lecture15 clh class
TRANSCRIPT
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Allowed and Forbidden Transitions
Only a fraction of all possible transitions are observed.
Allowed transitions-high probability, high intensity, electric dipole
interaction
Forbidden transitions
-low probability, weak intensity, non-electric dipoleinteraction
Selection rules for allowed transitions:
* The parity of the upper and lower level must be
different. (The parity is even ifSli is even. The parityis odd ifS li is odd.)* D l= 1* D J= 0 or 1, but J= 0 to J= 0 is forbidden.
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Additional Splitting Effects
Hyperfine splitting due to magnetic coupling of spin and orbital
motion of electrons with the nuclear spin.
Isotope shift. Sufficient to determine isotope ratios.
Splitting in an electric field (Stark effect): Relevant for arc and
spark techniques.Splitting in a magnetic field (Zeeman effect):
* In absence of a magnetic field, states that differonly by theirMJ values are degenerate, i.e., they
have equivalent energies.
* In presence of a magnetic field, this is not trueanymore.
* Can be used for background correction.
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Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds
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Pretsch/Buhlmann/Affolter/Badertscher,Structure Determination of
Organic Compounds
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Stark Splitting
www.wikipedia.org
For H:
split a E
For others:split a (E)2
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Zeeman Splitting
Ingle and Crouch,Spectrochemical Analysis
MJ Resultant totalmagnetic quantumnumber
MJ = J, J-1, , -J2J +1 possible values
Normal Anomalous
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Sample Introduction and Atomization
Atomization:
Convert solution vapor-phase free atoms
Measurements usually made in hot gas or enclosed furnace:
flames
plasmaselectrical discharges (arcs, sparks)heated furnaces
Free
Atoms
IonsMole-
cules
Nebulization
Desolvation
Volitalization
Adapted from Ingle and Crouch
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Atomic Emission Spectroscopy (AES)
See also: Fundamental reviews inAnalytical Chemistrye.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C.Anal.
Chem.2002, 74, 2691-2712 (Atomic Spectroscopy)
Beginning 19th century: alcohol flame (Brewster, Herschel, Talbot,Foucault)
mid 1800s: Discovery of Cs, Tl, In, Ga by atomic spectroscopy(Bunsen, Kirchhoff)
1877: Gouy designs pneumatic nebulizer
1920s: Arcs and sparks used for AES
1930s: First commercial AES spectrometer (Siemens-Zeiss)
1960s: Plasma sources (commercial in 1970s)
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Atomic Emission Spectroscopy (AES)2S1/2
2D3/2, 5/22P3/2
2P1/22S1/2
At RT, nearly all
electrons in 3s
orbital
Excite with flame,
electric arc, or
spark
Common electronictransitions
http://raptor.physics.wisc.edu/data/e_sodium.gif
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Example AE Spectra
http://www.technology.niagarac.on.ca/lasers/Chapter2.html
H2
Hg
He
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An Ideal AES Source
1. complete atomization of all elements2. controllable excitation energy3. sufficient excitation energy to excite all elements4. inert chemical environment5. no background
6. accepts solutions, gases, or solids7. tolerant to various solution conditions and solvents8. simultaneous multi-element analysis9. reproducible atomization and excitation conditions
10. accurate and precise analytical results11. inexpensive to maintain12. ease of operation
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Flame AES
Background signals due to flame fuel and oxidants line spectra:
OH 281.1, 306.4, 342.8 nmfrom O + H2 H + OH
H + O2 O + OH
O2250, 400 nm
CH
431.5, 390.0, 314.3 nmCObands between 205 to 245 nm
CN, C2, CH, NH bands between 300 to 700 nm
Unlike bands of atomic origin, these molecular bands are fairly broad.
Continuum emission from recombination reactionse.g. H + OH H2O + h CO + O CO2 + h
Flames used in AES nowadays only for few elements. Cheap butlimited. {Flame AES often replaced by flame AAS.}
Ingle and Crouch
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Inductively Coupled Plasma AESSpectral interference more likely for plasma than for flame due to largerpopulation of energetically higher states.
Modern ICP power: 15 kW (4 to 50 MHz)
4000 to 10,000 K: Very few molecules
Long residence time (23 ms)results in high desolvation
and volatilization rate
High electron density suppressesionization interference effects
Background: Ar atomic lines and,in hottest plasma region,Bremsstrahlung (continuum radiationfrom slowing of charged particles)
Price > $ 50 k
Operating cost relatively high due
to Ar cost (1015 mL/min) andtraining.www.wikipedia.org,Ingle and Crouch
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Microwave Plasma AES
Power 25 to 1000 W (ICP 10002000 W)
Frequency 2450 MHz (ICP 4 to 50 MHz)
Argon, helium or nitrogen
Temperature estimated to be 2000 - 3000 K
Low temperature causes problems with liquids
Useful for gases: GCmicrowave plasma AES
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Arcs and Sparks
Arc =An electrical discharge between two or moreconducting electrodes (1-30 A)Spark =An intermittent high-voltage discharge (fewmsec)
Limited to qualitative and semi-quantitative use (arcflicker)Particularly useful for solid samples (pressed intoelectrode)The burn takes > 20 sec (need multichannel detector)
Ingle and Crouch
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AES: Figures of Merit
Linearity over 4 to 5concentration decades
Reasons for deviationsfrom linearity:
- Self-absorption
- Extent of ionizationaffected by sample
- Flow rate- Atomization efficiency
Ingle and Crouch
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AES: Figures of MeritLinearity over 4 to 5 concentration decades
Precision: Typically a few % (lower in calibration solutions)
Limited by stability of source and random electrical noise
Accuracy: An optimized spectrometer should be capable ofprecision-limited accuracy
Limited in ICP AES by spectral overlap
Applicability: 3/4 of all elements (ICP)
Limitations in detection limits: * Major transitions in UV* Temperature too high for alkali
metals (ion emission in UV as they havefully occupied electron shells)
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Detection Limits for Flame AES
Ingle and Crouch,Spectrochemical Analysis
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Detection Limits for ICP AES
Ingle and Crouch,Spectrochemical Analysis
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AES: Instrumental Aspects
Ingle and Crouch,
Spectrochemical Analysis
1.
2.
3.
4.
5.
6.
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Atomic Absorption Spectroscopy (AAS)
See also: Fundamental reviews inAnalytical Chemistry
e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C.Anal.Chem.2002, 74, 2691-2712 (Atomic Spectroscopy)
1802 Wollaston observes absorption lines in solar spectrum
1914 Hollow cathode lamp
1955 Walsh describes analytical AAS
1959 1st Commercial Flame AAS
1960s Lvov and Massman describe graphite furnace
(commercial in 1970s)
Recall: A = -log10(T)
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Hollow Cathode Lamp
Kellner et al.,
Analytical Chemistry
Typical primary source of radiation: Hollow cathode lamp* Typically one lamp per element
* Different intensities for different elements
* Multielement lamps for multielement analysis
Continuum sources (e.g. Xe arc lamp) only for multielement analysis
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Flame AAS
Ingle and Crouch, Spectrochemical Analysis
Kellner et al.,
Analytical Chemistry
At
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Ingle and Crouch, Spectrochemical Analysis
Electrothermal Atomization
Heating current of
several hundred A
Heating rates of up to1000 C/s
LOD 100 times lowerthan flame AAS
Heated in three stages:
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Typical furnace material: Graphite Graphite Furnace AAS*Graphite tube 18-28 mm
*Samples 5-100 uL
*200 to 1000 cycles
*Temperature up to 3000 C to avoid graphite decomposition
*Carbon may be reducing agent for metal ions
*Argon flow avoids oxidation
Other furnace materials: Ta, W, Pt
*High melting point required
*Should not emit brightly at high temperature (disadvantage for W
and Ta)
Electrothermal Atomization
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Are you getting the concept?
Is ICP a good source for AAS?