FT-IR Instrument Components presentation
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The document discusses the components and functionality of FT-IR instruments, detailing elements such as the source types, Michelson interferometer, and detection methods. It highlights the advantages of FT-IR in terms of spectral resolution, selectivity, and application in qualitative analysis, while also noting its limitations in sensitivity compared to other techniques. Additionally, it mentions the use of ATR for enhanced sample interaction with the IR beam.
FT-IR Instrument Components presentation
- 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. 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- 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 IR beam sample Diamond tip