Introduction to Spectroscopic methods

Transcription

1 Introduction to Spectroscopic methods Spectroscopy: Study of interaction between light* and matter. Spectrometry: Implies a quantitative measurement of intensity. * More generally speaking electromagnetic radiation

2 Types of Analytical Spectroscopy Absorbance Fluoresence and Phosphoresence Emission (atomic with flames, arcs, sparks, and palsmas) Chemilumenesence and Biolumenesence Reflection

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4 The Electromagnetic Spectrum E = hν ν = c / λ

5 What about E? E = hν ν = c / λ

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7 Kinds of Spectroscopy Douglas A. Skoog,, F. James Holler and Timothy A. Nieman,, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia,, 1998.

8 LIGHT Electro-magnetic radiation

9 Light as a Wave E = A sin(ωt + φ) E m = Amplitude ω = angular frequency = 2πν2 = φ = phase at point x at t = 0 2πv λ Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

10 Light as a Wave Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, Frequency = ν Velocity of propagation = v = νλ Speed of light in a vacuum = c = 3.00 x 10 8 m/s Wavenumber = ν = kν = ν/v

11 Effect of the Medium on a Light Wave Frequency and Energy remain the same. Velocity and Wavelength change. Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

12 Electromagnetic Radiation Mutually perpendicular, oscillating electric and magnetic fields EM radiation is plane polarized when all E-field (or B-field) B oscillations occur in a single plane Chapter 6 12

13 Superposition Principle If two plane-polarized polarized waves overlap in space, the resulting electromagnetic disturbance is the algebraic sum of the two waves. Optical Interference: The interaction of two or more light waves yielding an irradiance that is not equal to the sum of the irradiances. Coherence: When two waves have an initial phase difference of zero or it is constant for a long time they are considered coherent.

14 Optical Interference Constructive Interference φ 2 φ 1 = δ = ±m2π Destructive Interference φ 2 φ 1 = δ = (2m+1)π Figure 3-43 Ingle and Crouch, Spectrochemical Analysis

15 If φ 1 φ 2 the phase changes. Eugene Hecht, Optics,, Addison-Wesley, Reading, MA, 1998.

16 Diffraction: The Bending of Light as It Passes Through an Aperture or Around a Small Object Dr. Quantum???? Fraunhofer Diffraction Narrow Slit Diffraction Douglas A. Skoog,, F. James Holler and Timothy A. Nieman,, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia,, 1998.

17 Diffraction of Waves in a Liquid Diffraction increases as aperture size λ Eugene Hecht, Optics,, Addison-Wesley, Reading, MA, 1998.

18 Diffraction Pattern From a Single Slit For Destructive Interference: Wsinθ = mλm m = ±1, ±2, ±3, Ingle and Crouch, Spectrochemical Analysis

19 Diffraction Pattern From Multiple Slits Douglas A. Skoog,, F. James Holler and Timothy A. Nieman,, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia,, 1998.

20 What happens when light hits a boundary between two media? Physics of Refraction Conservation Law α(λ) ) + ρ(λ) ) + T(λ) = 1 α(λ) = Fraction Absorbed ρ(λ) = Fraction Reflected T(λ) = Fraction Transmitted Eugene Hecht, Optics,, Addison-Wesley, Reading, MA, 1998.

21 Refractive index (η)( the velocity (v) of EM radiation depends on the medium through which it travels η = c/v (>1). the ratio of the velocity in vacuum over the velocity in the medium η depends on the frequency of the light Chapter 6 21

22 Refraction The direction of the light will change upon passing from a less dense medium to a more dense medium η 1 sinθ 1 = η 2 sinθ 2 (Snell s s Law) Chapter 6 22

23 Transmission: The Refractive Index η = c v η is wavelength (frequency) dependent. In glass η increases as λ decreases. Eugene Hecht, Optics,, Addison-Wesley, Reading, MA, 1998.

24 Refraction is a consequence of velocity change. Eugene Hecht, Optics,, Addison-Wesley, Reading, MA, 1998.

25 Snell s s Law of Refraction η 1 sinθ 1 = η 2 sinθ 2 If η 1 <η 2 light refracts toward the normal. If η 1 >η 2 light refracts away from the normal. Ingle and Crouch, Spectrochemical Analysis

26 Law of Specular Reflection Velocity is constant. θ 3 = θ 1 The angle of reflection is equal to the angle of incidence. Ingle and Crouch, Spectrochemical Analysis

27 Fraction of Light Reflected: Fresnel Equation For monochromatic light hitting a flat surface at 90 0 Very important it governs losses of light at air/sample cell Boundaries, at mirrors and lenses.

28 ρ(λ) at different interfaces Ingle and Crouch, Spectrochemical Analysis

29 Light as Particles Eugene Hecht, Optics,, Addison-Wesley, Reading, MA, E = h ν = h c λ h = Planck Constant = Js

30 The Photoelectric Effect Douglas A. Skoog,, F. James Holler and Timothy A. Nieman,, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, adiomete

31 Energy states of chemical species Plank Atoms, ions and molecules exist only in discrete states characterized by definite amounts of energy Absorbs or emits an amount of energy exactly equal to that of the difference between states Frequency or wavelength are related to the energy difference E E = hν = 1 0 hc λ

32 Chemiluminescence

33 Absorption

34 Photoluminescence (Fl,Ph)

35 Absorption Douglas A. Skoog,, F. James Holler and Timothy A. Nieman,, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia,, 1998.

36 Quantitative Aspects of Spectrochemical Measurements Transmittance T = P/P o (definition) P o - incident light power P - transmitted light power %T = P/P o x 100 % Absorbance A = - log T (definition) Beer s s Law (physical law applicable under certain conditions) A = ε b c (basis of quantitation) ε - molar absorptivity (L mol - 1 cm - 1 ) b - path length (cm) c - concentration (mol L - 1 )

37 Components of Optical Instruments

38 UV-Visible Visible Instrumentation (a) Single beam (b) Double beam in space (c) Double beam in time 38

39 Dispersion Devices Non-linear dispersion Temperature sensitive Linear Dispersion Different orders

40 Conventional Spectrophotometer Schematic of a conventional single-beam spectrophotometer

41 Conventional Spectrophotometer Optical system of a split-beam spectrophotometer

42 Conventional Spectrophotometer Optical system of a double-beam spectrophotometer

43 Energy States of Chemical Planck Black body radiation Atoms, ions, and molecules exist in discrete states Characterized by definite amounts of energy Changes of state involve absorption or emission of energy E1-E0 = hν = hc/ c/λ

44 Emission of Radiation Emission X * X + hνh P emitted = k c atomic emission Excitation needs energy! Particle bombardment (e-) Electrical currents (V) Fluorescence Heat Douglas A. Skoog,, F. James Holler and Timothy A. Nieman,, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia,, 1998.

45 Light Sources Lines, Bands, Continuum Gas, Liquid, Solid Continuum Spectra UV-VISIBLE VISIBLE

46 Incandescent Fluorescence LED Lamp

47 Newer lamps

48 Continuum Source Line Source Continuum + Line Source Al + Mg a b Ingle and Crouch, Spectrochemical Analysis

49 Saltwater in a flame

50 λ Ranges of Common Sources Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis,, Saunders College Publishing, Fort Worth, 1992.

51 Optical Source Characteristics Ingle and Crouch, Spectrochemical Analysis

52 Blackbody radiation

53 λ Wien s Displacement Law max = T K nm Stefan-Boltzman Law P = σt 4 σ = Wcm - 2 K -4 Blackbody Radiation Both λmax and radiation power (P) are related to TEMPERATURE and current! Eugene Hecht, Optics,, Addison-Wesley, Reading, MA, 1998.

54 Nernst Glower Rare earth oxides formed into a cylinder (1-2 2 mm diameter, ~20mm long). Pass current to give: T = K. Ingle and Crouch, Spectrochemical Analysis Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis,, Saunders College Publishing, Fort Worth, 1992.

55 Globar Silicon Carbide Rod (5mm diameter, 50 mm long). Heated electrically to K. Positive temperature coefficient of resistance Electrical contact must be water cooled to prevent arcing. Ingle and Crouch, Spectrochemical Analysis

56 Tungsten Filament Heated to 2870 K. Useful Range: nm Ingle and Crouch, Spectrochemical Analysis

57 Iodine added. Tungsten / Halogen Reacts with gaseous W near the quartz wall to form WI 2. W is redeposited on the filament. Gives longer lifetimes Allows higher temperatures (~3500 K).

58 Intensity Spectrum of the Tungsten-Halogen Lamp Weak intensity in UV range Good intensity in visible range Very low noise Low drift

59 Arc Lamps Electrical discharge is sustained through a gas or metal vapor. Continuous emission due to rotational/vibrational energy levels and pressure broadening. Ingle and Crouch, Spectrochemical Analysis

60 H 2 or D 2 Arc Lamps D 2 + E e- D 2 * D + D D + hν Energetics: E e- = E D2 * = E D + E D + hν Useful Range: nm. Ingle and Crouch, Spectrochemical Analysis

61 Intensity Spectrum of the Xenon Lamp High intensity in UV range High intensity in visible range Medium noise

62 Hg Arc Lamp Continuum + Line Source High Power Source. Often used in photoluminescence. Ingle and Crouch, Spectrochemical Analysis

63 Hollow Cathode Discharge Tube. Apply ~300 V across electrodes. Ar + or Ne + travel toward the cathode. If potential is high enough cations will sputter metal off the electrode. Metal emits photons at characteristic atomic lines as the metal returns to the ground state. Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis,, Saunders College Publishing, Fort Worth, 1992.

64 Hollow Cathode Discharge Tube. Line Widths are typically Å. Ingle and Crouch, Spectrochemical Analysis

65 Absorption of radiation Is a quantized process??? The energy absorbed is released, although not necessarily all as light energy (e.g. heat) Results in excitation of a molecule to a higher energy state E= E electronic + E vibrational + E rotational

66 Jablonski Diagram

67 Vibrational Relaxation???

68 Non-radiative relaxation Vibrational Relaxation: A molecule can give off some of its energy from absorbed light (usually uv-vis vis) ) by jumping to a lower energy vibrational state. The excess energy is used to make the conversion. No light is given off. Internal Conversion: The molecule transitions to a lower energy electronic state without giving off light. Excess energy is used to covert the molecule from one electronic state to another. Poorly understood External conversion: The molecule gives off energy to an external source, such as by collision with another similar molecule or solvent molecule. This s is called quenching Intersystem Crossing: The molecule goes from a singlet to triplet excited state and uses up energy changing the spin of an electron.

69 Singlet vs Triplet State

70 Molecule of Formaldehyde