Raman-Based Internal Standardization for Quantitative LA-ICP-MS Analysis of Individual Fluid Inclusions

A central challenge in quantitative laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of individual fluid inclusions is the lack of a directly measurable internal standard. Current approaches typically rely on Na concentrations inferred from microthermometric NaCl-equivalent salinity, yet in multicomponent fluids this approach can introduce considerable bias because the effects of non-chloride solutes on phase-transition (e.g., ice melting) temperatures remain poorly constrained. Here, we introduce an integrated Raman microspectroscopy and LA-ICP-MS workflow that overcomes this limitation by converting the Raman-derived concentration of boric acid (B(OH)3) to an equivalent elemental B concentration for internal standardization. Our results show that the ν1(BO3)/ν2(H-O-H) area ratio is markedly less sensitive to salinity than conventional normalization to the O-H stretching envelope. In quartz-hosted synthetic fluid inclusions, Aν1(BO3)/Aν2(H-O-H) follows a single linear calibration for 10 and 20 wt % NaCl solutions (y = (8.68 ×10-5)x, R2 = 0.9995), enabling direct determination of elemental B concentrations without salinity-specific calibration. Ab initio molecular dynamics Raman simulations support the molecular basis of this metric. Using Raman-derived B concentrations as the internal standard yields mean signed deviations of -11.9 to 6.4% and mean absolute relative deviations of 1.9-11.9% for Li, Na, Mg, K, Rb, Sr, and Cs, whereas conventional microthermometry-based Na standardization produces systematic positive deviations of 8.5-38.0%. This strategy improves quantification of B-bearing fluid inclusions and establishes a general analytical framework for quantitative multielement analysis of aqueous systems containing Raman-active dissolved species.