Quantitative Analysis of Mixed Volatile Fluids by Raman Microprobe Spectroscopy: A Cautionary Note on Spectral Resolution and Peak Shape

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Raman analyses of fluid inclusions can yield quantitative information on composition (from peak areas and heights) and density (from peak position and width). In this study, we examine the effect of instrumental spectral resolution on the ratios of these spectral parameters, and the selection of appropriate integration limits for the determination of peak areas in the CO2-CH4-N2 system. Spectral resolution was varied from about 1 to 9 cm−1 by co-varying the widths of all spectrometer slits. Changes in resolution produced a modest effect on peak-area ratios and a significant effect on peak-height ratios. Measured peak-width ratios varied strongly as a function of the spectral resolution. In addition, we observed a moderate shift in the measured peak position of N2, which can be related to the asymmetry of the band. These results indicate that accurate analysis requires careful attention to the selection of quantification factors, especially if the selected values were derived from studies at different spectral resolutions. Another factor that can have a significant effect on the calculated compositions of CH4- and H2-bearing fluid mixtures is the band broadening that occurs with increasing pressure.

Keywords: CO2-CH4-N2 gas mixtures; Quantitative analysis; Raman spectroscopy; Spectral resolution

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/0003702934067045

Affiliations: 1: Chemistry Division, Oak Ridge National Laboratory, P.O. Box 2008, Building 4500S, Oak Ridge, Tennessee 37831-6110 and Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 2: Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, One Brookings Drive, St. Louis, Missouri 63130 3: Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, One Brookings Drive, St. Louis, Missouri 63130; current address: School of Geology and Geophysics, The University of Oklahoma, 100 East Boyd Street, Norman, OK 73019-0628

Publication date: June 1, 1993

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