This thesis introduces a new measurement principle to monitor cleaning processes of stirrer vessels in the pharmaceutical industry. Until now, the cleanliness of a vessel is evaluated by quantifying the total organic carbon of the cleaning liquid at the end of the cleaning sequence. With infrared spectroscopy, in contrast, it is possible to monitor the cleaning liquid continuously, allowing to shorten the cleaning process. The sensor is based on an external quantum cascade laser emitting radiation in the range of 1030-1230 cm-1. Samples from the process stream are continuously injected into a 165 -m flow cell whereas the windows are made out of CaF2. Finally, the transmitted radiation is quantified by an MCT-detector. The performance and applicability of the sensor are evaluated at a laboratory vessel, an industrial-scale plant at the Vienna University of Technology as well as at a co-operation partner (CMC Biologics, Copenhagen). In order to save resources and time consuming fermentations, two typical carbon sources, namely glycerol and xanthan gum, are used as contamination substances. Moreover, the manufacturing process of ring cavity surface emitting lasers in the wavelength range of 1470-1530 cm-1 is shown. Its intention is to quantify proteins in the process stream in the future as well.