[Zurück]

B. Lendl:
"Potential of mid-IR Quantum Cascade Lasers for the analysis of liquids and solids";
Vortrag: MIRIFISENSE workshop, Zürich (eingeladen); 14.01.2014 - 15.01.2014.

The continuous advancement in QCL technology holds promise to cause a step-change in the performance characteristic of mid-IR measurements of liquids and solids. This is motivated by the important advantages of QCL light sources over thermal light sources used in standard FTIR spectrometers: These are i) significantly higher spectral power densities, ii) inherent polarized radiation, iii) pulsed operation with high repetition rates along with small size, room temperature operation, which hold promise for the realization of truly miniaturized sensors for solving dedicated analytical problems. The use of QCL technology for measuring condensed phase samples is lagging behind gas phase analysis. This is because of the current limited tuning range of QCLs and the differences between mid-IR measurements of condensed (liquids and solids) phase and gas phase samples. Firstly absorption bands in condensed phase are significantly broader when compared to ro-vibrational absorption lines in gas phase with the consequence that band overlapping is frequently encountered. Secondly, in condensed phase the matrix usually does show significant absorption, which again is different to gas phase spectroscopy. In fact considering the analysis of liquids, most of the absorption is caused by the solvent and not by the analyte. Therefore, in FTIR spectroscopy only very short interaction lengths of a few micrometers (8-40 µm) can be accepted when dealing with e.g. aqueous solutions. Thus FTIR based analyzers face an inherent robustness problem as clogging of a flow cell or formation of a thin impurities layers on ATR (attenuated total reflection) crystals do cause severe problems concerning sensor robustness. In this respect QCLs significantly improve the situation by allowing for optical path-lengths >150 µm, which solves the robustness problem e.g. in blood analysis, as it does improve the sensitivity of measurement. As an example the simultaneous determination of 9 parameters in human plasma and serum using an EC-QCL (1030-1230 cm-1) will be shown. The generic use of this sensor will be demonstrated by its application as a molecular specific detector in liquid chromatography and for monitoring the cleaning process of batch reactors in the pharmaceutical industry. Concerning the analysis of solids QCLs are the enabling technology for achieving near-field MIR-microscopy with spatial resolution of a few tens of nm and monolayer sensitivity. This is achieved by detecting the thermal expansion of a sample by an AFM cantilever and operation in resonance conditions which requires QCL pulse frequencies in the kHz range.