Introduction to Spectroscopic Methods: General Concepts and Instrumentation

1. Satish Pradhan Dnyanasadhana College, Thane-400604.Department of ChemistryM.Sc. Analytical Chemistry Sem-II1.1 General introduction to spectroscopic methods

2. Syllabus • 1.1 General introduction to spectroscopic methods:, Recapitulation Of Basic Concepts, general instrumentation: sources, wavelength selectors, sample containers, radiation transducers, signal processors and read out system, fiber optics, types of optical instruments, Fourier transform optical instruments.[5L] • 1.2 Molecular ultra violet and visible spectroscopy: • factors affecting molecular absorption, types of transitions, pH, temperature, solvent, effect of substituents, Derivative and dual wavelength spectroscopy, applications including simultaneous determinations.

6. Energy Increases Wavelength Decreases Electromagnetic Spectrum

7. W A V E L E N G T h I N C R E S E S Energy Increases

9. GENERAL INSTRUMENTATION

10. Sources of Energy

11. Sources of Energy

12. Type of Source • A continuum source emits radiation over a wide range of wavelengths, with a relatively smooth variation in intensity as a function of wavelength • Line sources, on the other hand, emit radiation at a few selected, narrow wavelength ranges

14. Chemical Sources of Energy

15. Electromagnetic spectrum

16. Electromagnetic spectrum

17. Electromagnetic spectrum

18. Electromagnetic spectrum

21. Wavelength selectors • Absorption filters work by selectively absorbing radiation from a narrow region of the electromagnetic spectrum. • Commercially available absorption filters provide effective bandwidths from 30–250 nm. The maximum throughput for the smallest effective band passes, however, may be only 10% of the source’s emission intensity over that range of wavelengths. • Interference filters uses constructive and destructive interference to isolate a narrow range of wavelengths. A simple example of an absorption filter is a piece of colored glass. A purple filter, for example, removes the complementary color green from 500–560 nm. • Interference filters are more expensive than absorption filters, but have narrower effective bandwidths, typically 10–20 nm, with maximum throughputs of at least 40%.

22. Wavelength Selection Using Monochromators One limitation of an absorption or interference filter is that they do not allow for a continuous selection of wavelength. • If measurements need to be made at two wavelengths, then the filter must be changed in between measurements. A further limitation is that filters are available for only selected nominal ranges of wavelengths. • An alternative approach to wavelength selection, which provides for a continuous variation of wavelength, is the monochromator. • The construction of a typical monochromator is shown in Figure 10.12. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with

24. 1.Filters MCL PCL

25. 2. PRISM MONOCHROMATOR FOCUSING LENS COLLIMATING LENS LAMBDA-1 LAMBDA-2 PRISM ENTRANCE SLIT EXIT SLIT PRISM MONOCHROMATOR

26. PRISM MONOCHROMATOR

27. GRATING MONOCHROMATOR

28. MAGINIFIED VIEW ENLARGED VIEW GRATING MONOCHROMATOR

29. Interferometers as wavelength selector In FTIR Spectroscopy • Interferometer: A device that allows all wavelengths of light to be measured simultaneously, eliminating the need for a wavelength selector • interferometer simultaneously allows source radiation of all wavelengths to reach the detector. Radiation from the source is focused on a beam splitter that transmits half of the radiation to a fixed mirror, while reflecting the other half to a movable mirror. • The radiation recombines at the beam splitter, where constructive and destructive interference determines, for each wavelength, the intensity of light reaching the detector. • As the moving mirror changes position, the wavelengths of light experiencing maximum constructive interference and maximum destructive interference also changes. • The signal at the detector shows intensity as a function of the moving mirror’s position, expressed in units of distance or time. The result is called an interferogram, or a time domain spectrum. The time domain spectrum is converted mathematically, by a process called a Fourier transform, to the normal spectrum (also called a frequency domain spectrum) of intensity as a function of the radiation’s energy.

30. Interferometers as wavelength selector In FTIR Spectroscopy Source Beam splitter Moving Mirror Sample Detector Fixed mirror

31. Sample cell

32. Detectors • The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. • Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. • Ideally the detector’s signal, S, should be a linear function of the electromagnetic radiation’s power, P, • S = kP + D • where k is the detector’s sensitivity, and D is the detector’s dark current, or the background electric current when all radiation from the source is blocked from the detector.

33. Transducer A device that converts a chemical or physical property, such as pH or photonintensity, to an easily measured electrical signal, such as a voltage or current.

35. PHOTOCELL DETECTOR (--) D D G C E- G B A +

36. PHOTO TUBE DETECTOR Photo Cathode Collector Anode - AMPLIFIER RECORDER

39. The pneumatic transducer, for example, consists of a small tube filled with xenon gas equipped with an IR-transparent. • window at one end, and a flexible membrane at the other end. A blackened surface in the tube absorbs photons, increasing the temperature and, therefore, the pressure of the gas. The greater pressure in the tube causes the flexible membrane to move in and out, and this displacement is monitored to produce an electrical signal.

44. Types Of Optical Instruments

45. Monochromatic light Photocell /PMT detector 1. Instrument: Single Beam Colorimeter Read Out Device U.V.Light & visible light Filter or Monochromator

46. 2. Atomic Absorption Spectrophotometer Rotating Chopper Hollow Cathode Lamp P.M.T.Detector Flame Amplifier Read Out Grating Power Supply Sample Solution

47. 3. Flame Photometer Slit Collimating Mirror P.M.T.Detector Amplifier Read Out Fuel Oxidant Prism Monochromator Sample Solution

48. Monochromatic light 2. Instrument: Single Beam Fuorimeter U.V.Light & visible light Primary filter Secondary filter Photocell /PMT detector Read Out Device

49. Turbidimeter 4 Photocell Detector Read Out Device Sample Cell visible light Filter Technique is used when concentration of suspended particles are high In this intensity of transmitted light is measured

50. Nephelometer 5 Graduated Disc Collimating Lens Light Trap visible light Photocell Detector Technique is used when concentration of suspended particles are less In this intensity of scattered light is measured Sample Cell Read Out Device