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S Schaden, M. Haberkorn, J. Frank, B. Lendl:
"QUANTITATIVE ANALYSIS OF CO2 IN WATER USING MID-IR QUANTUM CASCADE LASER";
Poster: ICAVS II, Nottingham (UK); 19.08.2003 - 24.08.2003.

The determination of carbon dioxide in aqueous samples is an industrially important task. Current process analyzer for CO2 expands the volume of a sample drawn from the process stream while stirring. A pressure sensor measures the increase in gas pressure caused by the released gas. These values are then related to the CO2 concentration in the liquid via the law of Henry. This measuring procedure entails several drawbacks such as poor selectivity, moving parts in the process analyzer as well as a delay between data points of 10 seconds. In this contribution we present a powerful direct optical method for CO2 analysis based on a quantum cascade lasers lasing at room temperature 2332 cm-1. For measurement a 119µm flow-cell equipped with CaF2 windows was used. Standards of different CO2 concentrations were prepared by purging defined rations of CO2 and nitrogen through a gas wash bottle at a controlled temperature (20 °C). The bottles were connected to the flow cell via an automated pump system. As a detector a photovoltaic MCT detector with a response time of 3,5 ns was used. The whole optical system was purged with nitrogen to remove any gaseous CO2 from the optical path. The laser was operated at -30 °C to shift the laser line of the available QCL as much as possible in the absorption band of the carbon dioxide. 35 ns long pulses were produced at a frequency of 20 kHz resulting in a duty cycle of 0,07 %. The detector signal was recorded using a boxcar averager and stored using a home made software program. A linear calibration extending from 367 to 1350 mg/l CO2 was obtained. The limit of determination was calculated as three times the r.s.d. of the noise level over time and found to be 39 mg/L.