oa Liquid Analysis Based On Quantum Cascade Lasers (QCL) Beyond Conventional Absorption Measurements

Mid-infrared (MIR) spectroscopy is a versatile and important tool for the analytics of liquids. Conventional absorption spectroscopy is based on exact intensity measurements to reveal information about the measured components. Therefore, stable light sources are needed for the detection of weakly absorbing, i.e. diluted substances. Often the strong absorption of the solvent itself makes exact measurements even more difficult. Moreover, it limits traditional spectroscopies using thermal light sources to selected spectral regions and solvents. The development of commercial widely tunable quantum cascade lasers (QCLs) has opened up new possibilities in mid-infrared vibrational spectroscopy. QCLs were first demonstrated in 1994 [1] and offer orders of magnitude (~104) more power compared to thermal light sources. This facilitates the use of measurement cells with path lengths > 100 µm for transmission measurements - an important requirement for fast process analytical applications. For instance a strongly absorbing aqueous solution of the amino acid proline (OD > 3) could successfully be measured [2]. Despite their high optical power and compact rugged design, QCLs are often plagued by intensity fluctuations, which limit the achievable sensitivity in absorption measurements. Here, two spectroscopic methods based on QCLs are presented. Both make use of the total internal reflection and can be adapted for fast process applications like online monitoring of water quality. First an QCL-based version of the so called Fiber Evanescent Field Analysis (FEFA) [3] is demonstrated [4], a special ATR-technique, where the light is guided inside an optical fiber. Due to the good collimation of the radiation emitted by the laser coupling losses can minimized. This makes QCLs an ideal light source for the FEFA-spectroscopy. Shown are the application and the results of this technique to the online-detection of water contaminations with pesticides. The second method is a form of MIR refractometry also making use of the total internal reflection. Changes in the beam profile after the reflection are used to simultaneously determine the refractive index and the absorption of the analyte without the need for an absolute detection of the intensity. This makes the method immune against intensity fluctuations of the laser. The principle is demonstrated with measurement results for dichloromethane (DCM), which exhibits a single absorption band in the examined spectral region. The work was funded by the Fraunhofer program ATTRACT (Grant 692247) and the cooperation project IRLSENS (BMBF, FKZ 13N11034). References: [1] J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, A. Y. Cho; Science 264 (1994) 553 - 556 [2] S. Lüdeke, M. Pfeifer, P. Fischer; J. Am. Chem. Soc. 133 (2011) 5704 - 5707 [3] T. Beyer, P. Hahn, S. Hartwig, W. Konz, S. Scharring, A. Katzir, H. Steiner, M. Jakusch, M. Kraft, B. Mizaikoff; Sensors and Actuators B 90 (2003) 319-323 [4] A. Lambrecht, M. Pfeifer, W. Konz, J. Herbst, and F. Axtmann; Analyst 139 (2014) 2070 - 2078 [5] M. Pfeifer, A. Ruf, P. Fischer; Optics Express 21 (2013) 25643 - 25654