Fourier Transform Infrared (FT-IR) Spectroscopy

Fourier Transform Infrared �(FT-IR) Spectroscopy

1. Fourier Transform Infrared (FT-IR) Spectroscopy Presented by: NIVEDITHA G 1 st M.Pharm , Dept. of Pharm aceutics . Nargund College of Pharmacy. 1

2. Contents : 1. FTIR 2. Interferometer 3. Advantages 4. Disadvantages 5. Interpretation of IR. 2

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. 3

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). 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.

5. 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 H 2 O will absorb infrared light when the vibration (stretch or bend) results in a molecular dipole moment change 5

6. Energy levels in Infrared Absorption Infrared absorption occurs among the ground vibrational states, the energy differences, and corresponding spectrum, determined by the specific molecular vibration(s). The infrared absorption is a net energy gain for the molecule and recorded as an energy loss for the analysis beam. h n Excited states Ground (vibrational) states h( n 1 - n ) h( n 1 - n ) h( n 2 - n 1 ) (overtone) Infrared Absorption and Emission n 1 n 2 n n 3 6

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

8. Infrared Spectroscopy For isopropyl alcohol, CH(CH 3 ) 2 OH, the infrared absorption bands identify the various functional groups of the molecule . 8

9. TYPES OF INFRA RED SPECTROSCOPY Dispersive I.R. Fourier Transform I.R. (With Interferometer) (With Monochromator ) 9

10. Albert Michelson (1852-1931) Michelson wanted to measure the speed the the earth moves through the ether (the medium in which light travels). By measuring the interference between light paths at right angles, one could find the direction & speed of the ether. Michelson’s first interferometer (1881) 10

11. 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 transformed into a spectrum using a Fourier Transform (FT). In order to measure an IR spectrum, FTIR takes only a few seconds. Moreover, the detector receives up to 50% of the energy of original light source. (much larger than the dispersion spectrometer.) FTIR Spectrometer 11

12. Interferogram is made by an interferometer. Interferogram is transformed into 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 12

13. Intensity Distribution and Temperature Dependency versus Wavelength of Black Body Radiation Energy 2 5 20 10 10 5 10 4 10 3 10 2 10 1 10 -1 10 -2 10 -3 10 -4 0.1 0.2 0.5 1 50 100 Wavelength l / m m 6000K 4000K 2000K 1000K 500K 300K 200K Spectral irradiance W l IR light source IR Light Source 13

14. H V MIRROR2 BEAM SPLITTER SOURCE DETECTOR V H H H V H MICHELSON INTERFEROMETER MIRROR 1 14

15. FOURIER TRANSFORM IR SPECTROMETER INTERFEROMETER… In the FT-IR instrument, the sample is placed between the output of the interferometer and the detector. The sample absorbs radiation of particular wavelengths. An interferogram of a reference is needed to obtain the spectrum of the sample. After an interferogram has been collected, a computer performs a Fast Fourier Transform , which results in a frequency domain trace (i.e. intensity vs. wave number). 15

16. Interferometer He-Ne gas laser Fixed mirror Movable mirror Sample chamber Light source (ceramic) Detector (DLATGS) Beam splitter FT Optical System Diagram 16

17. Fixed mirror Ｂ Movable mirror Fixed mirror Ａ Movable mirror Fixed mirror Ｃ Movable mirror Same-phase interference wave shape Opposite-phase interference wave shape Same-phase interference wave shape l Ｄ Interference pattern of light manifested by the optical-path difference Signal strength Ｉ (X) -2 l - l l 2 l Interference of two beams of light 17

18. Relationship between light source spectrum and the signal output from interferometer Monochromatic light (b) Dichroic light Continuous spectrum light All intensities are standardized. Light source spectrum Signal output from interference wave Time t Time t Time t I(t) I b (u) Wavenumber u Wavenumber u Wavenumber u S I S A z A z FTIR seminar Interference is a superpositioning of waves 18

19. FTIR seminar Interferometer interferogram Output of a Laser interferometer Primary interferometer interferogram that was sampled Optical path difference x Sampling of an actual interferogram 19

20. 4000 400 SB Fourier transform Optical path difference [x] ( Interferogram ) (Single beam spectrum) Wavenumber [cm -1 ] Single strength Fourier Transform 20

21. 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 21

22. 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. 22

23. 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. 23

24. 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 24

25. 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. 25

26. 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 . 26

27. The detector used in an FT-IR instrument must respond quickly. Pyroelectric detectors or liquid nitrogen cooled photon detectors- IDEAL. Thermal detectors- TOO SLOW. 27

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29. Interferometer consists of………. Drive mechanism Beam splitter Source and transducers. 29

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31. Drive mechanism.. Speed and planarity- CONSTANT. A motor driven micrometer screw measures displacement of the mirror. 31

32. Beam splitter Constructed of transparent materials 50% radiation is reflected and 50% is transmitted. Mylar sandwiched between two plate of a low refractive index solid. G ermanium or silicon deposited on cesium iodide or bromide , NaCl , KBr - MID IR REGIONS. Iron(III) oxide is deposited on calcium fluoride- NEAR IR REGION. 32

33. SOURCES AND Transducers SOURCES: Tungsten- Halogen lamp. High Pressure Mercury Lamp Globar (or) Nernst sources TRANSDUCERS: Pyroelectric transducers. Ex:- Deuterated triglycine sulfate (DCT) pyroelectric Transducer Photoconduction transducers: Ex:- Mercury cadmium telluride(MCT) or indium antimonite, lead sulfide photoconductive transducers. 33

34. TGS Operates at room temperature MCT Operates at the temperatur of liquid nitrogen D* ( l , f) (cmHz 1/2 W -1 ) 10 10 10 9 10 8 Wavenumber [cm -1 ] 4000 600 Transducer Properties 34

35. Most common pyro-electric compound is Tri- glycerine sulfate (T.G.S). Different types are: Lithium niobate Lead zircobate Triglycerine sulfate: Tri- glycerine sulfate (only upto 45 c) Deuterium tri- glycerine sulfate ( deuterated ) These substances have permanent dipole moment. Advantages: First choice of detector Response is fast and used for multiple scanning. 35

36. Advantages 1.Better sensitivity and brightness . 2.High wavenumber accuracy . 3. Resolutio n. 4. Stray light . 5. Wavenumber range flexibility . Disadvantages CO2 and H2O sensitive Single-beam, requires collecting blank Can’t use thermal detectors – too slow Destructive Too sensitive that it would detect the smallest contaminant FT-IR Advantages and Disadvantages 36

37. 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. 37

38. 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. 38

39. 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 . 39

40. Interpretation of IR Spectrum 40

41. A 100 % transmittance in the spectrum implies no absorption of IR radiation. When a compound absorbs IR radiation, the intensity of transmitted radiation decreases. This results in a decrease of per cent transmittance and hence a dip in the spectrum. The dip is often called an absorption peak or absorption band. FEATURES OF AN IR SPECTRUM Different types of groups of atoms (C-H, O-H, N-H, etc…) absorb infrared radiation at different characteristic wavenumbers. 41

42. No two molecules will give exactly the same IR spectrum (except enantiomers). Simple stretching: 1600-3500 cm -1 Complex vibrations: 400-1400 cm -1 , called the “fingerprint region ” I.R SPECTRUM 42

43. Describing IR Absorptions IR absorptions are described by their frequency and appearance. Frequency (n) is given in wavenumbers (cm -1 ) Appearance is qualitative: intensity and shape Conventional abbreviations: Vs very strong S strong M medium w weak Br broad Sh sharp OR shoulder 43

44. In general, the IR spectrum can be split into four regions for interpretation: 4000  2500 cm -1 : e.g . O  H, N  H, C  H 2500  2000 cm -1 : e.g . C≡C, C≡N 2000  1500 cm -1 : e.g. C=C, C=O 1500  600 cm -1 : This region often consists of many different, complicated bands and is called the fingerprint region. 44

45. FINGER PRINT REGION 1500 and 600 cm -1 . Absorption in this fingerprint region is characteristic of the molecule as a whole. C-O-C stretching vibration in ethers and esters at about 1200 cm -1 C- Cl stretching vibration at 700 to 800 cm -I . Sulfate , Phosphate, Nitrate, and Carbonate also absorb at wavenumbers below 1200 cm -I 45

46. Finger print region can be subdivided into three regions as follows: 1500 – 1350 cm -1 1350 – 1000 cm -1 Below 1000 cm -1 Region 1500 – 1350 cm -1 : doublet near 1380 cm -1 (m) and 1365 cm -1 (s)→ tertiary butyl group in compound. nitro compound, one bond → near 1350 cm -1 . Region 1350 - 1000 cm -1 : alcohols, esters, acid anhydrides Region below 1000 cm -1 : cis and trans alkenes. The higher value (970 – 960 cm -1 ) indicates that hydrogen atoms in alkenes are trans and viceversa . . E.g.: a band in the region 750 – 700 cm -1 show mono sunstituted benzene. 46

47. Summary of IR Absorptions 47

48. BASE VALUES (+/- 10 cm -1 ) O-H 3600 N-H 3400 C-H 3000 C N 2250 C C 2150 C=O 1715 C=C 1650 C O ~1100 large range 48

49. Specific groups Alkanes – combination of C-C and C-H bonds Show various C-C stretches and bends between 1360-1470 cm -1 (m) C-C bond between methylene carbons (CH 2 ’s) 1450-1470 cm -1 (m) C-C bond between methylene carbons (CH 2 ’s) and methyl (CH 3 ) 1360-1390 cm -1 (m) Show sp 3 C-H between 2800-3000 cm -1 (s) cm -1 Infrared Spectroscopy Octane 49

50. Specific groups Alkenes – addition of the C=C and vinyl C-H bonds C=C stretch occurs at 1620-1680 cm -1 and becomes weaker as substitution increases vinyl C-H stretch occurs at 3000-3100 cm -1 Note that the bonds of alkane are still present! The difference between alkane and alkene or alkynyl C-H is important! If the band is slightly above 3000 it is vinyl sp 2 C-H or alkynyl sp C-H if it is below it is alkyl sp 3 C-H 1-Octene Infrared Spectroscopy 50

51. Specific groups Alkynes – addition of the C≡C and vinyl C-H bonds C≡C stretch occurs between 2100-2260 cm -1 ; the strength of this band depends on asymmetry of bond, strongest for terminal alkynes, weakest for symmetrical internal alkynes (w-m) C-H for terminal alkynes occurs at 3200-3300 cm -1 (s) Remember internal alkynes ( R-C ≡ C-R ) would not have this band! 1-Octyne Infrared Spectroscopy 51

52. Specific groups Aromatics Due to the delocalization of electrons in the ring, where the bond order between carbons is 1 ½, the stretching frequency for these bonds is slightly lower in energy than normal C=C These bonds show up as a pair of sharp bands, 1500 (s) & 1600 cm -1 (m), where the lower frequency band is stronger C-H bonds off the ring show up similar to vinyl C-H at 3000-3100 cm -1 (m) Ethyl benzene Infrared Spectroscopy 52

53. Specific groups Aromatics If the region between 1667-2000 cm -1 (w) is free of interference (C=O stretching frequency is in this region) a weak grouping of peaks is observed for aromatic systems Analysis of this region, called the overtone of bending region, can lead to a determination of the substitution pattern on the aromatic ring Monosubstituted 1,2 disubstituted ( ortho or o -) 1,3 disubstituted ( meta or m -) 1,4 disubstituted ( para or p -) Infrared Spectroscopy 53