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Fourier Transform Infrared Spectrometry

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ASTM E1252

Transmittance scan vs. Absorbance scan


Transmittance: Transmittance scans visually display the light intensity reaching the equipment’s detector at each wavenumber. A 100% transmittance value means the sample absorbed no light, the measured light intensity before and after the sample are the same. A 60% transmittance value at any wavenumber means the sample absorbed 40% of the supplied infrared light energy at that wavenumber. Conversely, only 60% of the light energy reached the detector.

Transmittance Scan of Acetal
FTIR Transmittance Scan of Acetal

Absorbance: An absorbance scan looks like an inverted transmittance scan. A 100% absorbance value means no infrared light reached the detector at the indicated wavenumber. The following equation shows the mathematical relationship between “A” absorbance and “T” transmittance.
A = log10 (1/T)

Previous Acetal Transmittance Scan Converted to an Absorbance Scan
FTIR Absorbance Scan

Scans of Different Kinds of Polymers
Totally different polymers have very different looking spectral scans. It is very obvious the infrared scan of Polyethylene (blue) does not look like the scan of Polycarbonate (red).

Scans of different kinds of polymers

Similar Scans or Different Types of Specific Polymer
Scans of Different Types of a Specific Polymer, Such As PBT (red) and PET Polyesters (blue), Are Very Similar.

Scans of different kinds of polymers

About Infrared Light

Infrared light’s electromagnetic region lies between visible light and microwaves. There are three identified regions of infrared light. They are labeled Near-infrared, Mid-infrared, and Far-infrared. The Mid-infrared region is the most commonly used.

Generalized Infrared Light Regions:


Near-infrared Region
13,300 - 4000 Wavenumbers (cm-1)
(Wavelengths of 0.75 - 2.5µ) Where µ = 10-3mm

Mid-infrared Region
4000 - 400 Wavenumbers (cm-1)
(Wavelengths of 2.5 - 25µ) Where µ = 10-3mm

Far-infrared Region
400 - 10 Wavenumbers (cm-1)
(Wavelengths of 25 - 1000µ) Where µ = 10-3mm

The term Wavenumbers (cm-1) is defined as the number of waves per centimeter in a series of waves having a constant wavelength. The reciprocal of Wavenumbers (cm-1) is Wavelength, which is expressed in centimeters. A Wavelength expressed in centimeters can be easily converted to microns by multiplying the wavelength by 10,000.

Glossary of FTIR Terms

Infrared Source - Supplies an infrared light beam
Beam Splitter - Splits the infrared light beam into two beams and then recombines them
Laser – A precision laser used to calibrate the velocity of a moving mirror
Interferometer - Generates an interferogram signal beam
Detector - Measures the interferogram signal strength
Computer - Converts the measured interferogram signal strengths into a traditional plot of intensity vs. frequency

How an FTIR Spectrometer Works

An infrared light source directs a light beam at a beamsplitter, which splits it into two beams. One beam is reflected off of a flat, stationary mirror while the second beam is reflected off of a moving, flat mirror. The equipment maintains the non-stationary mirror’s back and forth movement at a constant velocity. This is achieved by the automated monitoring of the mirror velocity against the precise wavelength of an internal laser beam.

Both mirrors reflect their light beams back to the beamsplitter where they are recombined by an interferometer. The new beam, created by the merging of the two reflected beams, is called an interferogram. The merging process generates interferences that are both constructive and destructive. The interferences are caused by the different distances the infrared light beams traveled from their reflective mirrors.

About the Interferogram Signal
A typical dispersive infrared spectrometer exposes a sample to a single frequency of light at any given time. This procedure is repeated time after time until the sample is exposed to all of the available infrared light frequencies.

FTIR spectrometers expose a sample to all available infrared light frequencies simultaneously. Therefore, each portion of the interferogram signal contains encoded sample information that was derived from every infrared frequency the sample was exposed to.

While the detector is measuring the interferogram signal strength, the encoded sample information is being accessed simultaneously. It is this unique property that makes FTIR spectrometers much faster than the older dispersive type instruments.

A computer is necessary to decode the interferogram signal. The mathematical transformation of an interferogram signal into a visual representation of signal strength versus infrared light frequency is called Fourier Transformation.

Dipole and Dipole Moments Are Critical to Samples Being Able to Absorb Infrared Light

A dipole is typically represented as a magnetic bar with a positive and negative end. In such a situation, a defined dipole moment can be represented by an arrow beginning at the positive end and pointing toward the negative end of the magnet. The strength of a dipole moment is based upon the amount of charge, either positive or negative, and the distance between the centers of the opposite charges.

The molecular structure of a polymer is composed of individual atoms bonded together. Often the bond electrons, located between two different kinds of atoms, are not shared equally.

Since electrons have a negative charge, the atom that has the electrons for a greater percentage of time is assigned a partial negative charge. The other bonded atom is assigned a partial positive charge. A dipole moment can be assigned to a direction from the partially positive atom to the partially negative atom.

Diagram of Dipoles and a Dipole Moment
Below is a simplistic diagram showing examples of labeled dipoles and their associated dipole moments. The + and - signs represent partial positive and negative atomic charges.

diagram of dipoles and dipole moments

Some Samples Do Not Absorb Infrared Light
Solid, liquid, and even gas samples will absorb infrared light when:
(1) The infrared light frequency matches the natural vibration of an atomic bond causing it to vibrate more.
(2) A net change in dipole moment occurs.

Atomic Stretching and Bending
Whenever a sample absorbs infrared light, the absorbed light energy is converted into atomic bond vibrations. This diagram shows simplistic diagrams of the different types of atomic vibrations that can occur. The vibrations are classified as either stretching or bending.

diagram of different types of atomic vibrations

Stretching vibrations are categorized as being either symmetrical (movement in the same direction) or asymmetrical (movement in opposite directions). Bending vibrations are classified as scissoring, rocking, twisting, or wagging.

Infrared Absorption Frequency Charts and Frequencies of Absorbed Infrared Light
Specific groupings of bonded atoms, located within a material’s molecular structure, absorb infrared light within loosely defined frequency ranges. The defined absorption frequency range remains fairly constant and is not significantly dependant on the material type.

Infrared Absorption Frequency Charts are available in textbooks and on the Internet to help identify structures causing specific absorbance peaks. The charts list different functional group types, the characteristic frequency range at which the groups absorb infrared light, and the expected shape of the absorbance bands. The peak shapes are usually described in general terms such as strong, broad, and weak.

The charts are good reference tools, but material identification is usually done by comparing an entire FTIR spectral scan with other similar spectral scans.

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**Please note that this test description is intentionally generic in nature and aimed at providing a descriptive summary to enhance test understanding. For more information please contact a Intertek PTL Technical Representative at . Due to copyright restrictions, we are not able to provide copies of standards. Standards can be obtained from appropriate standards authorities.

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