TMHsiung@2014 1/40 Chapter 24 Introduction to Spectrochemical Methods.

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Chapter 24 Introduction to

Spectrochemical Methods

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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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1) Electromagnetic Spectrum

2. Interaction of Radiation and Matter

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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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3) Spectroscopies without Energy Exchange

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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) Absorption process

(3) Energy transition process illustrate

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ii) Emission or Chemoluminescence process

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iii) Photoluminescence process

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i) Transmittance and Absorbance

3. Absorption of Radiation1) The Absorption Process

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Continued

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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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(cont’d)

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(cont’d)

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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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i) Energy Level Diagram3) Molecular Fluorescence

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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).

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Homework (Due 2014/4/10)

Skoog 9th edition, Chapter 24 Questions and Problems24-524-6 (a) (b)24-924-23

End of Chapter 24