Practical issues in FTIR spectroscopy Lab/demo section ...

T BioMolecular Vibrational spectroscopy : Part 1: Principles of Infrared, Raman Spectra and Techniques Tim Keiderling University of Illinois at Chicago Practical issues in FTIR spectroscopy Lab/demo section -Warwick CD Workshop T FTIR optical layout static polarization Polarizer (rotate , - typically wire grid) Sample Interfer- ometer Detector Source Digilab schematic Moving mirror Fixed mirror HeNe laser alignment Voice coil Air Bearing Aperture T Alternate, simpler FT-IR : Bomem Michelson Design often found in routine instruments, no air needed, highly stable T Acquisition of an Infrared Spectrum -1 0 1 2 4000 3980 3960 3940 3920 0 10 20 30 40 4000 3000 2000 1000 -2 -1 0 1 2 3 4000 3980 3960 3940 3920 10 20 30 40 50 4000 3000 2000 1000 Interferogram of the source & sample Interferogram of the source Spectrum of the source (blank) Spectrum of the source & sample FT FT Fast Fourier transform done in computer Convert Interferogram to Spectrum (single beam) T Acquisition of an Infrared Spectrum 0 10 20 30 40 4000 3000 2000 1000 10 20 30 40 50 4000 3000 2000 1000 40 0 20 60 80 100 4000 3000 2000 1000 transmittance spectrum of the sample ( polystyrene film ) Spectrum of source only (no sample present) Spectrum of source & sample Divide by: gives Absorbance spectrum -log10T I/I0 4000 3000 2000 1000 I I0 Single beam sample Single beam background Transmittance Absorb

• Need extremely high intensity source, spectrometer with high degree of discrimination against stray light, extremely sensitive detection system able to detect small numbers of ... Useful for micro cells or for small crystals or solids . T Steps in Measurement, Processing FTIR

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