Infrared spectroscopy

introduction and principle of infrared spectroscopy

1. Infrared spectroscopy Submitted to : Diksha thakur mam Submitted by : Shabnam Role no. :1303 Class : Msc forensic science 2 nd year

2. Contents 1. Introduction 2. Infrared – region 3. Principle 5. Absorption of IR radiation by molecule 6. Molecular vibrations in IR region 7. Instrumentation 8. FTIR 9. Applications of IR spectroscopy in forensic science.

3. Introduction Infrared spectroscopy is the spectroscopy that deals with IR ( infrared region) of electromagnetic spectrum , that is light with longer wavelength and lower frequency than the visible light . Definition – It is the study of interaction between infrared light and matter.The process is based on the absorption spectroscopy. History – IR –Spectroscopy was discovered in 1800 by “ Sir William Hershel ”.

4. Conts . IR –Spectroscopy can be very sensitive to determination of functional groups within the sample since different functional groups absorbs different particular frequency of IR –radiation.

5. INFRARED-REGION IR –Spectroscopy is also classified into three region because of their characteristics property in different region of IR. 1. Far-infrared region (400 - 33 cm-1) –Vibration of molecules that containing heavy atoms, molecular skeletal vibrations and crystal lattice vibrations. 2. Mid-infrared region (4000 - 400 cm-1) –Useful for organic analysis. 3. Near-infrared region (12820 – 4000 cm-1)- Overtones, very useful for quantitative analysis.

6. Electromagnetic radiation Electromagnetic radiation is the properties of waves in many ways.Light waves can be represented as oscillating perpendicular electric and magnetic fields.Both are right angle to each other and to the wave of propagation light. Wavelenth is the crest-to-crest distance between two succesive maxima.SI unit is meter(m), centrimeter (cm) ,nanometer(nm). Amplitude maximum of vector from the origin to point displacement of oscillation.

7. Conts . Wavelenth Frequency (v) no. of crests passing a fixed point per second.Unit of v is Hertz (Hz) or inversely second (s-1). E = hv E= hc / lembda

8. Ecletromegnetic waves

9. Principle In any molecule it is known that atoms or groups of atoms are connected by bonds. These bonds are analogous to springs and not rigid in nature. Because of the continuous motion of the molecule they maintain some vibrations with some frequency characteristic to every portion of the molecule.This is called the natural frequency of vibrations. When energy is applied in the form of IR-radiation is applied and when Applied IR frequency=Natural frequency of vib .

10. Conts . Molecules are excited to high energy state on absorbs IR radiation, selectively absorbs the radiation resulting in vibration of a molecule of the compound , giving rise to closely packed absorption bands which is called as IR absorption spectrum. Bands corresponding to the functional groups and the bonds present in a chemical substance. Hence , an IR spectrum of a compound is considered as the fingerprint for its chemical identification.

11. IR spectru m

12. IR-active and IR-inactive molecules All the bonds in a molecule are not capable of absorbing IR energy. IR active - Those bonds which are accompanied by a change in dipole moment will absorb in the IR region and such transitions are called IR active transitions. IR inactive- The transitions which are not accompanied by a change in dipole moment of the molecule are not observed and are considered as IR inactive.

13. Absorption of IR-radiation by molecule Molecule with covalent bonds may absorbs IR radiation .This absorption is quantized , so only certain frequency of IR radiation are absorbed. When the radiation is absorbed , molecules moves higher energy radiation state.The energy absorbed which is associated with IR rotation , its sufficient to cause molecules to rotate and vibrate. Energy required to cause change in rotational level is small compared to the energy required to cause change in vibrational level.

14. Conts . Hence, each vibrational change has multiple rotational changes associated with it. Molecules absorbs radiation when a bond in the molecule vibrates at the same frequency as the incident radiant energy. Frequency absorbed depend on the masses of the atoms in the bond , geometry of molecule , strength of the bond and several other factors. Not all molecule can absorbs IR radiation , molecule must have change in dipole moment during vibrations in order to absorbs IR radiation.

15. Energy level diagram

16. Organic structure determination How the atoms are connected together ? Which bonds are single,double o triple? What functional group exist in a molecule? Identity of an organic compound can be established from its fingerprint region by comparing the sample spectrum with known spectrum of compound.

17. Vibrations in molecule Molecular vibrations are excited by IR radiation ,also takes in various modes of vibrations.Most common type of vibration is stretching and bending type. Stretch vibration – Involves the change in the bond length resulting change in interatomic distance. Bending vibration –Involves the change in bond angle or change in the position of the groups of atoms with respect to the rest of molecule.

18. Fundamental vibrations

19. Non – Fundamental vibrations Overtones –These are observed at twice the frequency of strong bond. E.g. Carbonyl group Combination tones- A combination band is the result of a two frequencies being excited is allow by symmetry.Overtone is not required a symmetry. Fermi- rasonance –It is the interactions between fundamental vibrations and overtones . e.g. carbon dioxide.

20. Number of vibrational modes: -For a non-linear molecule, no. of types of vibrations = 3N-6 -For a linear molecules, No. of types of vibrations =3N-5 Examples – HCl 3(2)-5 = 1 BF3 3(4)-6 = 6

21. Dispersive & Non dispersive spectrophotometer Dispersive Sequencing scanning of each wave number takes place. Double beam instrument are mostly used than single beam instrument. Non dispersive Filters are used for wavelength selection. Having sample specific detectors.

22. Instrumentation of IR Main part of IR spectrometer are: 1. Radiation source 2. Sample cell and sampling of substance. 3. Monochromator 4. Detectors 5. Recorder The material used in an IR spectroscopy must be transparent to IR radiation.Such as glass or quartz use for mid IR instrument because glass and quartz are not transparent to IR at wavelength larger than 3.5 micrometer. Ionic salts,potassium bromide,calcium floride,sodium chloride and zinc selenide also used in IR radiation.

23. Instrumental setup

24. INSTRUMENTATION 2. Radiation source – The intensity of radiation should be continuous over the wavelength range used.Cover wide wavelength range.Constant over the long period. Various source of IR radiation: a. Nernst glower b. Mercury lamp c. Tungsten lamp d. Glober source e. Nichrome wire f. Incandescent lamp

25. Nernst glower (mid range) - It is a cylindrical bar composed of zirconium oxide, cerium oxide and thorium oxide that are heated electrically to a temperature between 1500k to 2000k.Source are 20mm long and 2mm in diameter. On passing current through it causes heat and glow. Globar – It is more intense than nernst glower,globar is a bar of sintered silicon carbide which heated electrically to emit continuous IR radiation. Halogen lamp is used as source in NIR (>2000) region, its also contains a tungsten wire filament.

26. Mercury lamp –( far IR ) High pressure mercury discharge lamp ,it is constructed of a quartz bulb containing elemental Hg. Nichrome wire is also used , other metals such as rhodium are well used its heated electrically about 1100 C. Laser is also used which emits monochromatic radiation.Some lasers called tunable lasers which emit more than one wavelength of light but each wave length is monochromatic.

27. Sampling method 2. Solid-sampling – a. Include direct sampling. b. Palletization technique c. KBr & NaCl (used for preparation of pallet with sample. Particle size less than 2 micrometer Sample should be 1-2% of KBr / NaCl . Mulling – Paste and mull will be present,hexa chlorobutadiene . Solid sample for solution ( sample present in volatile solvent)

28. Conts . Liquid sampling – Two layers of NaCl pellet usually thin layer is used. Gas sampling- Gas cells are used. Length of gas cells are 10 cm. Made up of NaCl . Solution sampling technique- Chloroform Carbon tetrachloride Carbon disulphide

29. Monochromator Monochromator is an optical device that transmit a mechanically selectable narrow band of wavelength of light.The name was come from greek word mono – “single” and chroma – “color”, ator detonating a agent. Types of monochromator : 1. Prism monochromator 2. Grating monochromator

30. Types of mono- chromator Prism mono- chromator – Composed of glass and quartz ,having a property of reflection, coated by alkyl halide , greater range and simplicity. It also two types: Single beam mono- chromator - ( allow monopass prismatic radiation pass once through prism). Double beam mono- chromator – ( allow double prismatic pass twice through the prism.

31. Grating mono- chromator The device is based on separates the radiation by diffraction method which diffracts the different wavelengths at different angles represented by the multicolored line from the grating. It consists of a entrance slits , diffraction grating and spherical mirrors.

32. Detectors In IR two types of detectors used: 1. Thermal detector -: are following a) Bolometer : It is very sensitive electrical resistance thermometer that has been used to detect and measure weak thermal radiation. Bolometer used in older instruments consisted of thin metal conductor such as platinum wire. Incident radiation heats up this conductor which causes electrical resistance to change. Degree of resistance change measure the amount of radiation fallen on detector.

33. Thermocouple Thermocouple :- It is made up of welding together at each end two wires made from different metals. One welded joint called hot junction (which exposed to IR radiation) hotter than other welded joint cold junction (screened the protective box). Potential difference generated in wire is a function of temperature difference between the junctions intensity falling of IR-radiation on the hot junctions. Cannot be used for FTIR due to slow response.

34. THERMISTORS It is made up of fused mixture of metal oxides. The relationship between temperature and electrical resistance. As temp. increases its electrical resistance decreases (opposed to bolometer). This temp. and resistance difference allows thermistors to be used as IR detectors in same way as in bolometers.

35. It has small hollow cell filled with nano -absorbing gas such as xenon,in center a black film is present which absorb the radiation and causing an increase in temp. Thermal expansion causes a internal pressure of cell to increase. Wall of cell was thin convex mirror that is the part of optical system. As pressure increase the mirror is bulged which is detected by a mirror , the change in intensity cause change readout from detector. Response time is faster than bolometer , thermistor or thermocouple. GOLAY DETECTOR

36. Golay cell

37. PYROELECTRIC DETECTPORS Pyroelectric material change their electric polorization as function of temperature .This material may be insulator ,ferroelectric or semi conductor. Pyroelectric detector consist of thin crystal pyroelectric material placed between two electrode .Act as temperature dependent capacitor. Upon exposure of IR radiation the temperature and polarization of crystal change. The signal depend on the rate of polarization with temperature. It can be used as FTIR detector

38. Pyroelectric detector

39. Photon detectors It is semi conductor materials that are insulator when no radiation falls on them but become conductors when radiation falls on them. Exposure of radiation causes a very rapid change in their electrical resistance and therefore very rapid response to the IR signal. There is required must change to conductor from insulator.The IR also detected on the basis of bands gap.

40. FTIR spectrophotometer FTIR stands for Fourier transform infrared, the preferred method of IR spectroscopy. IR passed through the sample ,some of the radiation is absorbed by the sample and some of it is transmitted and detect by detector. Resulting spectrum represents molecular vibration and transmission ( in the form of peaks). FTIR spectrometry was developed in order to overcome the limitations encountered with dispersive instruments.

41. IMPORTANCE FT-IR preferred over dispersive or filter methods of IR spactral analysis.Due to:- 1. It is non- distructive technique. 2. It can increase speed ,in second collecting a scan. 3. High sensitive. 4. Greater optical throughput. 5. Mechanical simple device.

42. Purpose The main difficulty was the slow scanning process.A measuring all the frequencies simultaneously rather individually was needed. The instrument was formed which have a very simple optical device called an interferometer. Interferometer produce a unique type of signal which has all the infrared frequencies encoded into it. The signal can measured quickly,in seconds. Interferometer employ a beamsplitter which divides the incoming infrared radiation into two optical beams.

43. Instrument

44. IR-mirrors Fixed mirror – one beam reflects off a flat mirror which is fixed in place. Moving mirror – The other beam of light off on flat mirror which is move in very short distance away from beamsplitter . The beam reflect back from the mirror and recombined at the beamsplitter . Both radiation reflects on beamsplitter of same wavelength because travels same length which is known as zero path difference . If the move mirror has slightly more away from the beamsplitter than the fixed mirror.

45. Conts . The reflects back beam is not have a equal wavelength also called optical path difference. The signal which exist in interferometer is the result of these two beams interfering with each other. The resulting signal called interferogram . Zero path difference = constructive interfering. Optical path difference= Deconstructive interfering.

46. Conts .

47. FT-IR Spectrum

48. Advantage FT-IR Speed –All the measurements taken in second rather than minutes. Sensitivity –Sensitivity improved with FT-IR for many reasons optical throughput give much higher with lower noise levels. Mechanical simplicity- There is very simple mechanism of FT-IR.

49. Applications in forensic science IR is useful in forensic investigations. Forensic expert can also identify paints material ,sweat , fuels, hairs etc. which found on a crime scene. Can be use directly analysis on a scene ( on site analysis) with the help of portable IR equipment. Can be used to identify forged or altered documents by shining a beam of IR light on a documents ink.And can be used to analysis of fingerprint residues,explosives materials and counterfeit drug also analysed by the IR technique.

50. Refernces Undergraduate instrumental analysis sixth edition James W Robinson. https://www.slideshare.net/asmarf/7infraredspecroscopy6 www.wikipedia.com