FT-IR Instrument Components presentation

FT-IR Instrument Components presentation

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
•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)n = 10
-10
n, 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-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 aN
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 (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