Abstract
A method for the direct determination of carbon dioxide in aqueous solutions using a room-temperature mid-infrared (MIR) quantum cascade laser at 2330 cm<sup>–1</sup> is reported. The absorption values of different carbon dioxide concentrations were measured in a 119 μm CaF<sub>2</sub> flow-through cell. An optical system made of parabolic mirrors was used to probe the flow cell and to focus the laser beam on the mercury cadmium telluride (MCT) detector. Aqueous carbon dioxide standards were prepared by feeding different mixtures of gaseous N<sub>2</sub> and CO<sub>2</sub> through wash bottles at controlled temperature. The concentration of the dissolved CO<sub>2</sub> was calculated according to Henry's law, taking into account the temperature and the partial pressure of CO<sub>2</sub>. The carbon dioxide standards were connected via a selection valve to a peristaltic pump for subsequent, automated measurement in the flow-through cell. A calibration curve was obtained in the range of 0.338 to 1.350 g/L CO<sub>2</sub> with a standard deviation of the method<i> s</i><sub>xo</sub> equal to 19.4 mg/L CO<sub>2</sub>. The limit of detection was calculated as three times the baseline noise over time and was determined to be 39 mg/L.
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