Measurements of Bicarbonate in Water Containing Ocean-Level Sulfate Using a Simple Multi-Pass Optical Raman System

The concentration of dissolved inorganic carbon in the oceans at depths of a few meters to thousands of meters is a critical parameter for understanding global warming. The concentration is both pH dependent and depth dependent. Current analysis that employs pH meters must account for several other parameters, such as salinity, temperature, pressure, and the dissolved carbon's form, carbon dioxide, bicarbonate, or carbonate. Recently, Raman spectroscopy has been used to measure these forms directly in water at similar to 1000 ppm, which is unfortunately insufficient for typical ocean concentrations, such as similar to 115 ppm bicarbonate near the surface. Here, we employed a simple multipass optical system, a flat mirror to reflect the laser back through the sample, and a concave mirror opposite the entrance slit that effectively doubled the laser power and the collected Raman photons, respectively. This multi-pass optical Raman system with a 1.5W, 532 nm laser was used to measure 30 ppm bicarbonate in water that contained 2650 ppm sulfate to simulate ocean water, a bicarbonate concentration well below that near the ocean surface. Furthermore, spectral analysis employed the bicarbonate C=O symmetric stretch at 1360 cm(-1) instead of the C-OH stretch at 1015 cm(-1) to avoid the intense, overlapping sulfate SO4 symmetric stretch at 985 cm(-1). The calculated standard deviation of similar to 5 ppm for the described approach suggests that accurate measurement of bicarbonate in situ is possible, which has been, heretofore, either calculated based on pH or measured in a lab.

Automated summary

Researching the concentration of dissolved inorganic carbon in the oceans is crucial for understanding global warming, and current analysis needs to consider various factors. The recent use of Raman spectroscopy to directly measure the forms of inorganic carbon in water presents an effective method for accurately measuring low concentrations of bicarbonate in the ocean.