lecture15 clh class

7/29/2019 Lecture15 Clh Class

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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



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Detection Limits for ICP AES



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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?

lecture15 clh class

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