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B. Lendl:
"Quantum Cascade Lasers: A new light source for analytical chemistry";
Vortrag: Nanospec 2008, Cluj-Napoca; 07.09.2008 - 10.09.2008.

The advent of mid-IR quantum cascade (QC) lasers opens the path for the development of rapid, accurate and highly precise methods in analytical chemistry. QC-lasers can be engineered to emit photons at almost any wavelengths from 2.7 - 180 µm. In QC-lasers photon emission occurs within the conducting band via intersubband transitions. These intersubband transitions and thus the laser wavelength, can be controlled by careful bandstructure engineering at monomolecular resolution. The active part of the QC-laser is realized by molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) techniques. QC-lasers are small in size, tuneable and can be operated at room temperature with average output power in the mW range. These characteristics make them highly promising light sources for portable analyzers. In this presentation two portable analyzers based on QC-laser technology are introduced.
The first analyzer is based on photoacoustic (PA) detection for gas sensing. This sensor concept takes advantage of the inherent selectivity provided by mid-IR laser spectroscopy and the small gas volume required in photoacoustic spectroscopy allowing rapid response. Using a distributed feedback QCL emitting at 4,3 µm, the sensor concept as been successfully tested for carbon dioxide measurement under harsh conditions during biogas upgrade in a pilot plant. In this application a reliable and rapid gas sensor is required for process control to assure a continuous CO2 concentration below 2%. The obtained results compared favorably with a state of the art NDIR analyzer used as a reference with the distinct advantage of faster response times. Due to the generic nature of the developed PA-QCL sensor concept selective on-line measurement of other important analytes in biogas, such as NH3 and H2S are possible as well.
The second analyzer determines hydrocarbons (HC) in waste water. When extracted from water HCs can be quantified directly due to their characteristic C-H vibrations in the mid-IR spectral region. We found that when using QCL technology non-toxic and non-ozone depleting cyclohexane can be used as an extraction solvent instead of Freon 113, commonly used for this purpose. Successful realization of this new concept is made possible by the high spectral power density of QCLs and differences in the mid-IR spectra of cyclochexane and non-cyclic hydrocarbons. The developed sensor system meets the requirements of the petrochemical industry as it covers the relevant concentration range from 0.1 -1000 mg/L of HC in waste water, provides a portable solution to an existing sensing problem and improves on standard technology (gas chromatography) in terms of speed, portability, robustness and cost.