chapter 6. an introduction to spectrometric...
TRANSCRIPT
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Chapter 6. An Introduction to Spectrometric Methods
Spectroscopy: the science that deals with “interactions of matter
with electromagnetic radiation or other forms of energy”
Spectrometry: a more restrictive term,
- any procedure that uses light to measure chemical concentrations.
- the quantitative measurement of the intensity of electromagnetic
radiation at one or more wavelengths with photoelectric detector.
acoustic waves, beams of particles such as ions and electrons
(SIMS) (AES)
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Absorption of light: increases the energy of molecule
Emission of light: decreases the energy of molecule
ground state
excited state
absorption emission
Ground state: lowest energy state of a molecule
M + h• υ M*
(life time: 10-6 ~10-9 S)
M* M + light (fluorescence, phosphorescence)
or M* M + heat
Excitation
Relaxation
What Happens When a Molecule Absorbs Light ?
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Absorption
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Absorption of Radiation
Absorption
(10-14~10-15 s)
Vibration
(10-15 s)
Excited state
(life-time:10-8 s)
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Absorption of Radiation
Collisional broadening
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Emission or Chemiluminescence
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Emission of Radiation
Line spectra:
from atoms
Band spectra:
from molecules
For molecules:
E = Eelectronic + Evibrational + Erotational
For atoms:
E = Eelectronic
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Emission of Radiation
<Sodium atom line spectra> <Molecule band spectra>
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Photomluminescence
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T1: triplet excited state
S0: singlet ground state
S1: singlet excited state
IC: raditionless transition between states with the same quantum state (S1 S0)
ISC: raditionless transition between states with different quantum state (S1 T1)
Life time: 10-8-10-4 s Life time: 10-4-102 s
Photoluminescence
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Components of Optical Spectroscopy
Absorption
Photoluminescence
Scattering
Emission
Chemiluminescence No external light source
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When light is absorbed by a sample
the radiant power of the beam of light is decreased
Radiant power (P): the energy per second per unit area of the light beam
Transmittance (T): T = P/Po (T = 0 ~ 1)
Absorbance (A), or optical density: A = log (Po/P) = -log T
(if 90% light is absorbed, 10% transmitted: T = 0.1P/P = 0.1, A= - log T=1)
Absorption spectrum: absorbance vs wavelength
Absorption of Radiation in Analytical Chemistry
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The part of molecule responsible for light absorption: chromophore
Absorbance is directly proportional to the concentration
Beer-Lambert law: A = εbc
ε : molar absorptivity (extinction coefficient)
characteristic of a substance that tells how much light is absorbed
at a particular wavelength
b: path length
c: concentration
Beer’s law works for monochromatic radiation passing through a dilute solution
Absorption of Radiation: Beer’s Law
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Relation of emission intensity to concentration:
I = kPoC
I: emission intensity
Po: radiant power of incident light
C: concentration of emitting species
In FL: Higher radiation power higher intensity better detection
In Absorbance: Higher radiation power no change in absorbance
(Laser-induced fluorescence; LIF) good for the detection of trace amount
Emission intensity is not proportional to analyte concentration
at high concentration, or in the presence of significant amount of absorbing species
Self-absorption
Luminescence in Analytical Chemistry
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Chapter 7. Components of Optical Instruments
Optical spectroscopic methods are based upon six phenomena
- Absorption
- Fluorescence
- Phosphorescence
- Scattering
- Emission
- Chemiluminescence
While the instruments for measuring each differ in configuration,
most of their basic components are remarkably similar.
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Sample Cell & Prism
Components of Optical Spectroscopy
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Radiation Source
- should generate a beam of sufficient power (specially for FL)- output power should be stable
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Light Source
Light Detector
Electric discharge (spark) lamp filled with Hg vapor or Xe gas : UV and visible range & intense
Silicon carbide rod called a globarheated to near 1500 K
Components of Optical Spectroscopy
most common
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Continuum Source
Tungsten lamp (operate at 3000K) : visible and near IR
: most common
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Light Amplification by Stimulated Emission of Radiation
Laser:
- extremely intense (high intensity)- highly monochromatic (narrow bandwidth of 0.01 nm or less)- remarkably coherent
The first laser: in 1960
Application of laser:
- Spectroscopy- Detection of extremely low concentrations of species (LIF)
Laser
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Laser light is obtained by three step:- stimulated emission- population inversion- optical resonator or oscillator concept
Laser
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- A species in an excited electronic state may lose all or part of its excess energy of radiation by spontaneous emission. - It is also important to note that the instant at which emission occurs and the path of the resulting photon vary from excited molecule to excited molecule because that is a random process.- yields incoherent monochromatic radiation.
Rate of spontaneous emission = A21N2
Spontaneous Emission
A21
(slow)
(incoherent)different phase,
different direction
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The absorption process, which compete with stimulated emission.Absorption rate is depended on: - No of particles in state Ex, Ey- intensity of radiation, I(ν)- inherent probability of the transition (B12)—absorption coefficient
Rate of absorption = B12 I(ν) N1
(Stimulated) Absorption
B12
(fast)
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- The excited laser species are struck by photons that have precisely the same energies as the photons produced by spontaneous emission-Collision of this type cause the excited species to relax immediately to lower energy level and stimulate emission
- The emitted photon travels in exactly same direction and precisely in phase.- The stimulated emission is totally coherent with the incoming radiation
Rate of stimulated emission = B21 I(ν) N2
Stimulated Emission
B21
(coherent)same phase,
same direction
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In order to have light amplification, it is necessary that the # of photons produced by stimulated emission exceed the # lost by absorption.
Population Inversion - Pumping
N1>N2Non-inverted
population
N1<N2inverted
population
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Three-Level Laser Systems
B32>A31B32N3 > A21N2 : population inversion
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Four-Level Laser Systems
fast
fast
Fore-level system is much more efficient: small expenditure of pumping energy
The # in E2 state is generally negligible relative to # in E1 & E3 state
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-The active materials in dye lasers are solutions of organic compounds capable of fluorescing in the UV, Vis or IR (Four level system)- tunable over rage 20 to 50 nm, bandwidth < a few hundredth of nanometer
Dye Laser
Grating or Littrow prismRhodamine 6G
in organic solvent flowing
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Semiconductor Diode (Light Emitting Diode)
Eg: band gap energy between valence band and conduction band
small band gapthermal energy v = Eg/h
(1) A voltage is applied across a semiconductor diode in a forward direction (excitation).
(2) Some of the electrons relax and emit radiation.(3) Light emitting diode (LED): GaAs doped with P (660 nm)(4) Limited utility in spectroscopy: low intensity & limited λ.
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Wavelength Selector
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Components of Optical Spectroscopy
Wavelength Selector
Filters: Interference F (UV-Vis, IRAbsorption F (Vis)
Monochromators:Prism MGrating MHolographic MConcave M
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<Narrow bandwidth>- Enhance sensitivity in absorbance measurements- Enhance selectivity in absorption and emission methods
Ideal wavelength selector: provides an output of a single wavelength
An inverse measure of the quality of the wavelength selector
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Interference Filters (Fabry-Perot filters)
nλ’ = 2a (경로차)constructive interference
λ= 2 t η/n
λ: wavelength of transmitted radiationt: thickness of dielectric materialη: refractive index of dielectric material
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Interference Filters (Fabry-Perot filters)
- Available throughout UV, Vis, and IR region.- Effective bandwidths are about 1.5% of the wavelength at peak transmittanc
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Absorption Filters
- Absorbs certain portion of spectrum- Consists of colored glass or of a dye suspended in gelatin - Restrictedly used in the visible region- Wide bandwidth (30-250 nm) and low transmittance (~10%)
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Photometer
Single-beam
Double-beam
Simple, relatively inexpensive tools for performing absorption analyses
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Monochromators
1) Entrance slit that provides a narrow optical image
2) Collimator (mirror) that renders the rays spreading from the slit parallel
3) A component for dispersing this radiation
4) A focusing element to reform images of the slit
5) Exit slit to isolate the desired spectral band
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Prism Monochromator
Different λ→ different refractive index (n2) → (n1sin θ1 = n2sin θ2), (n1, θ1 the same) different refractive angle (θ2) different focal position
30o
λ1 λ2 θ1
λ1
θ2
Cornu prism λ2
λ2 < λ1
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Prism Monochromator
Transmittance prism: CornuReflection prism: Littrow
Reflection occurs
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Grating : Echellette-Type
Beam 1 and Beam 2의 경로차: CB + BD
Constructive interference:
n λ = CB + BD = d sin i + d sin r = d (sin i + sin r)i: incidence angle, r: reflection (diffraction) angle, n: diffraction order,d: groove distance
For constructive interferenced, i ; the same for all λdifferent λ different r
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Example:
1 mm당 1450개의홈을가지고있는 echellette 회절발에법선에대하여 48°의입사각으로다색
광을비추었다. 반사각 +20, +10, 0도에서나타나는복사선의파장을계산하여라.
d = (1mm/1450개의홈 ) * 106nm/mm = 689.7 nm/홈
r = 20 ° λ = (698.7 / n ) (sin 48 + sin 20) = 748.4 / n
r, 각도 n = 1 n = 2 n = 3
0102030405060708090
5136327488589561040
1161
1202
256316374429478520
580
601
171211249286318346
387
400
most intense high order lines are removed by filters
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Grating : Transmission
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Resolution in Monochromator
The limit of its ability to separate closely spaced peaks that have a slight difference in wavelength.
R = = nN
λ: average wavelength of the two peaksΔλ: differencen: diffraction orderN: number of grooves of the grating illuminated by radiation from
entrance slit
Better resolution:
- higher N- higher diffraction order
Δλ
λ
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Grating : Echelle-Type
- The blaze angle of an echelle grating is significantly higher than the echellette- The short side of the blaze is used rather than the long- The grating is relatively coarse: ~ 300/mm (1200-1400/mm, conventional type)- The angle of reflection r is much higher in echelle than in echellette.- r = i = β, n λ= 2 d sin β
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Echelle Monochromator
Used in atomic emission spectroscopy
(low dispersion)or grating
Two dispersion elements arranged in series: higher dispersion and resolutionthan an echellette of the samesize
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Echelle Monochromator in AES
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Polychromator
In polychromator, diffraction grating is locked in place, the exit slit is removed,and a multi-channel detector is permanently installed along the focal plane.
High speed and wavelength accuracy
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Effect of Slit WidthA narrow slit will optimize resolution, while wide one will optimize energy
Better resolutionLower power
Qualitative analysis: narrow slit width, Quantitative analysis: wide slit width
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Effect of Slit Width
The much greater spectral detailcan be realized with the narrowestslit setting
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Detector
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Components of Optical Spectroscopy
Light Detector
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Spectral Response of Light Detector
PbS photoconductivity detector
thermocouple
PMT
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- Good for the measurement of low radiation
- Photo-emissive surface (photocathode) + several dynodes
- Photocurrent is amplified: Gain of 106 – 109
- Very stable power supply is required
- Highly sensitive to UV-VIS range
- Extremely fast response time
- Sensitivity is limited by dark current electrons
(thermal emission is major source)
: cooling is good for the enhancement of PMT performance - 30 oC: no dark current
- PMT is limited to measuring low-power radiation (intense light causes irreversible damage to the photoelectric surfaces)
Photomultiplier Tube (PMT)
Current-to-voltage converter
dynode (+90 V)
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Absorption of radiation by semiconductor produces electrons and holes, thus leading to enhances conductivity
The most sensitive transducer for monitoring radiation in the near IR region (0.75 ~ 3 um)
Lead sulfide is the most widely used.
Is important in FT-IR
Photoconductive Detector
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Photodiode Array (PDA)- The individual photodiodes are a reverse-biased pn junction
- The number of transducer elements in a chip ranges from 64 to 4096 (mostly 1024)
Multi-channel Photon Detector: PDA
Reverse biased P-N junction
Radiation holes and electrons in the depletion region i d t
The Advantages of PDA1. Various elements of the dispersed spectrum
are measured simultaneously2. Fast measurement
The Shortcomings of PDA1. Low sensitivity2. Short dynamic range
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Photodiode Array UV-visible Spectrophotometer
PDA is placed in the focal planeof a spectrometer so that various elements of the dispersed spectrum can be transduced and measured simultaneously.
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(2) Charge Coupled Device (CCD)- 512 X 320 pixels
- Greater sensitivity to lower light level
Multichannel Photon Detector: CCD
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Multi-channel Photon Detector: CCD
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I. Thermocouple (or thermopile)Between the two junctions [copper and constantan (Cu + W)] a
potential develops that varies with the difference in temperature of the junctions.
II. Bolometer (or thermistor)
NiO, Pt or semiconductor: high change in resistance as temperature change
Thermal Transducers
- Used in IR (IR photon energy: not enough to produce photoelectron)
- IR: use photoconductivity detector or thermal transducer
- Radiation is absorbed by a small blackbody temperature raise
III. Pyroelectric Transducer
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Photon Counting
PMT
Discriminator
Counter
set threshold rangethreshold
The pulses whose heights exceed an appropriate threshold are counted: excludes most dark current
Advantages: - improved S/N ratio and sensitivity to lower radiation levels- less sensitive to PMT voltage and temperature fluctuations
Disadvantages: - equipment is complex and expensive
Application: fluorescence, chemiluminescence, Raman
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