Raman spectrometer for field determination of H2O in natural gas pipelines

Abstract

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.