1. INTRODUCTION TO SPECTROSCOPIC METHODS OF ANALYSIS LECTURE 1 1

2. WHAT IS SPECTROSCOPY? The study of the interaction between ELECTROMAGNETIC (EM) RADIATION and MATTER 2

3. SPECTROSCOPIC ANALYSIS covers ATOMIC SPECTROSCOPY MOLECULAR SPECTROSCOPY 3

4. T O U NDERSTAND S PECTROSCOPY W E M UST U NDERSTAND E LECTROMAGNETIC R ADIATION 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. WAVE PROPERTIES EM radiation is conveniently modeled as waves consisting of perpendicularly oscillating electric and magnetic fields, as shown below. x y z Electric Field Magnetic Field Direction of propagation 5

6. 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. 6

7. W AVE PARAMETERS We Use Symbols to Designate the Various Properties of Waves is the wavelength of the waves V is the frequency of the waves c is the speed of light We Use Symbols to Designate the Various Properties of Waves is the wavelength of the waves V is the frequency of the waves c is the speed of light Time or Distance - + Electric Field 0 Amplitude (A) Wavelength ( ) 7

8. D EFINITIONS : Period (p) – Period (p) – the time required for one cycle to pass a fixed point in space. Frequency (V) – Frequency (V) – the number of cycles which pass a fixed point in space per second. Amplitude (A) – Amplitude (A) – The maximum length of the electric vector in the wave (Maximum height of a wave). Wavelength ( ) – Wavelength ( ) – The distance between two identical adjacent points in a wave (usually maxima or minima). Wavenumber ( ) Wavenumber ( ) - The number of waves per cm in units of cm -1. 8

9. Radiant Power ( P ) - Radiant Power ( P ) - The amount of energy reaching a given area per second. Unit in watts (W) Intensity ( I ) - Intensity ( I ) - The radiant power per unit solid angle. 9 D EFINITIONS :

10. R ELATIONSHIP B ETWEEN T HESE V ARIABLES Speed of light = Wavelength x Frequency c = V = c/ V V = c/ For Electromagnetic Waves the Speed ( c) is a Constant c = 3.00 x 10 8 m/sec = 3.00 x 10 10 cm/sec 10

11. This Constant Speed Means a Direct, Inverse Relationship Between Wavelength and Frequency ∝ 1/ V The Higher the Frequency the Shorter the Wavelength. The Longer the Wavelength the Lower the Frequency. 11

12. T HE R ELATIONSHIP B ETWEEN F REQUENCY AND W AVELENGTH Wavelength is inversely proportional to frequency 12 800 nm Infrared radiation V = 3.75 x 10 14 s -1 Ultraviolet radiation V = 7.50 x 10 14 s -1

13. P ARTICLE PROPERTIES OF L IGHT : P HOTONS Wave theory failed to explain phenomena associated with the absorption and emission of radiation of radiant energy. Thus, EM is viewed as a stream of discrete particles, or wave packets, of energy called photons. We can relate the energy of photon to its wavelength, frequency and wavenumber by E = hV V - frequency = h c  - wavelength  υ - wavenumber = hcυ h – Planck’s constant =6.63x10 -34 J·s 13

14. T HE E LECTROMAGNETIC S PECTRUM 14

15. R EGIONS OF THE UV, V ISIBLE AND IR S PECTRUM RegionWavelength Range UV180 – 380 nm Visible380 – 780 nm Near-IR0.78 – 2.5 μm Mid-IR2.5 – 50 μm 15

16. P REFIXES FOR U NITS Prefix Symbols Multiplier giga- G 10 9 mega- M 10 6 kilo- k 10 3 deci- d 10 -1 centi- c 10 -2 milli- m 10 -3 micro- µ 10 -6 nano- n 10 -9 pico- p 10 -12 femto- f 10 -15 atto- a 10 -18 16

17. W AVELENGTH U NITS FOR V ARIOUS S PECTRAL R EGION RegionUnitDefinition (m) X-rayAngstrom unit, Å10 -10 m Ultraviolet/visibleNanometer, nm10 -9 m InfraredMicrometer, μm10 -6 m 17

18. INTERACTION OF ELECTROMAGNETIC RADIATION WITH MATTER Infrared primarily acts to set molecules into vibration. UV and visible light primarily acts to elevate electrons to higher energy levels. 18

19. INTERACTION OF ELECTROMAGNETIC RADIATION WITH MATTER The interaction of radiation with matter can cause redirection of the radiation and/or transitions between the energy levels of the atoms or molecules.energy levels 1. A transition from a lower level to a higher level with transfer of energy from the radiation field to the atom or molecule is called absorption.absorption 2. A transition from a higher level to a lower level is called emission if energy is transferred to the radiation field, or nonradiative decay if no radiation is emitted. emission 3. Redirection of light due to its interaction with matter is called scattering, and may or may not occur with transfer of energy, i.e., the scattered radiation has a slightly different or the same wavelength. scattering 19

20. T YPES OF SPECTRA 1. Absorption spectrum 2.Emission spectrum 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. 20

21. ATOMIC vs MOLECULAR TRANSITIONS 21

22. A TOMIC TRANSITION Atomic transitions are usually very discreet changes of electrons from one quantum state to another (energy levels, shells, spins, etc.). Only electronic transition is quantized. When an atom changes energy state, it absorbs or emits energy equal to the energy difference  E = E 1 – E 0 The wavelength or frequency of radiation absorbed or emitted during a transition proportional to  E Transitions between electronic levels produce line spectra. 22

23. A TOMIC TRANSITION E 0 – lowest energy electronic level or ground state E 1, E 2 – higher-energy electronic levels 23

24. M OLECULAR TRANSITION In molecules the electronic states are subdivided into vibrational states. The energy of a band in a molecular absorption spectrum is the sum of three different energy components. E = E electronic + E vibrational + E rotational Transitions between electronic-vibrational- rotational states give rise to spectra that appear to have bands. 24

25. 25 Vibrational energy level M OLECULAR TRANSITION Energy

26. A TOMIC ABSORPTION SPECTRUM The two peaks arise from the promotion of a 3s electron to the two 3p states 26 Absorption Spectrum of Na

27. M OLECULAR ABSORPTION SPECTRA The sharpness of molecular absorption spectra also depends on the state of the sample. Figure (b) shows an absorption band due to transitions between electronic-vibrational-rotational states Figure (d) shows a continuous spectra due to the sample is in the condensed state. In condensed states the spectra broaden due to molecular collisions. 27

28. E MISSION SPECTRUM Three types of spectra: Lines Bands Continuum spectra 28 Emission spectrum of a brine sample

29. COMPONENTS OF INSTRUMENTS FOR OPTICAL SPECTROSCOPY 29

30. G ENERAL D ESIGN OF O PTICAL I NSTRUMENTS Absorption Emission 30

31. F IVE B ASIC O PTICAL I NSTRUMENT C OMPONENTS 1)Source -A stable source of radiant energy at the desired wavelength (or 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 (monochromators, prisms, & filters). 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. 31

32. I.S OURCES OF R ADIATION Generate a beam of radiation that is stable and has sufficient power. A. Continuum Sources - emit radiation over a broad wavelength range and the intensity of the radiation changes slowly as a function of wavelength. This type of source is commonly used in UV, visible and IR instruments. Deuterium lamp is the most common UV source. Tungsten lamp is the most common visible source. Glowing inert solids are common sources for IR instruments. 32

33. B. Line Sources - Emit a limited number lines or bands of radiation at specific wavelengths. Used in atomic absorption spectroscopy Types of line sources: 1)Hollow cathode lamps 2)Electrodeless discharge lamps 3)Lasers -Light ­amplification by stimulated emission of radiation 33

34. II.W AVELENGTH S ELECTORS Wavelength selectors output a limited, narrow, continuous group of wavelengths called a band. Two types of wavelength selectors: A)Filters B)Monochromators 34

35. A.F ILTERS Two types of filters: 1)Interference filters 2)Absorption Filters B.Monochromators Wavelength selector that can continuously scan a broad range of wavelengths Used in most scanning spectrometers including UV, visible, and IR instruments. 35

36. III.R ADIATION T RANSDUCERS (D ETECTORS ) Early detectors in spectroscopic instruments were the human eye, photographic plates or films. Modern instruments contain devices that convert the radiation to an electrical signal. Two general types of radiation transducers: a.Photon detectors b.Thermal detectors 36

37. 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 A. Photon Detectors Commonly useful in ultraviolet, visible, and near infrared instruments. 37

38. Three types of thermal detectors : 1.Thermocouples 2.Bolometers 3.Pyroelectric transducers B. Thermal Detectors Used for infrared spectroscopy because photons in the IR region lack the energy to cause photoemission of electrons. 38

39. Sample containers, usually called cells or cuvettes must have windows that are transparent in the spectral region of interest. There are few types of cuvettes: - quartz or fused silica - silicate glass - crystalline sodium chloride QUARTZ OR FUSED SILICA - REQUIRED FOR UV AND MAY BE USED IN VISIBLE REGION SILICATE GLASS - CHEAPER COMPARED TO QUARTZ. U SED IN UV CRYSTALLINE SODIUM CHLORIDE - USED IN IR IV.S AMPLE H OLDER (C ONTAINER ) cuvette 39

40. SPECTROMETER - is an instrument that provides information about the intensity of radiation as a function of wavelength or frequency SPECTROPHOTOMETER - is 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. 40

41. REGIONSOURCESAMPLE HOLDER DETECTOR UltravioletDeuterium lampQuartz/fused silica Phototube, PM tube, diode array VisibleTungsten lampGlass/quartzPhototube, PM tube, diode array InfraredNernst glower (rare earth oxides or silicon carbide glowers) Salt crystals e.g. crystalline sodium chloride Thermocouples, bolometers Types of source, sample holder and detector for various EM region SUMMARY 41