Fourier Transform Infrared FT-IR Spectroscopy

1. Fourier Transform Infrared (FT-IR) Spectroscopy Theory and Applications

2. Introduction to FTInfrared Spectroscopy What is infrared spectroscopy? Theory of FT-IR FT-IR Advantages? New FT/IR4000-6000Series

3. 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 and far-infrared refers to the part that is closer to the microwave region. Mid-infrared is the region between these two. 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.

4. 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).

5. Infrared Spectroscopy The bonds between atoms in the molecule stretch and bend, absorbing infrared energy and creating the infrared spectrum.

6. Energy levels in Infrared Absorption Rayleigh scattered light undergoes no energy change. The cross-section for Raman scattering is very small- less than 10-6 Stokes scattering, the “normal” Raman peaks, looses energy: anti-Stokes scattering gains energy (anti-Stokes peaks usually much weaker) In spectroscopy we generally use the red-shifted Stokes side of the spectra as these peaks are the strongest, however the spectra are qualitatively symmetrical Raman scattering directly probes the electronic and lattice dynamic properties of materials, giving us both readily interpreted “fingerprint” spectra, and a wealth of more detailed physical informationRayleigh scattered light undergoes no energy change. The cross-section for Raman scattering is very small- less than 10-6 Stokes scattering, the “normal” Raman peaks, looses energy: anti-Stokes scattering gains energy (anti-Stokes peaks usually much weaker) In spectroscopy we generally use the red-shifted Stokes side of the spectra as these peaks are the strongest, however the spectra are qualitatively symmetrical Raman scattering directly probes the electronic and lattice dynamic properties of materials, giving us both readily interpreted “fingerprint” spectra, and a wealth of more detailed physical information

7. Infrared Spectroscopy A molecule can be characterized (identified) by its molecular vibrations, based on the absorption and intensity of specific infrared wavelengths.

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

9. 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.

10. 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.

13. Chapter 2 Principles of FTIR 2.3 IR Light Sources Infrared light heat radiation (All objects emit light corresponding to their temperature) The Sun is considered to be a black body with a temperature of 6,000 K. Black bodies are objects that absorb all light. Matter that absorbs light completely is known as a perfect black body, which has a rate of light absorption of 1. Matter with a rate of light absorption less than 1 is called a gray body, and comprises most of the matter around us. Matter that absorbs light well is also a good radiator of light according to Kirchhoff’s laws. Light emitted from a black body has a certain intensity distribution according to wavelength. This intensity distribution varies according to temperature as you can see in the above figure. FT//IR-400,600 series High-intensity ceramic light source Approx. 1,600 K An actual IR light source has the same intensity distribution as this black body radiation energy.Chapter 2 Principles of FTIR 2.3 IR Light Sources Infrared light heat radiation (All objects emit light corresponding to their temperature) The Sun is considered to be a black body with a temperature of 6,000 K. Black bodies are objects that absorb all light. Matter that absorbs light completely is known as a perfect black body, which has a rate of light absorption of 1. Matter with a rate of light absorption less than 1 is called a gray body, and comprises most of the matter around us. Matter that absorbs light well is also a good radiator of light according to Kirchhoff’s laws. Light emitted from a black body has a certain intensity distribution according to wavelength. This intensity distribution varies according to temperature as you can see in the above figure. FT//IR-400,600 series High-intensity ceramic light source Approx. 1,600 K An actual IR light source has the same intensity distribution as this black body radiation energy.

14. Chapter 2 Principles of FTIR 2.4 Structure of an Interferometer Fourier spectroscopy used in FT-IR is the general term for the use of a two-beam interferometer (primarily Michelson interferometers) in spectroscopy. A Michelson interferometer consists of a half-mirror (beam splitter) and two reflecting mirrors. One of the reflecting mirrors is fixed in place (fixed mirror) and the other has a mechanism for moving parallel to the optical axis (movable mirror). Light from the light source is collimated and directed into the interferometer, striking the beam splitter at an angle, thereby separating the light into transmitted light and reflected light. These two beams of light are each reflected by the fixed mirror and movable mirror, and then returned to the beam splitter where they are recombined into a single beam.Chapter 2 Principles of FTIR 2.4 Structure of an Interferometer Fourier spectroscopy used in FT-IR is the general term for the use of a two-beam interferometer (primarily Michelson interferometers) in spectroscopy. A Michelson interferometer consists of a half-mirror (beam splitter) and two reflecting mirrors. One of the reflecting mirrors is fixed in place (fixed mirror) and the other has a mechanism for moving parallel to the optical axis (movable mirror). Light from the light source is collimated and directed into the interferometer, striking the beam splitter at an angle, thereby separating the light into transmitted light and reflected light. These two beams of light are each reflected by the fixed mirror and movable mirror, and then returned to the beam splitter where they are recombined into a single beam.

15. Chapter 2 Principles of FTIR 2.5 Synthesis and Interference Phenomena of Two Beams of Light by a Beam Splitter Lets consider a case using a monochromatic light source. A: If the distance from the beam splitter to the fixed mirror and from the beam splitter to the movable mirror is the same (optical path difference x of the two beams = 0), the two beams when recombined will be in phase, and therefore interfere constructively. B: If we move the movable mirror to a position where the optical path difference x = l /2 (l : is the wavelength of light from the monochromatic light source), the two beams when recombined will be out of phase, and therefore interfere destructively. C: If we move the movable mirror to a position where the optical path difference x = l, the two beams when recombined will once again be in phase, and therefore interfere constructively. D: If we continuously move the movable mirror, thereby continuously changing the optical path difference x, and then read the optical path difference x on the x-axis and the amplitude of the two beams of light when recombined on the y axis, we will observe an interference pattern (interference phenomenon) in which constructive interference (light) and destructive interference (dark) appear alternately as you can see in the diagram for D.Chapter 2 Principles of FTIR 2.5 Synthesis and Interference Phenomena of Two Beams of Light by a Beam Splitter Lets consider a case using a monochromatic light source. A: If the distance from the beam splitter to the fixed mirror and from the beam splitter to the movable mirror is the same (optical path difference x of the two beams = 0), the two beams when recombined will be in phase, and therefore interfere constructively. B: If we move the movable mirror to a position where the optical path difference x = l /2 (l : is the wavelength of light from the monochromatic light source), the two beams when recombined will be out of phase, and therefore interfere destructively. C: If we move the movable mirror to a position where the optical path difference x = l, the two beams when recombined will once again be in phase, and therefore interfere constructively. D: If we continuously move the movable mirror, thereby continuously changing the optical path difference x, and then read the optical path difference x on the x-axis and the amplitude of the two beams of light when recombined on the y axis, we will observe an interference pattern (interference phenomenon) in which constructive interference (light) and destructive interference (dark) appear alternately as you can see in the diagram for D.

16. Chapter 2 Principles of FTIR 2.6 Signal Output from Interferometer This refers to the signal output from the interferometer when the movable mirror is moved at a constant velocity (v). (a): For a monochromatic light source, the signal output from the interferometer is modulated to a frequency corresponding to its wavenumber. (b): For a dichroic light source, each color of light is modulated to a frequency corresponding to its wavenumber, and the signal output from the interferometer is the sum of the two modulated signals and is slightly random. (c): For continuous spectrum light, the light spectrum is modulated to frequencies corresponding to the wavenumbers contained within the light spectrum, and the output signal from the interferometer is the sum of the modulated frequencies. Since the frequencies only interfere constructively in phase when the optical path difference x of the two light beams is zero, the signal intensity is maximum (center burst). As the optical path difference increases from this position, the modulated frequencies interfere destructively more and more, such that the signal intensity gradually attenuates until it reaches zero. This output signal is known as an interferogram. Since optical path difference x can be expressed as follows, the interferogram is the spectrum on the time (x) axis: x = 2vt (t: time)Chapter 2 Principles of FTIR 2.6 Signal Output from Interferometer This refers to the signal output from the interferometer when the movable mirror is moved at a constant velocity (v). (a): For a monochromatic light source, the signal output from the interferometer is modulated to a frequency corresponding to its wavenumber. (b): For a dichroic light source, each color of light is modulated to a frequency corresponding to its wavenumber, and the signal output from the interferometer is the sum of the two modulated signals and is slightly random. (c): For continuous spectrum light, the light spectrum is modulated to frequencies corresponding to the wavenumbers contained within the light spectrum, and the output signal from the interferometer is the sum of the modulated frequencies. Since the frequencies only interfere constructively in phase when the optical path difference x of the two light beams is zero, the signal intensity is maximum (center burst). As the optical path difference increases from this position, the modulated frequencies interfere destructively more and more, such that the signal intensity gradually attenuates until it reaches zero. This output signal is known as an interferogram. Since optical path difference x can be expressed as follows, the interferogram is the spectrum on the time (x) axis: x = 2vt (t: time)

17. Chapter 2 Principles of FTIR 2.7 Samping of an Actual Interferogram An interferogram is the sum of signals produced by modulating light of various wavenumbers emitted from a light source to frequencies proportional to the wavenumbers. Using a mathematical technique known as a Fourier transform, we can obtain from these interferograms the signal strength of each frequency component included in the modulated signals, thereby making it possible to find the intensity of light at each wavenumber. Plotting the intensity of each wavenumber on the y-axis and the wavenumbers themselves on the x-axis produces a single-beam spectrum. In this manner, FT-IR measures interferograms (time axis spectrum), producing a single-beam spectrum (wavenumber axis spectrum) through Fourier transform processing.Chapter 2 Principles of FTIR 2.7 Samping of an Actual Interferogram An interferogram is the sum of signals produced by modulating light of various wavenumbers emitted from a light source to frequencies proportional to the wavenumbers. Using a mathematical technique known as a Fourier transform, we can obtain from these interferograms the signal strength of each frequency component included in the modulated signals, thereby making it possible to find the intensity of light at each wavenumber. Plotting the intensity of each wavenumber on the y-axis and the wavenumbers themselves on the x-axis produces a single-beam spectrum. In this manner, FT-IR measures interferograms (time axis spectrum), producing a single-beam spectrum (wavenumber axis spectrum) through Fourier transform processing.

18. Chapter 2 Principles of FTIR 2.8 Fourier transform Interferograms obtained by a detector are analog signals. However, in order to apply a Fourier transform, which is a form of mathematical process, digital sampling is required. FT-IR introduces monochromatic light (632.8 nm) from a He-Ne laser into the interferometer and then uses in a sample instruction the signal output by the laser interferometers that works in unison with the movement of the movable mirror. Since the 632.8 nm He-Ne laser produces extremely stable monochromatic light, it is possible to accurately sample interferograms at equal intervals, thereby obtaining a spectrum with very precise wavenumbers.Chapter 2 Principles of FTIR 2.8 Fourier transform Interferograms obtained by a detector are analog signals. However, in order to apply a Fourier transform, which is a form of mathematical process, digital sampling is required. FT-IR introduces monochromatic light (632.8 nm) from a He-Ne laser into the interferometer and then uses in a sample instruction the signal output by the laser interferometers that works in unison with the movement of the movable mirror. Since the 632.8 nm He-Ne laser produces extremely stable monochromatic light, it is possible to accurately sample interferograms at equal intervals, thereby obtaining a spectrum with very precise wavenumbers.

19. Chapter 2 Principles of FTIR 2.9 Detector Properties 1) TGS (triglycine sulfate): pyro-electric detector Feature: Flat sensitivity in a wide infrared region. Operates at room temperature. 2) MCT (Hg1-XCdXTe): Semiconductor detector Feature: High sensitivity compared to the TGS detector. For this reason, it is the standard detector used in FTIR microscopy for handling weak light. Operates at the temperature of liquid nitrogen (77 K).Chapter 2 Principles of FTIR 2.9 Detector Properties 1) TGS (triglycine sulfate): pyro-electric detector Feature: Flat sensitivity in a wide infrared region. Operates at room temperature. 2) MCT (Hg1-XCdXTe): Semiconductor detector Feature: High sensitivity compared to the TGS detector. For this reason, it is the standard detector used in FTIR microscopy for handling weak light. Operates at the temperature of liquid nitrogen (77 K).

21. 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.

22. 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.

23. 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.

24. 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

25. 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.

26. 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.

27. 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.

28. 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.

29. 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.