s. From the results of standard analytic gaseous samples, the limits of detection of 33 ppm‐bar for H2, 38 ppm‐bar for CO, 19 ppm‐bar for CH4, 27 ppm‐bar for C2H6, 23 ppm‐bar for C2H4, and 20 ppm‐bar for C2H2 were obtained with exposure time of 200 s. These results indicate that the parabolic sample cell is a competitive Raman collector, which showing good balance in signal enhancement and background level. [ABSTRACT FROM AUTHOR]