Date of Award
2023
Document Type
Open Access Dissertation
Department
Chemistry and Biochemistry
First Advisor
Timothy J Shaw
Abstract
Carbon dioxide enters the ocean through a variety of natural and anthropogenic sources, and has a significant impact on ocean acidification, the marine buffering system, and bioavailability of life-essential carbonate ions. As carbon dioxide dissolves it reacts with water to form carbonic acid, which then rapidly dissociates into hydronium, bicarbonate, and carbonate ions. Some hydronium ions protonate carbonate to form bicarbonate, driving the carbonate concentration lower while there is a net increase in hydronium concentration. Monitoring the carbonate equilibrium system in the ocean is a complex analytical problem, where speciation is often measured or calculated indirectly ex situ. It is therefore desirable to have an in situ method of direct measurement of carbonate speciation. Raman spectroscopy is an attractive tool, as it is a non-destructive technique that requires no sample preparation and provides a unique chemical signature for molecular species. Current oceanographic Raman spectrometers are large and bulky and may suffer from misalignment during deployment. The monolithic spatial heterodyne Raman spectrometer (mSHRS) designed by the Angel Group is a fixed-grating interferometer with no moving parts, which is rugged and compact, weighing only ~80 g. Because the optical components are fused together in a solid-state package, the instrument is robust, making it insensitive to shock and vibration. High stability, combined with high light throughput, make the mSHRS ideally suited to field deployment. This work describes the first quantitative Raman measurements of the carbonate system in dilute aqueous solutions with an mSHRS. Using water as an internal standard, calibration curves were created as either the area ratio or intensity ratio of the analyte Raman band to the water Raman band versus the analyte to water concentration. Limits of detection (LOD) for carbonate and bicarbonate were determined on the mSHRS to bemM, respectively. These calibration curves were shown to reliably quantify carbonate species within error ofin situ. Furthermore, the carbonate to bicarbonate Raman band area ratios were used to calculate the pH of these solutions. As pH is considered a master descriptive variable in all equilibrium reactions that involve proton transfer, i.e. acid-base reactions, its accurate determination is invaluable in ocean chemistry studies. The Raman-calculated pH values were compared to those measured with a glass electrode and spectrophotometrically determined pH values, with similar results. Two configurations of the mSHRS system utilizing different magnifications are discussed. The performance of the mSHRS in terms of spectral resolution, signal-to-noise ratio (SNR), and carbonate LOD was assessed by comparison to measurements made using a conventional laboratory dispersive Raman spectrometer. While the resolution and SNR of the conventional instrument were better than the mSHRS, the carbonate LOD was lower by about ~1 mM.
Rights
© 2023, Amanda Carol Agrawal
Recommended Citation
Agrawal, A. C.(2023). Oceanographic Applications of a Monolithic Spatial Heterodyne Raman Spectrometer. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/7665