A Direct Bicarbonate Detection Method Based on a Near-Concentric Cavity-Enhanced Raman Spectroscopy System

Abstract

Raman spectroscopy has great potential as a tool in a variety of hydrothermal science applications. However, its low sensitivity has limited its use in common sea areas. In this paper, we develop a near-concentric cavity-enhanced Raman spectroscopy system to directly detect bicarbonate in seawater for the first time. With the aid of this near-concentric cavity-enhanced Raman spectroscopy system, a significant enhancement in HCO₃^- detection has been achieved. The obtained limit of detection (LOD) is determined to be 0.37 mmol/L-much lower than the typical concentration of HCO₃^- in seawater. By introducing a specially developed data processing scheme, the weak HCO₃^- signal is extracted from the strong sulfate signal background, hence a quantitative analysis with R² of 0.951 is made possible. Based on the spectra taken from deep sea seawater sampling, the concentration of HCO₃^- has been determined to be 1.91 mmol/L, with a relative error of 2.1% from the reported value (1.95 mmol/L) of seawater in the ocean. It is expected that the near-concentric cavity-enhanced Raman spectroscopy system could be developed and used for in-situ ocean observation in the near future.

Keywords: bicarbonate; direct detection; laser Raman spectroscopy; ocean application.

Figure 1

Schematic of the near-concentric cavity-enhanced Raman spectroscopy system for liquid sample detection.

Figure 2

Raman spectra of a range of Na₂SO₄ solution (0.10 mmol/L to 2.00 mmol/L). (a) Four spectra of typical concentration. (b) The linear relationship between concentrations and peak intensities of the SO₄²⁻ signal.

Figure 3

The detection ability of the NC-CERS system for HCO₃⁻. (a) The spectra of 5 NaHCO₃ aqueous solutions with different concentrations. (b) The linear relationship between concentrations and peak intensities of the HCO₃⁻ signal. (c) The Raman spectra of three mixed solutions which contains fixed NaSO₄ concentration (28.00 mmol/L) and different NaHCO₃ concentrations. (d) The detailed spectra (1000–1050 cm⁻¹) of mixed solutions.

Figure 4

The signal extraction method for quantitative analysis of HCO₃⁻. (a) The normalized spectrum of mixed solution with 28.00 mmol/L Na₂SO₄ and 1.93 mmol/L NaHCO₃. (b) Baseline correction. The red dotted line represents the baseline curve fitted by a polynomial function, and the blue line is the baseline corrected curve. (c) The signal extraction method for HCO₃⁻. The green dotted line represents the baseline B fitted by a double exponential function. The extracted signal is shown as black circle dots and fitted by using a Gaussian function (red line). (d) Fitted HCO₃⁻ Raman signals of 1000 m depth seawater and four mixed solutions with fixed 28.00 mmol/L Na₂SO₄ and different concentrations of NaHCO₃. (e) The calibration curve of HCO₃⁻. The pink point is the result of a blind sample, and the blue point is the result of a seawater sample.