应用拉曼光谱定量分析流体中硫酸盐质量摩尔浓度:内标选择和流体组分对分析结果的影响

Influences of internal standards and fluid composition on Raman spectroscopic determination of the molality of dissolved sulfate

硫酸盐是地质流体中重要的组成部分,流体包裹体是反映地质流体组分最直接和有力的指标,而应用拉曼光谱可以对封存于包裹体中的富硫酸盐流体成分进行定性和定量表征。本文制备了一系列含一定浓度的Na2SO4和Na2SO4-NaCl溶液的熔融毛细硅管胶囊,应用高分辨率激光拉曼光谱仪在室温条件下收集了这些溶液中SO42?的对称伸缩振动(v1-SO42-)、水的弯曲振动(v2-H2O)和伸缩振动(vs-H2O)光谱,建立了硫酸盐质量摩尔浓度(m(SO42-))与光谱参数之间的定量关系。研究结果表明,对于硫酸盐溶液而言,v2-H2O和vs-H2O谱峰均可以作为拉曼定量分析m(SO42-)的内标,v1-SO42-与v2-H2O或vs-H2O的峰强(I)或者峰面积(A)比均与m(SO42-)呈良好的线性正相关关系。然而,对于富含氯化物的硫酸盐溶液而言,应用v2-H2O光谱峰强或者峰面积为内标建立的校准曲线将导致所获得的m(SO42-)偏低。在这种情况下,应用vs-H2O光谱的峰面积为内标,或者将vs-H2O光谱拟合为位于3230cm^-1和3450cm^-1的两个高斯组分,并以其峰面积之和为内标,将会在很大程度上消除氯化物的影响,得到更加准确的m(SO42-)信息。考虑到拉曼定量因子与仪器状态和实验条件有关,提出了实验室可应用熔融毛细硅管制备一系列含一定硫酸盐质量摩尔浓度的溶液标样,每次实验之前收集标样的光谱并建立相应的m(SO42-)校准曲线,然后应用于样品m(SO42-)的拉曼定量分析。

Sulfate is an important component of geological fluids. Fluid inclusions are the most direct and powerful indicators of geological fluid composition, and Raman spectroscopy can be used to characterize and quantify the sulfate-rich fluid components encapsulated in fluid inclusions. In this study, a series of fused silica capillary capsules containing certain concentrations of Na2SO4 and Na2SO4-NaCl solutions were prepared. The stretching vibration spectrum of SO42?(v1-SO42?), the bending vibration spectrum (v2-H2O), and the stretching vibration spectrum (vs-H2O) of water were collected using a high-resolution laser Raman spectrometer at room temperature (23℃). Results show that, for sulfate solutions, the v2-H2O and vs-H2O peaks can be used as the internal standard for Raman spectroscopic quantification of the molality of sulfate (m(SO42-)). The peak intensity (I) and peak area (A) ratios between v1-SO42- and v2-H2O or vs-H2O both increased nearly linearly with m(SO42-) in Na2SO4 solutions. Therefore, several quantitative relationships between m(SO42-) and the spectral parameters (namely I(v1-SO42-)/I(v2-H2O), I(v1-SO42-)/I(vs-H2O), A(v1-SO42-)/A(v2-H2O), and A(v1-SO42-)/A(vs-H2O)) were established. However, applying the calibration curve with I(v2-H2O) or A(v2-H2O) as an internal standard to the NaCl-Na2SO4 solution will generate a lower m(SO42-) value. In this case, applying the calibration curve with A(vs-H2O) as the internal standard, or fitting the vs-H2O spectrum with two Gaussian components at 3230 cm^-1 and 3450 cm^-1, and then using the total peak areas as the internal standard will largely eliminate the influence of chloride and produce more accurate m(SO42-) data. Considering that the Raman quantitative factor is related to the instrument state and experimental conditions, we suggest that Raman spectra of a series of standard solutions should be collected and fitted to establish the m(SO42-) calibration curve prior to each experiment.