TMHsiung@2014 1/40 Chapter 24 Introduction to Spectrochemical Methods.
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Transcript of TMHsiung@2014 1/40 Chapter 24 Introduction to Spectrochemical Methods.
TMHsiung@2014 2/40
Contents in Chapter 24
1. Properties of Electromagnetic Radiation1) Wave properties2) Wave-Particle Duality
2. Interaction of Radiation and Matter1) Electromagnetic Spectrum2) Types of Quantum Transition3) Spectroscopies without Energy Exchange4) Spectroscopies Involving Energy Exchange
3. Absorption of Radiation1) The Absorption Process2) Absorption Spectra
4. Emission of Electromagnetic Radiation
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(a) Plane-polarized electromagnetc radiation
(b) 2D representation of electric factor
1. Properties of Electromagnetic Radiation1) Wave properties
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2) Wave-Particle Duality
λ: wavelengthν: frequencyv: light speedc: 3x108 m/s (in vacuum)n: refractive index
In vacuum: n= 1: wavenumber (cm–1)ΔE: energy gaph: 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.
Wave properties:
v c/n λν
ν 1/λ
Particle properties:
(in vacuum)
E h
hc/ hc
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λ change between different medium, ν remains constant*Speed of light = c/n, n (usually n > 1) is the refractive index of the medium
Continued
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Vacuum ultraviolet (VUV): 120–180 nmUltraviolet (UV): 180–380 nmVisible: 380–780 nmNear infrared regions (NIR): 0.78–2.5 μmMid infrared: 2.5–50 mFar infrared (FIR): 50–1000 m.
Continued
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2) Types of Quantum Transition
Type of Transition Part of spectrum
Nuclear -ray
Inner electron X-ray
Valence electron UV-vis
Rotation/Vibration Infrared
Rotation Microwave
Spin of electrons Electron Spin Resonance
Spin of nuclei Nuclear Spin Resonance
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4) Spectroscopies Involving Energy Exchange
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
microwave Microwave spectroscopy
radio waves nuclear magnetic resonance spectroscopy (NMR)
Emission (thermal excitation)
UV/Vis Atomic emission spectroscopy (AES)
(1) Classification
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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
Continued
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(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.
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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.
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i) Transmittance and Absorbance
3. Absorption of Radiation1) The Absorption Process
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ii) Beer’s Law
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
C PPo
b
C: Analyt’s concentrationb: Light path length
• 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.
*****
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Concentration(moles/L) Absorbance
0.001 0.210.002 0.390.005 1.010.01 2.02
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.
Solution:Beer's Law Plot
y = 201.85x
0.00
0.50
1.00
1.50
2.00
2.50
0 0.002 0.004 0.006 0.008 0.01 0.012
Concentration
Ab
sorb
an
ce
Y = 201.85X = bC
(a) =201.85 cm–1M–1
(b) C= 0.0075 M
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iii) Applying Beer’s Law to Mixtures
The absorbance at a specific wavelength for a mixture of n
components, Am, is given as:
n
iii
n
iim bcAA
11
Two component mixture for example:
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iv) Limitations to Beer’s Law
Linear range
* Beer’s law is valid only at low concentrations.
Generally, < 0.01 M
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(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.
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(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+ + In-
color 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.
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(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.
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h e
2) Absorption Spectrai) Atomic Absorption (line spectra)
When a atom absorbs specific quantized UV/Vis radiation, it undergoes a change in its valence electron configuration:
* 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.
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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.
ElectronicGround state
ElectronicExcited
ElectronicExcited
{
{
{
Vibrationlevel
Vibrationlevel
Vibrationlevel
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Example of UV-Visible absorption spectra
Analyte:1,2,4.5-tetrazine
Gaseous phase
Nonpolar solvent
Polar solvent
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iii) Visible Spectrum and Complementary Colors
Wavelength of max (nm) Color Absorbed Color Remaining
380-420 Violet Green-yellow
420-440 Violet-blue Yellow
440-470 Blue Orange
470-500 Blue-green Red
500-520 Green Purple
520-550 Yellow-green Violet
550-580 Yellow Violet-blue
580-620 Orange Blue
620-680 Red Blue-green
680-780 Purple Green
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1) Emission Spectra
4. Emission of Electromagnetic Radiation
Emission spectrum of a brine sample with an oxyhydrogen flame
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2) Atomic FluorescenceRadiant 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.
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ii) More Illustration
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.
a) Life time
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.
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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.
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e) Fluorescence versus phosphorescence
(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.
S0 S1 T1
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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 = 2.303kΦfP0εbCC: analyt’s concentrationb: light path lengthε: molar absorptivityk: efficiency constant of collecting and detecting the emissionP0: excitation incident powerΦf : number of photons emitted/number of photons absorbed (quantum yield).