General

1. General This is the subtitle

2. Introduction to Fourier Transform Infrared Spectroscopy

3. What is FTIR? FTIR stands for Fourier Transform Infra Red, the preferred method of infrared spectroscopy. A method for measuring all of the infrared frequencies simultaneously, rather than individually as with dispersive instruments

4. Why Infrared Spectroscopy ? An Infrared spectrum represents a fingerprint of a sample with absorption peaks which correspond to the frequencies of vibrations between the bonds of the atoms making up the material-Because each different material is a unique combination of atoms, no two compounds produce the exact same spectrum, therefore IR can result in a unique identification of every different kind of material!

5. Infrared Spectroscopy is simply the study of the interaction of Infrared light with matter!! Most powerful aspect is the ability to identify complete unknowns direct correlation between the wavenumbers at which a molecule absorbs infrared radiation and its chemical structure

6. Infrared Spectroscopy Energy of radiation increases with increasing frequency and decreases with increasing wavelength Absorption of electromagnetic radiation of different wavelengths produce different excitations in molecules. Infrared radiation, for instance, corresponds to the energies associated with “molecular vibrations” Energy of radiation increases with increasing frequency and decreases with increasing wavelength Absorption of electromagnetic radiation of different wavelengths produce different excitations in molecules. Infrared radiation, for instance, corresponds to the energies associated with “molecular vibrations”

7. Wavenumbers (cm-1) - standard  IR spectral units  Modern infrared instruments record the absorption of energy as a function of decreasing frequency (decreasing energy) from “left to right” W ( cm-1) = 1/? A frequency-related unit known as a wavenumber is expressed as “reciprocal centimeters”- number of vibrations occurring over 1 cm

8. WAVENUMBER SCALE

9. Origins of Infrared Absorptions: All atoms within molecules are in constant vibration all bonds with a permanent dipole (slightly electronegative charge) will absorb infrared radiation at the appropriate vibrational frequency (??/ ?X ? 0 ) the vibrational frequency will depend on the atoms associated with the bond as well as the bond strength

11. Nature of Molecular Vibrations: Normal Modes

12. Factors affecting “frequency” of  molecular vibrations Mass of the atoms - heavier molecules will result in slower movement and result in lower frequencies Elasticity of the spring (force constant) - a stronger spring (bond) will cause the vibration to be more rapid than a weaker one- HIGHER FREQUENCY Bond Strength - increasing # of bonds increases frequency of vibration

13. Infrared light from the source passes through the sample, then goes through a slit into a grating to disperse the light into a spectrum of its component wavenumbers. The grating is rotated so that different wavenumber slices pass through the slit and are sampled by the detector Dispersive IR Instruments

14. Direct ratio photometric system is adopted, by which reference signals and sample signals are measured independently. And then the ratio between the 2 signals are calculated. Light beams which are emitted from a light source pass through the target sample or the reference sample, and the transmitted light beams are subjected to chopping by a chopper with 2 different frequencies. The reference light and sample light are combined by a beam combiner, and the combined light is introduced into a monochromator. The light is the separated into monochromatic light by diffraction gratings and filters. The light from monochromator converted by detector into electrical signals, and then separated into reference signals and sample signals by a band-pass filter. The transmittance or absorbance ( = infrared spectrum) of the sample is calculated with both signals. Direct ratio photometric system is adopted, by which reference signals and sample signals are measured independently. And then the ratio between the 2 signals are calculated. Light beams which are emitted from a light source pass through the target sample or the reference sample, and the transmitted light beams are subjected to chopping by a chopper with 2 different frequencies. The reference light and sample light are combined by a beam combiner, and the combined light is introduced into a monochromator. The light is the separated into monochromatic light by diffraction gratings and filters. The light from monochromator converted by detector into electrical signals, and then separated into reference signals and sample signals by a band-pass filter. The transmittance or absorbance ( = infrared spectrum) of the sample is calculated with both signals.

15. Older Technology- limitations Slow Measurements: Because dispersive instruments measure each frequency individually, a single measurement may take as long as 15 minutes Relatively Insensitive: Older instruments employed detectors which required a large amount of sample. Today we can detect as little as 0.01% of a compound in a particular matrix with FTIR Mechanically Complex- Because dispersive instruments have lots of moving parts, they are susceptible to mechanical failure increasing possible instrument down time and the slits throw most of the sample beam away at any one time anyhow

16. Advantages of FTIR (technique) Universal technique sensitivity 10-6 grams fast and easy relatively inexpensive rich information sensitive to “molecules”-anything that contains chemical bonds vast majority of molecules in the universe absorb mid-infrared light, making it a highly useful tool

17. Disadvantages of FTIR Cannot detect atoms or monoatomic ions - single atomic entities contain no chemical bonds Cannot detect molecules comprised of two identical atomS symmetric-such as N2 or O2. Aqueous solutions are very difficult to analyze- water is a strong IR absorber Complex mixtures - samples give rise to complex spectra

18. Michelson Interferometer

19. Constructive Interference 

20. Destructive Interference

21. At all path differences other than ZPD (zero path difference), a variation of “constructive” and “destructive”interference takes place - this “modulated” radiation is denoted by how many times a wave may switch between being light and dark known as the frequency A radiation beam, emitted from a light source, is split by the beam splitter into 2 different paths. The split beams are reflected on the fixed or movable mirrors, combining once again at the beam splitter. Here, the 2 waves of beams interfere with each other, varying in intensity with the movement of the mirror. The figure shows the interference effect for a beam with wavelength of only 2.5 mm. In (A), the beam reflected by the fixed mirror is in phase with the beam reflected by the movable mirror, so both beams intensify each other. This produces a greater intensity of light as is shown in (B). Shifting the movable mirror by 1/4 of the wavelength (0.625mm), I.e. x (optical path difference) = 1/2 of wavelength, offsets the intensity. On the other hand, at position x = 0 (optical path difference), a beam of 5mm (which is shown) is as intense as the 2.5mm beam, becoming weakest at the point where x = 2x1.25 = 2.5mm. This change of intensity against the change of optical difference differs from that of the 2.5mm beam. Since the actual interferogram represents the simultaneous interference of beams of all wavelengths, it presents a waveform which shows the strongest intensity at x = 0, with a symmetrical gradual reduction on both sides of 0. A radiation beam, emitted from a light source, is split by the beam splitter into 2 different paths. The split beams are reflected on the fixed or movable mirrors, combining once again at the beam splitter. Here, the 2 waves of beams interfere with each other, varying in intensity with the movement of the mirror. The figure shows the interference effect for a beam with wavelength of only 2.5 mm. In (A), the beam reflected by the fixed mirror is in phase with the beam reflected by the movable mirror, so both beams intensify each other. This produces a greater intensity of light as is shown in (B). Shifting the movable mirror by 1/4 of the wavelength (0.625mm), I.e. x (optical path difference) = 1/2 of wavelength, offsets the intensity. On the other hand, at position x = 0 (optical path difference), a beam of 5mm (which is shown) is as intense as the 2.5mm beam, becoming weakest at the point where x = 2x1.25 = 2.5mm. This change of intensity against the change of optical difference differs from that of the 2.5mm beam. Since the actual interferogram represents the simultaneous interference of beams of all wavelengths, it presents a waveform which shows the strongest intensity at x = 0, with a symmetrical gradual reduction on both sides of 0.

23. Interferogram of a Broadband Source

24. Relationship between Interferogram and Power Spectrum

25. FTIR HhOWHHrinciples The Fourier transform infrared spectrophotometer (FTIR) does not use a dispersive element. Instead, an interferometer generates interference light. Rather than obtaining a spectrum directly from the detector, an interference waveform, called an interferogram, is generated. This waveform is obtained by the reciprocating back-and-forth movement of the interferometer mirror. Fourier transform, a mathematical calculation, is used to produce a spectrum from the interferogram. The calculation is performed by computer at high speed. The Fourier transform infrared spectrophotometer (FTIR) does not use a dispersive element. Instead, an interferometer generates interference light. Rather than obtaining a spectrum directly from the detector, an interference waveform, called an interferogram, is generated. This waveform is obtained by the reciprocating back-and-forth movement of the interferometer mirror. Fourier transform, a mathematical calculation, is used to produce a spectrum from the interferogram. The calculation is performed by computer at high speed.

26. Fourier Transform of an Interferogram The wavelength and intensity data at each point in the scanned spectrum are encoded so that all of the spectral information is acquired at once; it is mathematically decoded through Fourier Transform. Essentially, wavelength and intensity are related mathematically to mirror position and velocity.

27. What is Apodization?????? (comes from Greek word “a podi” or “no feet”) Calculating A Fourier Transform involves performing a mathematical integral on the IFG. Ideally the limits of this integral should be +/- infinity (infinity for OPD and # of data points collected to Fourier Transform properly) - This is impossible, so the IFG and the integral must be truncated at some point, the limits being 0 OPD and the maximum OPD An unfortunate outcome of truncating the signal is the lineshapes become distorted causing “sidelobes” which are sinusoidal undulations in the baseline. These lobes, or “feet” are suppressed by multiplying the IFG by an “Apodization Function”

28. Main Side-Effect of Apodization!! - Spectral Resolution Will be Reduced - AF’s vary in how well they suppress sidelobes, and how much they degrade resolution AF that provides the highest resolution, and does the worst job of suppressing “sidelobes”, is the boxcar apodization (only use with gases or where utmost resolution is required) AF recommended for ‘Condensed Phase samples” is Happ-Genzel or “Medium Beer-Norton

29. Relationship between the Interferogram and the Spectrum As an example, let’s consider 3 wavenumbers (or wavelengths), n1, n2 and n3 in the incident white light getting into the interferometer. As the mirror in the interferometer moves, these beams create waveforms of different cycles. The interferogram is made up of the 3 beams added together, representing a symmetric waveform in which the beam intensity is strongest where the path difference is 0 (x = 0). The actual signal at the FTIR detector is an interferogram, where the interference waveforms of all wavenumbers, such as n1, n2 and n3 are added. The wave heights A(n1), A(n2) and A(n3) from the interference signals of beams n1, n2 and n3 correspond to the intensities of the respective wavenumbers. A(n1), A(n2) and A(n3) can be calculated by the Fourier transform of the interferogram. In general, any waveform is composed of the sinusoidal function of different cycles. Therefore, the intensity of each sinusoidal function can be determined by breaking down the waveform into its respective sinusoidal functions. This method is known as Fourier transform. The intensities of the light beams of respective wavenumbers determined by the Fourier transform are plotted continuously to obtain a spectrum, as shown in above figure. If the intensity of the beam of wavenumber n2 is reduced by sample absorption, becoming A’(n2), the intensity of the beam at n2 is also diminished, as shown in above figure. As an example, let’s consider 3 wavenumbers (or wavelengths), n1, n2 and n3 in the incident white light getting into the interferometer. As the mirror in the interferometer moves, these beams create waveforms of different cycles. The interferogram is made up of the 3 beams added together, representing a symmetric waveform in which the beam intensity is strongest where the path difference is 0 (x = 0). The actual signal at the FTIR detector is an interferogram, where the interference waveforms of all wavenumbers, such as n1, n2 and n3 are added. The wave heights A(n1), A(n2) and A(n3) from the interference signals of beams n1, n2 and n3 correspond to the intensities of the respective wavenumbers. A(n1), A(n2) and A(n3) can be calculated by the Fourier transform of the interferogram. In general, any waveform is composed of the sinusoidal function of different cycles. Therefore, the intensity of each sinusoidal function can be determined by breaking down the waveform into its respective sinusoidal functions. This method is known as Fourier transform. The intensities of the light beams of respective wavenumbers determined by the Fourier transform are plotted continuously to obtain a spectrum, as shown in above figure. If the intensity of the beam of wavenumber n2 is reduced by sample absorption, becoming A’(n2), the intensity of the beam at n2 is also diminished, as shown in above figure.

30. Multiplex (Fellgett) Advantage The spectral element (m) of a spectrum measured from wavenumber n1 to n2, with a resolution of D n, can be calculated by (n1-n2 )/ D n. A dispersive IR examines only one spectral element of a spectrum at a time. If T is the time required to examine the spectral elements between n1 and n2, the time required to observe a spectral element will be T /m. The multiplex advantage of FTIR is the simultaneous observation of all the infrared spectral elements (m) over the entire wavenumber range (n1 to n2) . This means that the time spent for observing a spectral element in FTIR is T, not T / m. Because the signal-to-noise (S/N) ratio improves proportionally to the square root of the observation time of each spectral element, the S/N ratio of an FTIR is superior to that of dispersive IR by a factor of m1/2. The spectral element (m) of a spectrum measured from wavenumber n1 to n2, with a resolution of D n, can be calculated by (n1-n2 )/ D n. A dispersive IR examines only one spectral element of a spectrum at a time. If T is the time required to examine the spectral elements between n1 and n2, the time required to observe a spectral element will be T /m. The multiplex advantage of FTIR is the simultaneous observation of all the infrared spectral elements (m) over the entire wavenumber range (n1 to n2) . This means that the time spent for observing a spectral element in FTIR is T, not T / m. Because the signal-to-noise (S/N) ratio improves proportionally to the square root of the observation time of each spectral element, the S/N ratio of an FTIR is superior to that of dispersive IR by a factor of m1/2.

31. Jacquinot ( Throughput) Advantage The aperture advantage is a result of the larger optical throughput of the FTIR interferometer relative to a dispersive IR spectrometer. The optical throughput, ED , for a dispersive IR spectrometer is given in Eq.(a): ED = AxB (a) where: A is the area of entrance slit, and B is the incident solid angle of the beam For an FTIR, A’ is defined as the area of aperture size. Since the FTIR does not have the slit used in the dispersive IR spectrometer, the optical throughput for the FTIR is about 2 orders greater than that of the dispersive IR spectrometer. A related advantage is known as the stray light advantge. Stray light is present at some low level in all spectrometers, and is a source of noise. Since an FTIR always has a greater throughput than a dispersive instrument, a given amount of stray light represents a lower percentage of the signal in an FTIR than a dispersive IR. Thus, the relative amount of noise due to stray light is less. The aperture advantage is a result of the larger optical throughput of the FTIR interferometer relative to a dispersive IR spectrometer. The optical throughput, ED , for a dispersive IR spectrometer is given in Eq.(a): ED = AxB (a) where: A is the area of entrance slit, and B is the incident solid angle of the beam For an FTIR, A’ is defined as the area of aperture size. Since the FTIR does not have the slit used in the dispersive IR spectrometer, the optical throughput for the FTIR is about 2 orders greater than that of the dispersive IR spectrometer. A related advantage is known as the stray light advantge. Stray light is present at some low level in all spectrometers, and is a source of noise. Since an FTIR always has a greater throughput than a dispersive instrument, a given amount of stray light represents a lower percentage of the signal in an FTIR than a dispersive IR. Thus, the relative amount of noise due to stray light is less.

32. Another difference between FTIR and the dispersive IR spectrophotometer is the use of the He-Ne laser in the FTIR. This offers better wavenumber accuracy for the FTIR spectrum than that of dispersive IR. Another difference between FTIR and the dispersive IR spectrophotometer is the use of the He-Ne laser in the FTIR. This offers better wavenumber accuracy for the FTIR spectrum than that of dispersive IR.

33. The Signal-Averaging Advantage Fast scans with an FTIR enable the coaddition of many scans in order to reduce the random measurement noise-use the computer to do this quite easily The positive and negative fluctuations in the random noise level cancel themselves out as more scans are added together

34. FTIR 8300/8400/8700/8900 SOURCE Purpose: provide radiant energy in the infrared region of the electromagnetic spectrum Ceramic Source/Globar- bright and very intense - inert solid is heated electrically to 1500-2000 K -guaranteed for 5000 hours

36. BEAMSPLITTER: KBr is almost universally used as a substrate material in FTIR beamsplitters-this material does not split the beam since it transmits in the IR Instead, a thin coating of Germanium is sandwiched between two pieces (disks) of KBr, and it is this Ge coating that splits the beam. So they are referred to as Ge on KBr beamsplitters Disadvantage: KBr’s natural tendency to absorb moisture-fogging

37. FTIR 83/84/87/8900 Interferometer :   Mirror is the only moving part FJS -Flexible Joint Support- unique-patented Shimadzu system Swing system with smooth film support system. Base plate swings while keeping parallel against the top plate. Mirror is mounted in front of the base. Mirror runs parallel with theTop plate Advantage: Long endurance and precision because of noncontact; free from wear High linearity and precision almost equal to an air bearing system

39. Friction-less Mirror Drive Requires No Maintenance  Unique, patented Shimadzu design employs a friction-less electro-magnetic moving mirror drive supported by two tough, durable polyimide sheets (a flexible joint support system). Stability and linearity of response is equivalent to the Air-bearing Drive without the need for pure, dry air

40. FTIR83/84/87/8900 DETECTORS DLATGS (deuterated L-Alanine doped tri-glycine Sulfate) - Shimadzu patented design pyroelectric bolometer - changes in the amount of infrared radiation striking the detector cause the temperature of the DTGS element to change, measured as a voltage change -Temperature controlled high sensitivity detector-better sensitivity than traditional DTGS detector MCT Detector (optional) - AIM Microscope required ----mercury cadmium telluride (HgCdTe)- consists of an alloy of 3 elements, and it is a semi-conductor. Energy is detected by use of a “bandgap” (photons exceeding the energy bandgap will be detected) DTGS detector-(deuterated tri-glycine Sulfate) known as a pyroelectric bolometer-Changes in the amount of infrared radiation striking the detector cause the temperature of the DTGS element to change. The dielectic constant of materials such asDTGS change with temperature. The resultant change in capacitance with temperature is measured as a voltage across the detector element -not very sensitive MCT detectetor-detector element absorbs infrared photons, and as a result electrons are promoted from the valence band(or bonding orbital) to the conduction band(or anti-bonding orbital). Once electrons are in the conduction band they can respond to an applied voltage, giving rise to an electrical current. The electrical current is a measure of the number of electrons, and so is directly proportional to the number of infrared photons hitting the detector. Thus, the current generated by the detector element is a direct measure of infrared intensity. The energy difference between the valence and conduction band is called the bandgap.Photons with less energy than the bandgap will not be detected since they do not promote electrons to the conduction band Narrow band-sensitive to 700cm-1 Wide band-down to 400cm-1 but much noisier(5-10 times more) Disadvantage: saturate easily and need Liquid N2 to run DTGS detector-(deuterated tri-glycine Sulfate) known as a pyroelectric bolometer-Changes in the amount of infrared radiation striking the detector cause the temperature of the DTGS element to change. The dielectic constant of materials such asDTGS change with temperature. The resultant change in capacitance with temperature is measured as a voltage across the detector element -not very sensitive MCT detectetor-detector element absorbs infrared photons, and as a result electrons are promoted from the valence band(or bonding orbital) to the conduction band(or anti-bonding orbital). Once electrons are in the conduction band they can respond to an applied voltage, giving rise to an electrical current. The electrical current is a measure of the number of electrons, and so is directly proportional to the number of infrared photons hitting the detector. Thus, the current generated by the detector element is a direct measure of infrared intensity. The energy difference between the valence and conduction band is called the bandgap.Photons with less energy than the bandgap will not be detected since they do not promote electrons to the conduction band Narrow band-sensitive to 700cm-1 Wide band-down to 400cm-1 but much noisier(5-10 times more) Disadvantage: saturate easily and need Liquid N2 to run

41. He-Ne Laser Laser acts as an internal wavenumber standard. - He-Ne gives off light at precisely 15,798.637 cm-1. All infrared frequencies are measured relative to it. Laser is used to track the position of the moving mirror. - so the optical path difference can be measured precisely and any adjustments will be made instantaneously to the fixed mirror positioning

42. Construction of Dynamic Alignment System The dynamic alignment system ensures the stability of the interferometer. The dynamic alignment keeps the interference condition continually optimized during measurement. This feature ensures high stability and minimizes the need of maintenance of the instrument. Construction of Dynamic Alignment System The dynamic alignment system ensures the stability of the interferometer. The dynamic alignment keeps the interference condition continually optimized during measurement. This feature ensures high stability and minimizes the need of maintenance of the instrument.

43. Dynamic Auto-Alignment for Maximum Reproducibility Dynamic feedback loop - maintains the optimum alignment conditions during measurement, a consistently stable and reproducible spectrum is obtained. Continuous laser referencing - operates continuously at 5,000 to 10,000 Hz Auto-Adjust Function - provides quick and easy fine tuning of the interferometer. It eliminates the need for tedious mechanical adjustments

44. Dynamic Auto-Alignment (cont’d) Recalls last position at power-down Minimizes start-up time 5 min or less for Qualitative Analysis ~30 min for Quantitative Analysis Eliminates the need to keep the FTIR on continuously

45. Triple Protection of the Interferometer

46. Interferometer & Optical Component Protection features : A high-tech proprietary anti-moisture coating is applied directly to the KBr beamsplitter surface Optical compartment is sealed and desiccated with Calcium Oxide - will not re-release moisture back into the optical compartment KRS-5 Exit Window does not react with water, it is impervious to moisture and will never fog- Disadvantage: only 70% transmissivity

47. Common Causes of Instrument Problems? Beam blocked by something Low throughput accessory Poor (vibration,humidity) operational environment Failing Source Fogged Beamsplitter Poorly Aligned (tuned) moving mirror Failing detector Electronic Failures

48. Initial Suggestions to Follow Follow infra red beam (Are laser spots in the sample compartment visible on your hand?) - Make sure no obstructionsin light path/ Does problem exist with Accessories removed? Turn power on/off - Look for laser lights. If not present, replace laser tube/laser power supply

49. More Suggestions ! Inspect beamsplitter for fogging, and replace if necessary Check the Moving Mirror in Hayato 105 (Service Mode) and see if it moves freely-it should since we are forcing it to move- If it does not then inspect with different scanning speeds to observe movement- if none- replace the CPU boards- FTIR Main/Digital II