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S. Radel, M. Brandstetter, B. Lendl, E. Benes:
"ATR mid-infrared spectroscopy of suspended particles spatially controlled by an ultrasonic standing wave";
Vortrag: ICU 2009 - International Congress on Ultrasonics, Universidad de Santiago de Chile, Santiago/Chile; 11.01.2009; in: "ICU 2009 - International Congress on Ultrasonics - Book of Abstracts", (2009), S. 65.

The growing use of biotchnology as a manufacturing route for e.g. antibiotics and other medical compounds stimulates the development of reliable sensors for bioprocess purposes. In order to keep control of the monitored bioprocess continued analysis with fast response times is needed. Furthermore it is necessary, that a sensor delivers information reflecting the condition of the suspension of cells and medium within the bioreactor as close as possible. Fullfilling these requirements we set out to develop a robust, fast in-line sensor delivering chemical information of the particles and the liquid of a suspension, respectively in combining two rather separate technologies: ATR vibrational spectroscopy and ultrasonic particle manipulation.
Vibrational spectroscopy is an optical measurement technique increasingly popular in process analytical chemistry because of its ability to directly provide molecular specific (bio-)chemical information about a given sample. The ATR (Attenuated Total Reflection) spectroscopy is a widely used method for mid-infrared vibrational spectroscopy especially in connection with highly absorbing samples like e.g. aqueous solutions. Only a thin film of some micrometers in the proximity of the ATR sensitive element is spectroscopically analyzed, beyond this evanescent field the instrument is "blind".
This opens the possibility to employ the radiation forces within an ultrasonic standing wave to manipulate the whereabouts of suspended particles relative to the ATR. More precisely the field is used to either push the particles towards the optical sensor or away from it. Infrared spectra showing the population and depopulation of the sensitive zone of the ATR with yeast cells within a few seconds will be shown.
Furthermore the effect of the presence of an ultrasonic standing wave on the ATR absorbance measurements of water was investigated, because if the specific absorbance of water turned out to be changed by the ultrasonic field, the acquired spectra potentially can be misinterpreted.
Hence absorbance spectra of water irradiated by an ultrasonic standing wave as well as of samples of temperate water in the range between 23°C and 45°C were recorded. Variations in the infrared absorbance when applying an ultrasonic standing wave could be observed and were linked to the spectral variations caused by changes in the water temperature. By comparison to conventional heated samples it was concluded that the observed variations in the spectral absorbance of water were most likely induced by local temperature changes in the proximity of the ATR probe due to the energy dissipation in the ultrasonic standing wave.
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