CHM260 - Spectroscopy Method


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CHM260 - Spectroscopy Method

  1. 1. LECTURE 1 1
  2. 2. The study of the interaction betweenELECTROMAGNETIC (EM) RADIATION and MATTER 2
  3. 3. covers ATOMIC MOLECULAR SPECTROSCOPY SPECTROSCOPY(atomic absorption) (molecular absorption) 3
  4. 4. What is Electromagnetic Radiation? is a form of energy that has both Wave and Particle Properties. For example: Ultraviolet, visible, infrared, microwave, radio wave. 4
  5. 5. 5
  6. 6. 6
  7. 7. 7
  8. 8.  EM radiation is conveniently modeled as waves consisting of perpendicularly oscillating electric and magnetic fields, as shown below. Direction of propagation 8
  9. 9. o At 90° to the direction of propagation is an oscillation in the ELECTRIC FIELD.o At 90° to the direction of propagation and 90° from the electric field oscillation (orthagonal) is the MAGNETIC FIELD oscillation. 9
  10. 10.  Period (p) the time required for one cycle to pass a fixed point in space. Frequency (V @ f ) the number of cycles which pass a fixed point in space per second. Unit in Hz or s-1 Amplitude (A) The maximum length of the electric vector in the wave (Maximum height of a wave). Wavelength (λ) The distance between two identical adjacent points in a wave (usually maxima or minima). 10
  11. 11.  Wavenumber (ν) The number of waves per cm in units of cm-1. Radiant Power ( P ) The amount of energy reaching a given area per second. Unit in watts (W) Intensity (I) The radiant power per unit solid angle. 11
  12. 12. Speed of light = Wavelength x Frequency Speed of light = Wavelength x Frequency c = λV c = λVWhere asWhere as λ is the wavelength of the waves λ is the wavelength of the waves V is the frequency of the waves V is the frequency of the waves c is the speed of light c is the speed of light c = 3.00 x 1088 m/s = 3.00 x 1010 cm/s c = 3.00 x 10 m/s = 3.00 x 1010 cm/s 12
  13. 13. 800 nm Infrared radiation Ultraviolet radiation V = 3.75 x 1014 s-1 V = 7.50 x 1014 s-1Wavelength is inversely proportional to frequency λ ∝ 1/V The Higher the Frequency the Shorter the Wavelength . The Longer the Wavelength the Lower the Frequency. 13
  14. 14. EMR is viewed as a stream of discrete particles ofenergy called photons.We can relate the energy, E of photon to itswavelength, frequency and wavenumber by hc E = hV = = hcν λ h = Planck’s constant h = 6.63 x 10 -34 J.s 14
  15. 15. hcE = hV = hcν = λTherefore wavenumber, ν ν = 1/λ = V/cUnit of wavenumber is cm-1
  16. 16. What is the energy of a 500 nm photon?V = c/λ = (3 x 108 m s-1)/(5.0 x 10-7 m)V = 6 x 1014 s-1 @ HzE = hV = (6.626 x 10-34 J•s)(6 x 1014 s-1) = 4 x 10-19 J 16
  17. 17. 17
  18. 18. Region Wavelength Range UV 180 – 380 nmVisible 380 – 780 nmNear-IR 780 – 2500 nmMid-IR 2500 – 50000 nm 18
  19. 19. Region Unit Definition (m) X-ray Angstrom unit, Å 10-10 mUltraviolet/visible Nanometer, nm 10-9 m Infrared Micrometer, μm 10-6 m 19
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  21. 21.  Atoms are the basic blocks of matter. They consist of heavy particles (called protons and neutrons) in the nucleus, surrounded by lighter particles called electrons. 21
  22. 22.  An electron will interact with a photon. An electron that absorbs a photon will gain energy. An electron that loses energy must emit a photon. For absorption to occur, the energy of the photon must exactly match an energy level in the atom (or molecule) it contacts. ◦ Ephoton = Eelectronic transition We distinguish two types of absorption ◦ Atomic ◦ Molecular 22
  23. 23.  Absorption EMR energy transferred to absorbing molecule (transition from low energy to high energy state). Emission EMR energy transferred from emitting molecule to space (transition from high energy to low energy state). Scattering redirection of light with no energy transfer.
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  25. 25.  Electrons bound to atoms have discrete energies (i.e. not all energies are allowed). Thus, only photons of certain energy can interact with the electrons in a given atom. Transitions between electronic levels of the electrons produce line spectra. 25
  26. 26.  Consider hydrogen, the simplest atom. Hydrogen has a specific line spectrum. Each atom has its own specific line spectrum (atomic fingerprint). 26
  27. 27. The energy of photon that can promote electronsto excite/jump to a higher energy level dependson the energy difference between the electroniclevels. 27
  28. 28. Each atom has a specific set of energy levels, andthus a unique set of photon wavelengths with whichit can interact. 28
  29. 29.  Absorption and emission for the sodium atom in the gas phase.  The diagram illustrate the transitions (excitation and emission) of electrons between different energy levels in sodium atom.ΔEtransition = E1 - E0 = hv = hc/λ 29
  30. 30.  The energy, E, associated with the molecular bands: Etotal = Eelectronic + Evibrational + Erotational In general, a molecule may absorb energy in 3 ways:1. By raising an electron (or electrons) to a higher energy level. (electronic)2. By increasing the vibration of the constituent nuclei. (vibrational)3. By increasing the rotation of the molecule about the axis. (rotational)
  31. 31. hνEn En hν hνEo Eo Absorption Emission
  32. 32. RotationalabsorptionVibrationalabsorption
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  34. 34.  Absorption spectrum ◦ A plot of the absorbance as a function of wavelength or frequency. Emission spectrum ◦ A plot of the relative power of the emitted radiation as a function of wavelength or frequency. 34
  35. 35. Absorption Spectrum of Na The two peaks arise from the promotion of a 3s electron to the two 3p states 35
  36. 36. Electronic Transition Vibrational Transition Superimposed on the Electronic Transition Absorption Band – A series of closely shaped peaks 36
  37. 37.  In solvents the rotational and vibrational transitions are highly restricted resulting in broad band absorption spectra. 37
  38. 38.  Three types of spectra: ◦ Lines ◦ Bands ◦ Continuum spectra Emission spectrum of a brine sample 38
  39. 39. 39
  40. 40. Absorption SpectroscopyEmission Spectroscopy 40
  41. 41. 1. Source A stable source of radiant energy at the desired wavelength (or λ range).2. Sample Holder A transparent container used to hold the sample (cells, cuvettes, etc.).3. Wavelength Selector A device that isolates a restricted region of the EM spectrum used for measurement 41 (monochromators, prisms, & filters).
  42. 42. 4. Photoelectric Transducer (Detector) Converts the radiant energy into a useable signal (usually electrical).5. Signal Processor & Readout Amplifies or attenuates the transduced signal and sends it to a readout device such as a meter, digital readout, chart recorder, computer, etc. 42
  43. 43. Generate a beam of radiation that is stable and has sufficientpower.A. Continuum Sources emit radiation over a broadwavelength range and the intensity of the radiationchanges slowly as a function of wavelength. This type of source is commonlyused optical instruments. Deuterium lamp is the mostcommon UV source. Tungsten lamp is the most 43common Visible source.
  44. 44. B. Line SourcesEmit a limited number lines or bands of radiationat specific wavelengths. Used in atomic absorptionspectroscopy.Types of line sources:1.Hollow cathode lamps2.Electrodeless discharge lamps3.Lasers (Light­amplification by stimulatedemission of radiation) 44
  45. 45. Sample containers usually is called cells orcuvettes, must have side/windows that aretransparent in the spectral region of interest.There are few types of cuvettes1. quartz or fused silica (below 350nm) required for UV & VIS region2. silicate glass (350 – 2000nm) cheaper compared to quartz. Used in VIS3. crystalline sodium chloride used in IR 45
  46. 46. Wavelength selectors provides a limited, narrow,continuous group of wavelengths called a band.Two types of wavelength selectors: A) Filters B) Monochromators 46
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  48. 48. Early detectors in spectroscopic instruments werethe human eye, photographic plates or films.Modern instruments contain devices that convertthe radiation to an electrical signal.Two general types of radiation transducers: a. Photon detectors b.Thermal detectors 48
  49. 49. A. Photon DetectorsCommonly useful in ultraviolet, visible and nearinfrared instruments.Several types of photon detectors are available: 1. Vacuum phototubes 2.Photomultiplier tubes 3.Photovoltaic cells 4. Silicon photodiodes 5.Diode array transducers 6. Photoconductivity transducers 49
  50. 50. B. Thermal DetectorsUsed for infrared spectroscopy because photons in the IR region lack the energy to cause photoemission of electrons.Three types of thermal detectors: 1. Thermocouples 2. Bolometers 3. Pyroelectric transducers 50
  51. 51. SPECTROMETERis an instrument that provides information about the intensity of radiation as a function of wavelength or frequency.SPECTROPHOTOMETERis a spectrometer equipped with one or more exit slits and photoelectric transducers that permits the determination of the ratio of the radiant power of two beams as a function of wavelength as in absorption spectroscopy. 51
  52. 52. REGION SOURCE SAMPLE DETECTOR HOLDERUltraviolet Deuterium lamp Quartz /fused Phototube, silica Photo Multiplier tube, diode array Visible Tungsten lamp Silicate Glass Phototube, /Quartz Photo Multiplier tube, diode array Infrared Nernst glower (rare Salt crystals Thermocouples, earth oxides or silicon (crystalline bolometers carbide glowers) sodium chloride) 52