Oceanographic Applications of a Monolithic Spatial Heterodyne Raman Spectrometer

• Main Author: Agrawal, Amanda Carol
• Format: Text
• Language: unknown
• Published: Scholar Commons 2023
• Subjects: marine bicarbonate; marine carbonate system; monolithic spatial heterodyne Raman spectrometer; ocean acidification; Raman spectroscopy; spatial heterodyne spectrometer; Carbonic acid; Ocean acidification
• Online Access: https://scholarcommons.sc.edu/etd/7665; https://scholarcommons.sc.edu/context/etd/article/8570/viewcontent/Agrawal_sc_0202A_19051_1_.pdf

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 ...