3. Infrared Spectroscopy
Near infrared
（ 780 nm ～ 2.5 µm ）
Mid Infrared
（ 2.5 µm ～ 25 µm ）
Far Infrared
（ 25 µm ～ 2000 µm ）
12500 ～ 4000 cm-1
4000 ～ 400 cm-1
400 ～ 10 cm-1
Peak absorptions are
shown as the region facing
downwards
Infrared
radiation
source
sample
100% transmission
4. Ⅰ ） Dispersive Infrared Spectrophotometer
Ⅱ ） Fourier Transform Infra-Red Spectrophotometer
(Fourier Transform Infra-Red Spectroscopy, FTIR)
← Current technology
FTIR
PerkinElmer Spectrum 100
Dispersive IR
PerkinElmer Model 297
Infrared Spectrophotometer
Types of infrared spectrophotometer
5. Dispersive Infrared Spectrophotometer
light
source
reference
sample
Chopper mirror
Thermocouple detector
grating
Radiation from the source
is 「 dispersed 」 by the grating into single
wavelength components.
The sample beam ratioed to the reference
beam produces a plot of the sample
spectrum 。
spectrum
sample
compartment
6. Fourier Transform Infra-Red Spectrophotometer (FTIR)
spectrum
light source
sample
He-Ne Laser
moving
mirror
interferogram
beamsplitter
fixed mirror
detector
interferometer
Sample
compartment
Fourier transform
Radiation emitted from the source is split into two with a beamsplitter in the interferometer. The fixed and
moving mirrors reflect each of the beam back to the beamsplitter, where the two beams recombine into one and
falls on the detector 。 The two beams combine constructively or destructively, varying as the optical path
difference, when the moving mirror is moved. When the combined beam is transmitted through the sample, it is
detected as an interferogram and contains all infrared information on the sample. The infrared spectrum is
obtained from the interferogram by the mathematical process of Fourier transformation 。
7. What is a beamsplitter ？
interferometer = produces optical path difference in radiation → changes phase difference of radiation
requires precise movement!!
radiation
source
fixed mirror
moving mirror
beamsplitter
0λ/4λ/2
I0R0
I0R0
2R0T0I0
I0T0
I0T0
I0
I0(R0
2
+T0
2
)
opdδ= 0
opdδ= λ/2
Optical path difference – opd
opdδ= λ
As the moving mirror moves continuously to a further
distance away, the intensity of the combined beam at the
center of the beamsplitter changes from I ～ 0 ～Ｉ
～・・・
radiation from fixed mirror
radiation from moving mirror
combined radiation
think about the interference of monochromatic radiation
8. intensity is strongest at 0 optical path difference
Light interference of multiple wavelengths(interferogram)
0 λλ/2
I
OPD
① monochromatic radiation of wavelengthλ
② two wavelengths radiation
λ1
λ2
0
OPD
③ multiple wavelengths
λ1
λ2
λ3
λ4
λ5
λ6
0
OPD
Intensity
Intensity
Intensity
9. Role of Laser
0
OPD
Intensity
centerburst （ in theory at OPD=
0 ）
interferogram obtained
sampled interferogram
determine the sampling interval
→ using the periodicity of the laser
interferogram
The interferogram is a function of 「 distance 」
10. What is Fourier transform ？
（ Fourier’s theory ）
The period of the trigonometric function is the
wavelength. The magnitude is the intensity 。
Revert to their respective wavelength interferograms
All functions can be represented as the sum of trigonometric
functions
λ1
λ2
λ3
λ4
λ5
λ6
interferogram
Fourier transform
12. Energy in infrared region and interferogram
continuous wavelength
radiation
- 500.0 - 400 - 300 - 200 - 100 0 100 200 300 400 500.0
- 0.65
- 0.6
- 0.5
- 0.4
- 0.3
- 0.2
- 0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.65
ポイント
EGY
4000.0 3000 2000 1500 1000 400.0
0.7
10
20
30
40
50
60
70
80
90
100.8
c m- 1
EGY
interferogram
FT
sum of
trigonometric
(cos) functions
13. δδν
δν
dfF e
i−∞
∞−∫= )()(
General formula for Fourier series
)()( δν fF ⇔
function of distance
wavelength function δ must be integrated from －∞ to ＋∞
The interferometer must be moved an infinite distance （ → ） to obtain
an interferogram which will meet the requirements of the Fourier transform.
An apodization function which is simply a weighting is applied to the
interferogram, convert this interferogram to an interferogram 「 similar 」 to
that obtained with the moving mirror moving to an infinite distance, before
performing the Fourier transform.
function of wavelength
spectrum interferogram
As it is impossible ・・・
＜ Problems encountered in actual FTIR measurements ＞
What is apodization function ？
14. Apodization function
FT with limited range FT with apodization function
applied to the limited range
side lobesside lobes
A decay function with increasing |x|
The interferogram is made to converges to zero at the edge.
There are several apodization functions, such as Norton-Beer and Triangular,
which affects the peak width and peak height differently.
15. Phase Correction
The centerburst is not at
optical path difference
「 0 」
（ due to delay in acquisition of
electronic signals, optical beam distortions
etc)
「 phase correction 」
strongest interferogram intensity position
(centerburst) is set to optical path
difference "0" position
（＋） deviation
from the optical
path difference
「 0 」 position
（－） deviation
from the optical
path
difference 「 0 」
position
※In general, several points centered around the centerburst are
subjected to the correction
17. Short measurement time
All frequencies are measured simultaneously in an interferometer with Fourier
transform. 。 The measurement time is reduced in a multi-wavelength
measurement
Reaction process measurement
Rapid measurement time allows a chemical reaction or kinetics to be monitored 。
High signal to noise ratio
Throughput advantage (Jacquinot Advantage)
Throughput advantage of FTIR is 100 times better than a dispersive IR.
Fellgett advantage (Multiplex Advantage)
FTIR can measure the entire wavelength range simultaneously
Measuring very small samples
An infrared microscope system allows these small samples to be measured easily 。
Measuring dark samples
Samples with high carbon content (such as black rubber) can be measured 。
Improvement in wavelength accuracy (Connes Advantage)
FTIR is calibrated with a He-Ne laser 。
He-Ne laser has a very stable frequency. Therefore, FTIR will have excellent long term
Advantages of FTIR
Advantages of FT-IR （ compared to dispersive IR ）
