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.