Chapter 24 Introduction to Spectrochemical Methods TMHsiung@2014 1/40 Contents in Chapter 24 1. Properties of Electromagnetic Radiation 1) Wave properties 2) Wave-Particle Duality 2. Interaction of Radiation and Matter 1) Electromagnetic Spectrum 2) Types of Quantum Transition 3) Spectroscopies without Energy Exchange 4) Spectroscopies Involving Energy Exchange 3. Absorption of Radiation 1) The Absorption Process 2) Absorption Spectra 4. Emission of Electromagnetic Radiation TMHsiung@2014 2/40 1. Properties of Electromagnetic Radiation 1) Wave properties (a) Plane-polarized electromagnetc radiation (b) 2D representation of electric factor TMHsiung@2014 3/40 2) Wave-Particle Duality Wave properties: v c/n λν ν 1/λ Particle properties: (in vacuum) E h hc/ hc λ: ν: v: c: n: wavelength frequency light speed 3x108 m/s (in vacuum) refractive index In vacuum: n= 1 : wavenumber (cm–1) ΔE: energy gap h: Plank’s constant, 6.626x10–34J·s Light measurement: Power (P): The flux of energy per unit time. Intensity (I) -The flux of energy per unit time per area. TMHsiung@2014 4/40 Continued λ change between different medium, ν remains constant *Speed of light = c/n, n (usually n > 1) is the refractive index of the medium TMHsiung@2014 5/40 2. Interaction of Radiation and Matter 1) Electromagnetic Spectrum TMHsiung@2014 6/40 Continued Vacuum ultraviolet (VUV): 120–180 nm Ultraviolet (UV): 180–380 nm Visible: 380–780 nm Near infrared regions (NIR): 0.78–2.5 μm Mid infrared: 2.5–50 m Far infrared (FIR): 50–1000 m. TMHsiung@2014 7/40 2) Types of Quantum Transition Type of Transition Nuclear Inner electron Part of spectrum Valence electron Rotation/Vibration Rotation Spin of electrons UV-vis Infrared Microwave Electron Spin Resonance Spin of nuclei Nuclear Spin Resonance -ray X-ray TMHsiung@2014 8/40 3) Spectroscopies without Energy Exchange TMHsiung@2014 9/40 4) Spectroscopies Involving Energy Exchange (1) Classification Type of Energy Transfer Electromagnetic Spectrum Region Spectroscopic Technique Absorption -ray Mossbauer spectroscopy x-ray x-ray absorption spectroscopy (XAS) UV/Vis UV/Vis spectroscopy Atomic absorption spectroscopy (AAS) infrared Infrared spectroscopy (IR) Raman spectroscopy Emission (thermal excitation) microwave Microwave spectroscopy radio waves nuclear magnetic resonance spectroscopy (NMR) UV/Vis Atomic emission spectroscopy (AES) TMHsiung@2014 10/40 Continued Type of Energy Transfer Electromagnetic Spectrum Region Spectroscopic Technique Photoluminescence x-ray x-ray fluorescence (XRF) UV/Vis Fluorescence spectroscopy Phosphorescence spectroscopy Atomic fluorescence spectroscopy (AFS) Chemiluminescence UV/Vis Luminescence spectroscopy TMHsiung@2014 11/40 (2) Glossary for Spectroscopies Involving Energy Exchange i) Optical spectroscopy (Involving Energy Exchange): Methods based on the absorption, emission, luminescence of electromagnetic radiation that is proportional to the amount of analyte in the sample. ii) Absorption spectroscopy: Measuring the quantized energy absorbed by atoms/molecules. iii) Emission spectroscopy: Exciting atom by heat (thermal), then, the emitted quantized energy from excited state to ground states is measured. TMHsiung@2014 12/40 iv) Photoluminescence: Exciting atom/molecule by light, then, the emitted quantized energy is measured. a) Fluorescence: The ground state with the same spin as excited state. b) Phosphorescence: The ground state with the opposite spin as excited state. v) Chemoluminescence (chemiluminescence): The luminescence (emission light) is the result of a chemical reaction. TMHsiung@2014 13/40 (3) Energy transition process illustrate i) Absorption process TMHsiung@2014 14/40 ii) Emission or Chemoluminescence process TMHsiung@2014 15/40 iii) Photoluminescence process TMHsiung@2014 16/40 3. Absorption of Radiation 1) The Absorption Process i) Transmittance and Absorbance TMHsiung@2014 17/40 Continued TMHsiung@2014 18/40 ***** ii) Beer’s Law Po C P C: Analyt’s concentration b: Light path length b Beer’s Law: A = abC a: absorptivity, unit is of cm–1conc–1. Analyte in molar concentration: A = bC : molar absorptivity, unit is of cm–1M–1 • Beer’s law is the linear relationship between a sample’s absorbance and concentration. • Values for a or depend on the wavelength of electromagnetic radiation. • Wavelengths corresponding to maxima absorbance in the spectra called λmax. TMHsiung@2014 19/40 TMHsiung@2014 20/40 Example: The following data was obtained from an optical absorption instrument with a cell path length 1 cm. (a) Find the molar absorptivity coefficient. (b) Determine the concentration of an unknown solution that has an absorbance of 1.52. Concentration (moles/L) 0.001 0.002 0.005 0.01 Absorbance 0.21 0.39 1.01 2.02 Solution: Y = 201.85X = bC (a) =201.85 cm–1M–1 (b) C= 0.0075 M Beer's Law Plot Absorbance 2.50 2.00 y = 201.85x 1.50 1.00 0.50 0.00 0 0.002 0.004 0.006 0.008 0.01 0.012 Concentration TMHsiung@2014 21/40 iii) Applying Beer’s Law to Mixtures The absorbance at a specific wavelength for a mixture of n components, Am, is given as: n n i 1 i 1 Am Ai i bci Two component mixture for example: TMHsiung@2014 22/40 (cont’d) TMHsiung@2014 23/40 (cont’d) TMHsiung@2014 24/40 iv) Limitations to Beer’s Law Linear range * Beer’s law is valid only at low concentrations. Generally, < 0.01 M TMHsiung@2014 25/40 (cont’d) i) Fundamental Limitations: At higher concentrations: (1) The individual particles of analyte no longer behave independently (recalled “activity”) of one another resulting in changing the value of . (2) Since absorptivity depend on the sample’s refractive index, when the refractive index varies with the analyte’s concentration, the values of will change. TMHsiung@2014 26/40 (cont’d) ii) Chemical Limitations Deviations from Beer’s law also occur when the analyte dissociates, associates, or reacts with a solvent to produce a product having a different absorption spectrum from the analyte. Example: HIn = H+ + Incolor 1 color 2 The above reaction causes the color to be pH dependent (indicators for instance). Thus, must buffer our solution to a constant pH to eliminate pH related chemical deviations. TMHsiung@2014 27/40 (cont’d) iii) Instrumental limitation (1) Beer’s law is followed only with truly monochromatic, the polychromatic radiation cause deviations from Beer’s law. (2) Stray radiation (any radiation reaching the detector that does not follow the optical path from the source to the detector) cause deviations from Beer’s law. TMHsiung@2014 28/40 2) Absorption Spectra i) Atomic Absorption (line spectra) When a atom absorbs specific quantized UV/Vis radiation, it undergoes a change in its valence electron configuration: h e * Transitions between two different orbital are termed electronic transitions. For example, Na consists of a few, discrete absorption lines corresponding to transitions between 3s→3p, 3s→4p etc. TMHsiung@2014 29/40 ii) Molecular Absorption (band spectra) * Molecular absorptions spectra are generally broad band (band spectra) because vibrational and rotational levels are "superimposed" on the electronic levels. Vibration level { Electronic Excited Vibration level { Electronic Excited Vibration level { Electronic Ground state TMHsiung@2014 30/40 Example of UVVisible absorption spectra Gaseous phase Nonpolar solvent Analyte: 1,2,4.5-tetrazine Polar solvent TMHsiung@2014 31/40 iii) Visible Spectrum and Complementary Colors Wavelength of max (nm) Color Absorbed Color Remaining 380-420 420-440 Violet Violet-blue Green-yellow Yellow 440-470 470-500 500-520 520-550 Blue Blue-green Green Yellow-green Orange Red Purple Violet 550-580 580-620 620-680 Yellow Orange Red Violet-blue Blue Blue-green 680-780 Purple Green TMHsiung@2014 32/40 4. Emission of Electromagnetic Radiation 1) Emission Spectra Emission spectrum of a brine sample with an oxyhydrogen flame TMHsiung@2014 33/40 2) Atomic Fluorescence Radiant emission from atoms that have been excited by absorption of electromagnetic radiation. * Resonance fluorescence: fluorescence emission at a wavelength that is identical with the excitation wavelength. TMHsiung@2014 34/40 3) Molecular Fluorescence i) Energy Level Diagram TMHsiung@2014 35/40 ii) More Illustration a) Life time A* A Lifetime of an analyte in the excited state (A*): • ~10–5–104 s for electronic excited states • ~10–15 s for vibrational excited states. b) Relaxation types of excited state (1) Nonradiative relaxation, e.g., vibrational deactivation, excess energy is released to solvent molecules: A* → A + heat (2) Released as a photon of electromagnetic radiation: A* → A + h c) Strokes shift: Difference in wavelengths of incident and emitted radiation. TMHsiung@2014 36/40 Continued d) Vibration Deactivation versus Internal Conversion (1) Vibrational deactivation (relaxation): A nonradiative relaxation when a excited molecule nonradiatively loses vibrational energy in a same electronic level, lifetime is rapid (10–13 to 10–11 s). (2) Internal conversion: A nonradiative relaxation in which the analyte moves from a higher electronic level to a lower electronic level. TMHsiung@2014 37/40 e) Fluorescence versus phosphorescence S0 S1 T1 (1) Fluorescence: Emission of a photon when the analyte returns to a lower-energy state with the same spin as the higher energy state, i.e., S1→S0, in which the electron life time in the excited state is ~10–5–10–8 s. (2) Phosphorescence: Emission of a photon when the analyte returns to a lower-energy state with the opposite spin as the higher-energy, i.e., T1→S0, in which the electron life time in the excited state is ~10–4–104 s. TMHsiung@2014 38/40 f) Fluorescence intensity equation*** (1) Fluorescence is generally observed with molecules where the lowest energy absorption is a π → π* transition, and those chromophores are called fluors or fluorephores. (2) For low concentrations of the fluorescing species, where εbC is less than 0.01, the intensity of fluorescence (If) is expressed as: If = 2.303kΦfP0εbC C: analyt’s concentration b: light path length ε: molar absorptivity k: efficiency constant of collecting and detecting the emission P0:excitation incident power Φf : number of photons emitted/number of photons absorbed (quantum yield). TMHsiung@2014 39/40 Homework (Due 2014/4/10) Skoog 9th edition, Chapter 24 Questions and Problems 24-5 24-6 (a) (b) 24-9 24-23 End of Chapter 24 TMHsiung@2014 40/40