This paper reports methods for obtaining time-dependent reduced isotropic Raman spectra of aqueous species in quartz capillary high-pressure optical cells under hydrothermal conditions, as a means of determining quantitative speciation in hydrothermal fluids. The methods have been used to determine relative Raman scattering coefficients and to examine the thermal decomposition kinetics of the non-complexing anions bisulfate (HSO₄⁻), perchlorate (CIO₄⁻), perrhenate (ReO₄⁻), and trifluoromethanesulfonate, or “triflate” (CF₃SO₃⁻) in acidic and neutral solutions at temperatures up to 400 °C and 30 MPa. Arrhenius expressions for calculating the thermal decomposition rate constants are also reported. Thermal stabilities in the acidic solutions followed the order HSO₄⁻ (stable) > ReO₄⁻ > CIO₄⁻ > CF₃SO₃⁻ with half-lives (t_1/2) > 7 h at 300 °C. In neutral solutions, the order was HSO₄⁻ (stable) > CF₃SO₃⁻ > ReO₄⁻ > CIO₄⁻, with t_1/2 > 8 h at 350 °C. CF₃SO₃⁻ was extremely stable in neutral solutions, with t_1/2 > 11 h at 400 °C.

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Non-Complexing Anions for Quantitative Speciation Studies Using Raman Spectroscopy in Fused Silica High-Pressure Optical Cells under Hydrothermal Conditions