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Submitted By kingxis
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© ABB - 1 11/11/2003

Henry Buijs

IR Spectroscopy and FTIR

Molecular Vibrations
Material is made up of atoms

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A group of atoms linked together by chemical bonds is called a molecule

Atoms of hydrogen and oxygen

Molecule of water

Molecular Vibrations
The atoms of a molecule are always in motion

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These motions are called vibrations

symmetrical vibration
H-O-H

asymmetrical vibration
H-O-H

Properties of Molecular Vibrations
The rate of vibration is millions of cycles per second (it never stops)
Each vibration has a unique frequency
The frequency of a vibration depends on:
The strength of the chemical bond between atoms
The mass of each atom

So we can learn a lot about a molecule when we study its vibrations!

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Infrared spectroscopy is a method for the analysis of molecular vibrations IR Spectroscopy
Analysis based on the absorption at different wavelengths of an infrared beam by a sample

Source radiation

Transmitted radiation

Sample

100
80

100

Relative intensity of IR beam

80

60

60

40

40

20

20

0
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transmittance spectrum of the sample

0

4000

3000

2000

1000

4000

3000

2000

1000

Principles of Infrared Spectroscopy
When a frequency of light corresponds to a molecular vibration it is absorbed by the sample

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The fraction of light transmitted by the sample compared with the light incident as a function of frequency gives the infrared spectrum of the sample

Infrared Spectrum of a Polystyrene Film
Wavelength is expressed in number of waves per cm.
(e.g. 1000 /cm is the same as wavelength λ=1/1000th cm)
Vertical scale is percent remaining intensity transmittance spectrum of the sample

100

80

60

%

40
20

0

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4000

3000

2000

Mid IR

Near IR

UV

Infrared
Visible

1000

Wavenumbers (cm-1)

Properties of an IR Spectrum
There is much detail in a spectrum.
Much of it is unique to a sample
A band as narrow as 6 cm-1 at
3100 cm-1 (0.2% of wavelength)

90
80
70
60
50
40
30

90

20

80
70

10

60

0

50
3500

3000

2500

2000

1500

1000

500

40
30
20
10

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0
3250 3200 3150 3100 3050 3000 2950 2900 2850 2800

But What Is Really Happening Here?
In fact recording a spectrum is a little like tuning in to radio stations...
We turn the button (change the frequency)
Sometimes we catch a signal, and we hear music (we “absorb” the signal at this frequency)
We know which stations to find, because we know the frequency where they are (we know which vibrations to pickup)
Sometimes the signal is strong, sometimes weaker (the peak is higher or lower)

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Sometimes we hear nothing at all!

Absorbing and Non-Absorbing Species
Absorbing:
Any molecular vibration which displaces an electric charge will absorb infrared radiation.
Diatomic molecules with dissimilar atoms HCl, CO
Asymmetric vibrations in polyatomic molecules
Functional groups
Non-absorbing:
Atomic Species:

He, Ne, Ar, Kr

All Atomic Species

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Molecular Species: H2, N2, O2, F2, Cl2, Br2
These cannot be analyzed by infrared

Mid IR Spectroscopy

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Wavelength range 2500 nm to 25000 nm
Frequency range 4000 to 400 cm-1
Mid IR spectra provide distinctive patterns for many compounds
Permits distinction of several hundred thousand different compounds
Most compounds have high absorptivity in mid IR
Condensed phase compounds
>50% transmittance for sample thickness in 10 to 100 micrometer range
Need sample accessories that assure short light path through sample Capillary liquid transmission cell, ATR and diffuse reflectance Gas phase compounds
>50% transmittance for sample thickness in 10 to 100 cm range
Mid IR is suitable for quantitative gas phase analysis

IR Spectrum of a Liquid Vegetable Oil (path: 12 microns)

Absorbance

1.2

.8

.4

0
4000

3000

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Frequency

2000
(cm -1 )

1000

IR Spectra of Hand Cream and Water (path: 12 microns)

Absorbance

1

hand cream

.5

water
0

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4000

3000

2000

Frequency (cm -1 )

1000

Absorbance

Infrared Spectrum of Gaseous HCl (10-cm path cell, resolution 1 cm-1)

2970 2960 2950 2940

3200

3000

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Frequency

2800
(cm -1
)

2600

Near IR Spectroscopy
Wavelength range 700 nm to 2500 nm
Frequency range 14000 to 4000 cm-1
Near IR spectra have less distinctive patterns than Mid IR spectra Harmonics and combination frequencies of mid IR bands

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Principally harmonics of H-C, H-N, H-O vibrations
Absorptivity is lower than in mid IR
Low absorptivity is offset with longer path sampling
Sampling with pathlength of several millimeters is much easier and permits greater reproducibility
Disposable glass vial sampling
Sampling of flowing sample in process pipe
Quantitative analysis using chemometrics

Overtones (Harmonics) and Combinations
Infrared spectrum of H2O:
OH stretching: 3400 cm-1
OH bending: 1630 cm-1

First overtone: 2 x 3400 = 6800 cm-1
Second overtone: 3 x 3400 = 10200 cm-1

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Combination: 3400 + 1630 = 5030 cm-1

Mid and Near Infrared Spectra of Isopropanol
The pathlength is increased by a factor of 150 in the
NIR to get intensity similar to the mid-IR
4

Absorbance

Isopropanol, mid-IR
Horizontal ATR
2

Isopropanol, NIR
2 mm cuvette

0

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10000

8000

6000

Frequency

4000
-1 )
(cm

2000

NIR Spectra of Surfactants
Hydrocarbon region
From the bottom to the top:
- OH value = 107.4, sap value = 159.5
- OH value = 185.6, sap value = 153.2
- OH value = 217.6, sap value = 145.7

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Normalized Absorbance

OH region

moisture
OH region
Hydrocarbon region

9000

8000

7000

Frequency

(cm-1 )

6000

5000

Fundamentals and Overtones

The C-H Stretching Vibration for CHCl3 Liquid

Transition υ = 0 to υ=

Observed wavenumber (cm-1)

1
2
3
4
5
6

3019
5912
8677
11318
13850
16270

Anharmonic oscillator calculation
(cm-1)
3019
5912
8679
11320
13835
16224

Absorbance for a
1 cm cell

Pathlength for measurement

170
17
0.55
0.02
0.001
0.0001

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Note the reduction in absorbance. This requires very short path at 3019cm-1, convenient 0.5 mm and 10 mm path at 5912 and 8677 respectively.

5 µm
0.5 mm
1.0 cm
5.0 cm
N/A
N/A

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Henry Buijs

FTIR principles

Dispersive Infrared Spectroscopy

Prism dispersion

Diffraction grating

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Geometric wavelength separation

The Michelson Interferometer
Optical part of FT-IR
2 flat mirrors and a beamsplitter
One mirror moves
Changes optical path of split light beams
Recombine light and get variable interference
Fixed mirror

Compensator
Beamsplitter

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Each wavelength has a unique modulation frequency

Wavelength separation by modulation

Moving mirror

FTIR technology
Many Interferometer types are used by different FT-IR manufacturers. Why?
To achieve more stable and reproducible operation
The original Michelson interferometer is sensitive to misalignment
Tilt of 1 micrometer across the IR beam (25 micro radians) will change interferogram by >10%
The scan mechanism can wear out or become irratic fixed mirror

1 micro meter

moving mirror

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compensator
Scan Mechanism beamsplitter © ABB Inc. - 24 -

The ABB Michelson FT-IR

FT-IR with Laser Controlled Measurement
Modulation frequencies of all wavelengths are locked to modulation frequency of an internal He-Ne laser
Provides ultra precise measurement of spectral frequencies
Provides reproducible measurement of spectral frequencies fixed mirror

IR beam

moving mirror

Laser beam

compensator
Scan Mechanism

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beamsplitter

Measurement with FT-IR
Typical interferogram
4 volts at centerburst
0.004 volts at end of scan

4

0

Volts signal

Volts signal

2

.005

-2

-4

0

-.005
24000

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Interferogram / Arbitrary X

23500

23000

Measurement with FT-IR
Measurement
Two interferograms (near end of scan) difference Volts signal

.015

.01

.005

0

31200

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Interferogram / Arbitrary X

31100

31000

Measurement with FT-IR
Difference between 2 interferograms (16 coadded scans )
(peak signal (4volts)/p. to p. noise =40000:1)

Volts signal

10E-05

0

-10E-05

31400

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Interferogram / Arbitrary X

31300

31200

Measurement with FT-IR
Interferogram of blocked beam
(3 levels of ADC)

Volts signal

.001

50E-05

0

-50E-05

-.001
15000

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Interferogram / Wavenumber (cm-1)

10000

5000

0

S/N achievable
Table 1 figure A
B
C
D

RMS noise in
Micro A
0A
1.634A
1.634A +gain
2.683A +gain

At 4000 cm-1
2500nm
4.3
168
11
125

At 5000 cm-1
2000nm
2.6
160
6
83

At 5800 cm-1
1725nm
3.25
148
8.5
105

At 6600cm-1
1515nm
4.6
177
14
145

3
2.6845
2.684
2.6835
2.683

2

1

D
5400

1.6355
1.635
1.6345
1.634
5200

5000

4800

5000

4800

1.635
1.6345
1.634
1.6335
1.633

B
5400

5200

4600

4600

C
5400

5200

5000

4800

4600

50E-06
0

A

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-50E-06
0
12000

5400
10000

8000

5200

5000

4800

4600
6000

Absorbance / Wavenumber (cm-1)

4000

Interferometer Features
Patented Michelson type interferometer
Cube corner retroreflectors mounted on a "wishbone" swing arm. Factory prealigned interferometer and input/output optics. Does not require any adjustment by user. Scanning by rotating the swing arm on a flex pivot, driven by an induction motor

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This provides smooth, constant-velocity, perturbation-free scanning without wear.

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The ABB Michelson FT-IR

Acquisition of an Infrared Spectrum
The light coming from the source goes through the interferometer; the movement of the mirrors causes an interference pattern which is called the interferogram
Next the light passes through the sample; the interferogram of the source is modified as the light is absorbed by the sample
The interferogram is measured by the detector

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The interferogram is transformed into a spectrum by a mathematical operation called the Fourier Transform algorithm Acquisition of an Infrared Spectrum
3

50

Interferogram of the source

Spectrum of the source

40

2

FT

1

30

0

20

-1

10

-2
4000

3980

3960

3940

3920

4000

Interferogram of the source & sample

2

40

3000

2000

1000

Spectrum of the source & sample

30
1

FT

0

10

-1
© ABB Inc. - 34 -

20

0
4000

3980

3960

3940

3920

4000

3000

2000

1000

Acquisition of an Infrared Spectrum
50
40

Spectrum of source only
(no sample present)

Spectrum of source & sample
40

30

divided by

30

20
20
10
10
0
4000

3000

2000

1000

4000

3000

gives
100
80

transmittance spectrum of the sample
( polystyrene film )

60
40
20

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0
4000

3000

2000

1000

2000

1000

Advantages of FT-IR Over Dispersive

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Optical efficiency
Measurement efficiency
Precision of measurement

Optical Efficiency of FT-IR
A dispersive spectrometer has much greater light restriction than FT-IR for the same size and resolution spectrometer
A Dispersive spectrometer has a slit width of 0.25mm by 5mm high
FT-IR has a round aperture of 7.5mm diameter

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Ratio of areas for FT-IR/dispersive is 35

Dispersive spectrometer
Slit 0.25x 5mm
1.25mm2

FT-IR
Jacquinot stop 7.5 mm diam
44 mm2

Optical efficiency of FT-IR
FT-IR has sufficient throughput to simultaneously illuminate 8 sampling accessories via fiber optic cables

The ABB FTPA2000-200 (Networkir)

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modulated outputs
8 channels

Direct FO coupled Detectors
8 channels

Measurement Efficiency of FT-IR
Multiplexing
Grating observes 1 spectral element at a time
1000 to 2500 nm with 4 nm resolution = 375 elements

FTIR observes all spectral elements at the same time
4000 to 10000 cm-1 at 16 cm-1 =375 elements
Typical advantage 20x

All wavelengths are modulated simultaneously

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A single detector measures all wavelengths continuously

Precision Requirements in IR Spectroscopy
For quantitative analysis via IR spectroscopy, the spectra of a sample should be reproducible
For a reproducibility of 1%,
The absorbance scale must be reproducible to

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Cyanobacterium Lab Report

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Will Tv Succumb to the Internet

...Şencan & Mustafa Karaboyacı & Mehmet Kılıç Received: 26 November 2013 / Accepted: 23 April 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract This study aimed to determine the Pb+2 adsorption capacities of hazelnut shell and activated carbon obtained from hazelnut shell. It also aimed to determine the effect of ZnCl2 in the activation process. The hazelnut was pyrolyzed at 250 and 700 °C. For determining the capture speed of the adsorbents, the pseudo-first- and second-order kinetic studies were performed. The Freundlich and Langmuir isotherm models were used to determine adsorption equilibrium. The surface characterization of hazelnut shell and activated carbon was determined by Brunauer-Emmett-Teller (BET) analysis and FTIR spectrum. Pb+2 adsorption capacity of obtaining activated carbon was determined by ICP-OES analysis. The raw hazelnut shell’s BET surface area is 5.92 m2/g and the surface area of activated carbons which is pyrolyzed at 250 and 700 °C were determined (270.2 and 686.7 m2/g, respectively. The surface area of hazelnut shell, which pyrolyzed at 700 °C after being activated with ZnCl2, was determined to be 736.49 m2/g. Results show that physical adsorption process is dominant for the activated carbon pyrolysis at 700 °C but the chemical adsorption is dominant for the activated carbon pyrolysis at lower degrees and for raw hazelnut shell. Keywords Activated carbon . Heavy metals . Lead(II) . Kinetic . Isotherm . Hazelnut Responsible...

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