Spectroscopy

Spectroscopy

Fourier Transform Infrared (FT-IR) Spectroscopy

Theory and Applications

THE ELECTROMAGNETIC SPECTRUM

INFRAREDGAMMA RAYS X RAYS UV VISIBLE

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Introduction to FTInfrared Spectroscopy

What is infrared spectroscopy?Theory of FT-IR

FT-IR Advantages? New FT/IR4000-6000Series

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What is the basic principle of IR spectroscopy?

IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

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What is Infrared? Infrared radiation lies between the visible and microwave portions of the

electromagnetic spectrum. Infrared waves have wavelengths longer than visible and shorter than microwaves,

and have frequencies which are lower than visible and higher than microwaves. The Infrared region is divided into: near, mid and far-infrared.

Near-infrared refers to the part of the infrared spectrum that is closest to visible light, 14000–4000 cm−1 (0.8–2.5 μm wavelength) and far-infrared refers to the part that is closer to the microwave region, 400–10 cm−1 (25–1000 μm).

Mid-infrared is the region between these two, 4000–400 cm−1 (2.5–25 μm). The primary source of infrared radiation is thermal radiation. (heat) It is the radiation produced by the motion of atoms and molecules in an object. The

higher the temperature, the more the atoms and molecules move and the more infrared radiation they produce.

Any object radiates in the infrared. Even an ice cube, emits infrared.

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An IR spectrum show the energy absorptions as one 'scans' the IR region of the EM spectrum.  As an example, the IR spectrum of butanal is shown below. In general terms it is convenient to split an IR spectrum into two approximate regions:

• 4000-1000 cm-1 known as the functional group region, and•< 1000 cm-1 known as the fingerprint region

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•identification of inorganic and organic materials

•purity control of materials

•reaction kinetics e.g. conversion of polymers, curing acrylics,

hybrid systems

•troubleshooting

•identification of monomers and polymers; resins, hardeners,

stabilizers, plasticizers, fillers, adhesives, oils and waxes

•identification of solvents and extracts

•identification and quantification of contaminants on surfaces

Applications

Infrared (IR) spectroscopy is used to obtain information on the molecular structure of virtual all type of samples in any physical state (solid, liquid or gas). The technique is widely spread and is applied in the polymer, pharmaceutical, medical, food and chemical industry.

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What is Infrared? (Cont.)

Humans, at normal body temperature, radiate most strongly in the infrared, at a

wavelength of about 10 microns (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter). In

the image to the left, the red areas are the warmest, followed by yellow, green and

blue (coolest).

The image to the right shows a cat in the infrared. The yellow-white areas are the warmest and the purple areas are the coldest. This image gives us a different view of a familiar animal as well as information that we could not get from a visible light picture. Notice the cold nose and the heat from the cat's eyes, mouth and ears.

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Infrared Spectroscopy

The bonds between atoms in the molecule stretch and bend, absorbing infrared energy and creating the infrared

spectrum.

Symmetric Stretch Antisymmetric Stretch Bend

A molecule such as H2O will absorb infrared light when the vibration (stretch or bend) results in a molecular dipole moment change

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Infrared Spectroscopy

A molecule can be characterized (identified) by its molecular vibrations, based on the absorption and intensity of specific

infrared wavelengths.

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Infrared Spectroscopy

For isopropyl alcohol, CH(CH3)2OH, the infrared absorption bands identify the various functional groups of the molecule.

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Capabilities of Infrared Analysis

Identification and quantitation of organic solid, liquid or gas

samples.

Analysis of powders, solids, gels, emulsions, pastes, pure

liquids and solutions, polymers, pure and mixed gases.

Infrared used for research, methods development, quality

control and quality assurance applications.

Samples range in size from single fibers only 20 microns in

length to atmospheric pollution studies involving large areas.

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Applications of Infrared Analysis

Pharmaceutical research Forensic investigations Polymer analysis Lubricant formulation and fuel additives Foods research Quality assurance and control Environmental and water quality analysis methods Biochemical and biomedical research Coatings and surfactants Etc.

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To separate IR light, a grating is used.

Grating

Light source

Detector

Sample

Slit

To select the specified IR light, A slit is used.

Dispersion SpectrometerIn order to measure an IR spectrum, the dispersion Spectrometer takesseveral minutes.Also the detector receives onlya few % of the energy oforiginal light source.

Fixed CCM

B.S.

Moving CCM

IR Light source

Sample

Detector

An interferogram is first made by the interferometer using IR light.

The interferogram is calculated and transformedinto a spectrum using a Fourier Transform (FT).

FTIRIn order to measure an IR spectrum, FTIR takes only a few seconds. Moreover, the detector receives up to 50% of the energy of originallight source.(much larger than the dispersionspectrometer.)

Comparison Beetween Dispersion Spectrometer and FTIR

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Interferogram is made by an interferometer.

Interferogramis transformedinto a spectrum using a FT.

BKG

SB

3000 2000 1000

[cm-1]

Sample

SB

Sample

3000 2000 1000

[cm-1]

Sample/BKG

IR spectrum

%T

3000 2000 1000 [cm-1]

The Principles of FTIR Method

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FTIR seminar

Interferometer

He-Ne gas laser

Fixed mirror

Movable mirror

Sample chamber

Light source

(ceramic)

Detector

(DLATGS)

Beam splitter

FT Optical System Diagram

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Fixed mirror

Ｂ Movable mirror

Fixed mirror

Ａ Movable mirror

Fixed mirrorＣ Movable mirror

Same-phase interference wave shape

Opposite-phase interference wave shape

Same-phase interference wave shape0

Movable mirror

Ｄ Interference pattern of light manifested by the optical-path difference

Continuous phase shift

Sig

na

l s

tre

ng

th

Ｉ (X)

-2 - 0 2

-2 - 0 2

FTIR seminar

Interference of two beams of light

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Relationship between light source spectrum and the signal output from interferometer

(a) Monochromatic light

(b) Dichroic light

(c) Continuous spectrum light

All intensities are standardized.

Light source spectrum Signal output from interference wave

Time t

Time t

Time tI(t)

I

Wavenumber

Wavenumber

Wavenumber

I

Az

Az

FTIR seminar

Interference is a superpositioning of waves

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FTIR seminar

Interferometer interferogram

Output of a Laser interferometer

Primary interferometer interferogram that was sampled

Optical path difference x

Sampling of an actual interferogram

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4000 400

SBFourier transform

Optical path difference[x]

(Interferogram) (Single beam spectrum)

Wavenumber[cm-1]

Sin

gle

s

tre

ng

th

Time axis by FFT Wavenumber

Fourier Transform

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FTIR seminar

TGSOperates at room temperature

MCTOperates at the temperatur

of liquid nitrogen

D* (

, f

) (c

mH

z1/2 W

-1)

1010

109

108

Wavenumber[cm-1]4000 600

Detector Properties

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1.Better sensitivity and brightness- Allows simultaneous measurement over the entire wavenumber range- Requires no slit device, making good use of the available beam2.High wavenumber accuracy- Technique allows high speed sampling with the aid of laser light interference fringes- Requires no wavenumber correction- Provides wavenumber to an accuracy of 0.01 cm-13. Resolution- Provides spectra of high resolution4. Stray light- Fourier Transform allows only interference signals to contribute to spectrum. Background light effects greatly lowers.- Allows selective handling of signals limiting intreference5. Wavenumber range flexibility- Simple to alter the instrument wavenumber range

CO2 and H2O sensitive

FT-IR Advantages and Disadvantages

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FT-IR Advantages Fellgett's (multiplex) Advantage

FT-IR collects all resolution elements with a complete scan of the interferometer. Successive scans of the FT-IR instrument are coadded and averaged to enhance the signal-to-noise of the spectrum.

Theoretically, an infinitely long scan would average out all the noise in the baseline.

The dispersive instrument collects data one wavelength at a time and collects only a single spectrum. There is no good method for increasing the signal-to-noise of the dispersive spectrum.

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FT-IR Advantages

Connes Advantage

an FT-IR uses a HeNe laser as an internal wavelength standard. The infrared wavelengths are calculated using the laser wavelength, itself a very precise and repeatable 'standard'.

Wavelength assignment for the FT-IR spectrum is very repeatable and reproducible and data can be compared to digital libraries for identification purposes.

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FT-IR Advantages Jacquinot Advantage

FT-IR uses a combination of circular apertures and interferometer travel to define resolution. To improve signal-to-noise, one simply collects more scans.

More energy is available for the normal infrared scan and various accessories can be used to solve various sample handling problems.

The dispersive instrument uses a rectangular slit to control resolution and cannot increase the signal-to-noise for high resolution scans. Accessory use is limited for a dispersive instrument.

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FT-IR Application Advantages

Opaque or cloudy samples

Energy limiting accessories such as diffuse reflectance or FT-

IR microscopes

High resolution experiments (as high as 0.001 cm-1 resolution)

Trace analysis of raw materials or finished products

Depth profiling and microscopic mapping of samples

Kinetics reactions on the microsecond time-scale

Analysis of chromatographic and thermogravimetric sample

fractions

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FT-IR Terms and Definitions

Resolution (common definition) –

The separation of the various spectral wavelengths, usually defined in wavenumbers (cm-1).

A setting of 4 to 8 cm-1 is sufficient for most solid and liquid samples. Gas analysis experiments may need a resolution of 2 cm-1 or higher. Higher resolution experiments will have lower signal-to-noise.

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FT-IR Terms and Definitions Resolution – FT/IR Case A spectrum is said to be collected at

a resolution of 1 cm-1 if 4 data points are collected within each spectral interval of 1 cm-1 .

In order to acquire a spectrum at higher, an increased number of data points is needed, requiring a longer stroke of the moving mirror.

For higher resolution instruments an aperture is needed in order to improve parallelism within interferometer.

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FT-IR Terms and Definitions

Apodization - a mathematical operation to reduce unwanted oscillation and noise contributions from the interferogram and to avoid aberrations coming from the “finite” nature of real (non theoretical interferograms). Common apodization functions include Beer-Norton, Cosine and Happ-Genzel.

Apodization

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FT-IR Terms and Definitions

Scan mode - Either single beam or ratio. Single beam can be a scan of the background (no sample) or the sample. Ratio mode always implies the sample spectrum divided by, or ratioed against, the

single beam background.

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FT-IR Terms and Definitions

Scan(s) - a complete cycle of movement of the interferometer mirror. The number of scans collected affects the signal-to-noise ratio (SNR) of the final spectrum. The SNR doubles as the square of the number of scans collected; i.e. 1, 4, 16, 64, 256, ….

Scan speed or optical path velocity - the rate at which the interferometer mirror moves. For a DTGS detector, the SNR decreases as the scan speed increases.

Scan range - spectral range selected for the analysis. The most useful spectral range for mid-infrared is 4000 to 400 cm-1.

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The highest S/N ratio in the world, 50,000:1 (FT/IR-6300) (Over sampling with 24-bit ADC)

DSP-driven interferometer and new ADC (18-bit to 24-bit)Digital control of the moving mirror drive using an advanced high speed digital signal processor (DSP) technologyThe outstanding performance of the ADC (Analog-to digital converter) and DSP (Digital signal processor) allows very rapid and accurate correction for the effects of velocity and position errors.

Autoalignment for all models (The interferometer optics can always be aligned by the PC)

In addition to proven technology for Rapid scanning and vacuum capabilities; a Step scan capability enables time-resolved studies similar to research models by Nicolet, Bruker and Bio-Rad.

IR imaging with IMV-4000 multi-channel microscope for all models (Rapid scanning with a linear array MCT detector )

PC communication and control using USB

Aperture of 7.1, 5.0, 3.5, 2.5, 1.8, 1.2, 0.9, 0.5 mm diameter for FT/IR-4100/4200

Spectra Manager II (cross-platform software suite for JASCO spectroscopy systems) (Spectra Manager CFR: 21 CFR Part 11 compliance)

Research model capability (Upgradeable wavelength extension, high resolution, step scan)

Improved Water Vapor and CO2 Compensation

New Features of FTIR4000-6000Series

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Polymer shellImproved instrument designCompact sizeSample compartment with same size as a higher class model

FT/IR-400 Plus

Aperture

No additional optics for IR microscope interface Standard apertures for optimum S/N and resolution capabilityEasy replacement of light source and detector

FT/IR-4100FT/IR-4200

Microscope

FTIR4000 Series

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FT/IR-4000 Series purge design

N2gas inlet

Control valve

Instrument purge is standard for all models of the FT/IR-4000 Series.

FTIR4000 Series Purge System

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Conventional method

Find the zero crossings, then interpolate a matching set of IR data points.

Over sampling method

Reduction of high frequency noise by over sampling with a 16 times greater number of sampling points enables improvement of the S/N ratio.

Pre-amp.

Analog circuit

Photo coupler

Voice Coil

HeNe laser

Photo coupler

Pre-amp.

ADC

DSPDAC

Clock

24-bit AD

Voice Coil

HeNe laser

Accurate mirror driveAnd reduce flutter at low wavenumber range.

FT/IR-4000 & 6000 series

S/N ratio (Oversampling system)

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Spectroscopy FT/IR-6100 / 6200 / 6300

FT/IR-600Plus

Polymer shellImproved instrument designCompact size

- Upgradeability- Wide wavenumber range- Full vacuum capability- Step scan upgrade

Microscope

FT-Raman

FT/IR-6000 Series Optical design

FTIR6000 Series

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FT/IR-6000 Series purge design

N2gas inlet

Purge control valve – front side

Instrument purge is standard for all models of the FT/IR-6000 Series.

FTIR6000 Series Purge/Vacuum System