Spontaneous Raman spectroscopy was used to measure changes in the ratio of water vapor to molecular nitrogen gas in laboratory air sample with relative uncertainty of 5 × 10−4 during 100 s observations. Experimental data were collected during 100 s-runs in order to obtain sufficient signal-to-noise ratios. Estimated detection limit of water vapor at signal-to-noise ratio equal to 1 was determined to be equal to 1.4 × 1013 molecules per cm3, which is equivalent to molar concentration of 7.4 × 10−7 at Standard Temperature and Pressure (STP). The achieved sensitivities make our Raman spectrometer suitable for noninvasive, rapid monitoring of gaseous species with very broad applications, for example in natural gas pipe-lines. An inexpensive, 70 mW power, multi-mode, diode-pumped solid-state laser operating near 532 nm served as an excitation source. The high sensitivity was achieved using a multi-pass cell built out of high reflectivity concave mirrors.
Chibirev, I., Mazzoleni, C., van der Voort, D. D., Borysow, J., & Fink, M. (2018). Raman spectrometer for field determination of H2O in natural gas pipelines. Journal of Natural Gas Science and Engineering, 55, 426–430. https://doi.org/10.1016/j.jngse.2018.05.015