Fundamentals of FT-IR Spectroscopy: Components and Functionality

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 mm), falls off to about 1 % max at 670 cm-1 (15 mm) • Nernst Glower – cylinder made of rear earth elements • Globar- SiC rod • CO2 laser • Hg arc (Far IR), Tungsten filament (Near IR)

4. Michaelson Interferometer • 1014 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 1010 times slower • 104-105 Hz can be measured electronically • f = (2vm/c)n = 10-10n, vm = 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. Stationary mirror HeNe laser Beam Splitter Source Moving mirror PMT Sample Detector

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- DT=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 DT; detection of DTs 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 a N1/2 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 (mM)

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