Cavity-Enhanced Raman Spectroscopy of Natural Gas with Optical Feedback cw-Diode Lasers

Multicomponent analysis in the gas phase is an important, but difficult task in analytical chemistry when some components are at great excess and others at trace levels. This is particularly relevant for monitoring natural gas which may contain different trace compounds, some of them toxic, at varying concentrations depending on the source. Common analytical techniques for multi-gas monitoring include gas chromatography (GC) or mass spectrometry (MS); whilst sensitive and selective, they require expensive equipment and have practical limitations, including difficulties detecting certain components and the need for sample preparation which prevents real-time, in-situ monitoring. GC is a sequential technique with long analysis time, and MS has difficulties distinguishing relevant isomers.

Spectroscopic techniques are indispensable in analytical applications since they are non-intrusive, require little or no sample preparation, provide real-time data and allow in-situ monitoring with spectroscopic selectivity and unprecedented sensitivity. Direct absorption techniques, like FTIR or diode laser near-IR spectroscopy, are widely used in analytical chemistry, but some molecules are quite difficult to detect, for example diatomic homonuclear molecules such as H₂ or N₂. This is of particular relevance for monitoring natural gas, where these gases can be minor components, or for monitoring the purity of biofuels or hydrogen gas produced by biotechnology or by alternative energies.

 Due to different selection rules, Raman spectroscopy can monitor all relevant components. The spectroscopic peaks in the Raman spectrum can be used for conclusive identification of single compounds or of individual components in mixtures. Applications of Raman spectroscopy for trace gas analysis, however, has not found wide-spread use so far due to the inherent weakness of Raman transitions, and is thus mainly employed in condensed phases. Raman detection of natural gas has been demonstrated before, but high power lasers and high sample pressures are needed to achieve sensitivity. Trace gas Raman spectroscopy at atmospheric pressure requires special Raman techniques which often need large laser systems and sophisticated equipment. Methods to increase sensitivity include stimulated Raman techniques such as PARS (photoacoustic stimulated Raman spectroscopy) and CARS (coherent anti-Stokes Raman spectroscopy), and fibre-enhanced or cavity-enhanced Raman spectroscopy.

 We have recently introduced a sensitive Raman technique where an inexpensive diode laser of low or moderate power is enhanced by several orders of magnitude in a high-finesse  optical cavity. Cavity-enhanced Raman spectroscopy with optical feedback diode lasers (CERS) is very selective due to high spectral resolution and its high sensitivity allows trace gas detection in multicomponent analysis. In the highlighted article, we describe advancements made on this technique which improves the optical stability and improves previously reported detection limits. We further introduce a relevant analytical application of the CERS technique, the multicomponent analysis of natural gas samples, and demonstrate that CERS with low power diode lasers is suitable for online monitoring of natural gas mixtures, including monitoring H₂, H₂S, N₂, CO₂, and alkanes in-situ and on-line, with noise-equivalent detection limits at 1 s integration time below 1 mbar at 1 bar total pressure, depending on Raman cross sections. Detection limits can be easily improved with longer integration times, using higher power diodes or working at higher sample pressures.