Characterization and quantitation of a tertiary mixture of salts by Raman spectroscopy in simulated hydrothermal vent fluid.

This article will demonstrate that Raman spectroscopy can be a useful tool for monitoring the chemical composition of hydrothermal vent fluids in the deep ocean. Hydrothermal vent systems are difficult to study because they are commonly found at depths greater than 1000 m under high pressure (200-300 bar) and venting fluid temperatures are up to 400 degrees C. Our goal in this study was to investigate the use of Raman spectroscopy to characterize and quantitate three Raman-active salts that are among the many chemical building blocks of deep ocean vent chemistry. This paper presents initial sampling and calibration studies as part of a multiphase project to design, develop, and deploy a submersible deep sea Raman instrument for in situ analysis of hydrothermal vent systems. Raman spectra were collected from designed sets of seawater solutions of carbonate, sulfate, and nitrate under different physical conditions of temperature and pressure. The role of multivariate analysis techniques to preprocess the spectral signals and to develop optimal calibration models to accurately estimate the concentrations of a set of mixtures of simulated seawater are discussed. The effects that the high-pressure and high-temperature environment have upon the Raman spectra of the analytes were also systematically studied. Information gained from these lab experiments is being used to determine design criteria and performance attributes for a deployable deep sea Raman instrument to study hydrothermal vent systems in situ.