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S Schaden, B. Lendl:
"Quantum Cascade Laser for Determination of Solutes in Liquid Phase";
Poster: 5th QCL-Workshop, Freiburg; 23.09.2004 - 24.09.2004.

QCLs have the potential to be applied in industrial relevant process analyzers. While most sensing applications of QCLs so far have been shown in gas phase, QCLs may also be advantageously applied for determination of solutes in liquid phase. Room temperature operation, small size and high spectral density make them ideal light sources for dedicated absorption measurements also in aqueous phase. Unfortunately, their high cost is hindering their application in industrial relevant sensors despite of their capability to solve relevant analytical problems e.g. in the food industry. It is the aim of this contribution to highlight this fact on the example of the direct determination of dissolved CO2 in beverages. Similar applications covering other analytes such as glucose, ethanol or artificial sweeteners could be targeted in a similar way. In the second part of the contribution the use of QCL as a new detection principle in capillary electrophoresis is shown. In both cases the laser beam was collimated and focused on a flow cell using offaxis parabolic mirrors. A photovoltaic MCT detector with a response time of 3,5 ns was used. The detector signal was recorded using a boxcar averager and stored using a home made software program. For CO2 determination a DFB-QCL (Alpes Laser) lasing near room temperature at 4,29 µm and a 119 µm flow-cell equipped with CaF2 windows have been used. Standards of different CO2 concentrations were prepared by purging defined ratios 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. The whole optical system was purged with nitrogen to remove any gaseous CO2 from the optical path. 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. A FP-QCL (Alpes Laser) based MID-Infrared absorption spectroscopy in capillary electrophoresis is also demonstrated here for the first time. To overcome the problem of total IR absorption of the fused-silica capillaries that are normally employed in CE separations, a micromachined IR-transparent flow cell was developed. The cell consisted of CaF2 plates separated by a polymer coating (SU-8). The flow channel was about 2 mm long, 150 µm broad and with an optical path of 60 µm. The cell was placed in an inhouse made cell holder with special design to connect the capillaries with commercial O-rings to the cell. Results of a separation of 4 analytes are reported.