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8 October 2003 Application of FTIR ellipsometry to detect and classify micro-organisms
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Here is presented a new method to detect and classify microorganisms from their polarimetric response in the mid-infrared range (2-12 μm) measured by FTIR ellipsometry. Apart from the ellipsometric measurements, the performance of the methods also stands on the simplicity of sample preparation and on the data analysis. Spectroscopic ellipsometry is a non-invasive optical characterization technique sensitive to the polarization of the light reflected or transmitted by a sample. The extreme sensitivity of ellipsometry allows the detection of minute changes on the sample surface, even at the monomolecular layer level. In the mid-infrared range each molecule exhibits a characteristic absorption fingerprint, thus making ellipsometry chemically selective. FTIR ellipsometry is used here for the first time to analyze bacteria grown in culture media. Sample preparation is extremely simple and consists of the evaporation of a droplet of an aqueous suspension of microorganisms on a planar surface. Ellipsometric measurements are performed on the solid residue left on the surface after the evaporation of the droplet. Data analysis can be divided in two steps. First, simplification of the measured spectra by Principal Component Analysis (PCA), which is one of the existing multivariate statistical techniques commonly used to eliminate redundant information. Second, classification of the simplified spectra using a standard clustering method. As a result, we show how this method can be employed to discriminate and identify bacteria at the species level. The results of this experiment are very promising for the application of ellipsometry for analytical purposes in biochemistry and in medicine.
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Enric Garcia-Caurel, Jacqueline Nguyen, Laurent Schwartz, and Bernard Drevillon "Application of FTIR ellipsometry to detect and classify micro-organisms", Proc. SPIE 5141, Diagnostic Optical Spectroscopy in Biomedicine II, (8 October 2003);

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