1. FT-IR Instrument

2. Components Source Michelson Interferometer Sample Detector

3. Sources Black body radiators Inert solids resistively heated to 1500-2200 K Max radiation between 5000-5900 cm -1 (2-1.7  m), falls off to about 1 % max at 670 cm -1 (15  m) Nernst Glower – cylinder made of rear earth elements Globar- SiC rod CO 2 laser Hg arc (Far IR), Tungsten filament (Near IR)

4. Michaelson Interferometer 10 14 Hz is too fast for the rapid changes in power to be directly measured as a function of time. Can not measure the FID signal directly Interferometer creates a replicate interference pattern at a frequency that is a factor of 10 10 times slower 10 4 -10 5 Hz can be measured electronically f = (2v m /c)  10 -10  v m = 1.5 cm/s

5. Michaelson Interferometer Beam splitter Stationary mirror Moving mirror at constant velocity Motor driven Micrometer screw He/Ne laser; sampling interval, control mirror velocity

6. Source Stationary mirror Moving mirror Sample Detector Beam Splitter PMT HeNe laser

7. Sample Sample holder must be transparent to IR- salts Liquids –Salt Plates –Neat, 1 drop –Samples dissolved in volatile solvents- 0.1-10% Solids –KBr pellets –Mulling (dispersions) Quantitative analysis-sealed cell with NaCl/NaBr/KBr windows

8. Detector Transducers –The heating effect of radiation Thermal transducer- black body, small, very low heat capacity-  T=10 -3 K, housed in vacuum, signal is chopped Thermocouples –Two junctions of dissimilar metals, An and Bi –One is IR detector, one is reference detector –Potential difference that develops in proportional to  T; detection of  Ts of 10 -6 K is possible

9. FT-IR detectors Pyroelectric tranducers (PTs) Pyroelectric substances act as temperature- dependent capacitors Triglycine sulfate is sandwiched between two electrodes. One electrode is IR transparent The current across the electrodes is Temperature dependent PTs exhibit fast response times, which is why most FT instruments use them

10. Photoconducting transducers Thin film of a semi-conducting material IR radiation promotes non-conducting electrons to a higher energy conducting state. The voltage drop across the thin film is a measure the Power of the IR beam. PbS for near IR can be operated at RT Hg/Cd/Te can be used in the mid-IR and far IR, but must be cooled to 77 K Superior response characteristics Great for GC-IRs

11. Setting up an experiment Factors you can control –Spectral Resolution –Number of scans averaged –These combine to determine the overall time required to collect a spectrum Signal/Noise ratio  N 1/2 If S/N ratio is 3 for 1 scan, you can expect the S/N to increase to 30 if you collect and average 100 scans

12. Selectivity Offers much more selectivity that UV-vis spectroscopy Absorption peaks are narrow in comparison and the energies of the absorption bands are unique for sets of functional groups Thus qualitative information is readily obtained from IR spectra Correlation charts and compilations of IR spectra for unknown matching But IR spectra do not have the specificity that NMR spectra or electron impact mass spectra tend to exhibit

13. Sensitivity This is perhaps the major shortcoming of this technique when compared to fluorescence, or especially mass spectrometry However, Beer’s law type analysis are possible and fairly routine using FT-IR Detection limits are in the ppm range (  M)

14. ATR Attenuated total reflectance More dense media to less dense media Complete reflectance Evanescent wave Penetrates several micrometers IR beam sample Diamond tip