Transcript FT-IR Instrument - University of Massachusetts Boston

FT-IR Instrument
Components
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Source
Michelson Interferometer
Sample
Detector
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)
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
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f = (2vm/c)n = 10-10n, vm = 1.5 cm/s
Michaelson Interferometer
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Beam splitter
Stationary mirror
Moving mirror at constant velocity
Motor driven Micrometer screw
He/Ne laser; sampling interval, control
mirror velocity
Stationary mirror
HeNe laser
Beam Splitter
Source
Moving mirror
PMT
Sample
Detector
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
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
FT-IR detectors
• Pyroelectric tranducers (PTs)
• Pyroelectric substances act as temperaturedependent 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
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
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
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
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)
ATR
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Attenuated total reflectance
More dense media to less dense media
Complete reflectance
Evanescent wave
Penetrates several micrometers
Diamond tip
sample
IR beam