c – 1s 2 2s 2 2p 2 step 1:consider two valence p electrons 1 st 2p electron has n = 2, l = 1, m l...

C – 1sC – 1s222s2s222p2p22

Step 1:Consider two valence p electronsStep 1:Consider two valence p electrons11stst 2p electron has n = 2, l = 1, m 2p electron has n = 2, l = 1, m ll = 0, = 0, ±1, m±1, mss = ±½ → 6 possible sets of = ±½ → 6 possible sets of

quantum numbersquantum numbers22ndnd 2p electron has 5 possible sets of quantum numbers (Pauli Exclusion 2p electron has 5 possible sets of quantum numbers (Pauli Exclusion

Principle)Principle)For both electrons, (6x5)/2 = For both electrons, (6x5)/2 = 15 possible assignments since the electrons 15 possible assignments since the electrons

are indistinguishableare indistinguishable

Spectroscopic Description of Spectroscopic Description of All Possible Electronic States – Term SymbolsAll Possible Electronic States – Term Symbols

Step 2: Draw all possible Step 2: Draw all possible microstates. Calculate Mmicrostates. Calculate ML L and and

MMSS for each state. for each state.

C – 1sC – 1s222s2s222p2p22

Step 3: Count the number of microstates for each MStep 3: Count the number of microstates for each MLL—M—MSS possible possible

combinationcombination


Step 4: Extract smaller tables representing each possible termStep 4: Extract smaller tables representing each possible term

C – 1sC – 1s222s2s222p2p22

Step 5: Use Hund’s Rules to determine the relative energies of all Step 5: Use Hund’s Rules to determine the relative energies of all possible states.possible states.1. The highest multiplicity term within a configuration is of lowest 1. The highest multiplicity term within a configuration is of lowest energy.energy.2. For terms of the same multiplicity, the highest L value has the 2. For terms of the same multiplicity, the highest L value has the lowest energy (D < P < S).lowest energy (D < P < S).3. For subshells that are less than half-filled, the minimum J-value 3. For subshells that are less than half-filled, the minimum J-value state is of lower energy than higher J-value states.state is of lower energy than higher J-value states.4. For subshells that are more than half-filled, the state of maximum 4. For subshells that are more than half-filled, the state of maximum J-value is the lowest energyJ-value is the lowest energy..

Based on these rules, the ground electronic configuration for carbon has Based on these rules, the ground electronic configuration for carbon has the following energy order: the following energy order: 33PP00 < < 33PP11 < < 33PP22 < < 11DD22 < < 11SS00


Write term symbols in analogous manner except consider the Write term symbols in analogous manner except consider the orbital to which an electron is promoted.orbital to which an electron is promoted.

For example, excitation of Na promotes one valence electron For example, excitation of Na promotes one valence electron into the 3p orbital. In this case, n = 3, S = ½, 2S+1 = 2, L into the 3p orbital. In this case, n = 3, S = ½, 2S+1 = 2, L = 1 (P term), J = 3/2, 1/2.= 1 (P term), J = 3/2, 1/2.

There are two closely spaced levels in the excited term of There are two closely spaced levels in the excited term of sodium with term symbols sodium with term symbols 22PP1/21/2 and and 22PP3/23/2

Spectroscopic Description of Spectroscopic Description of Excited States – Term SymbolsExcited States – Term Symbols

This type of splitting (same L but This type of splitting (same L but different J) is called different J) is called fine structure.fine structure.

Transition from Transition from 22PP1/21/2 → → 22SS1/21/2

Allowed and Forbidden TransitionsAllowed and Forbidden Transitions

Only a fraction of all possible transitions are observed.Only a fraction of all possible transitions are observed.Allowed transitionsAllowed transitions

-high probability, high intensity, electric dipole -high probability, high intensity, electric dipole interactioninteractionForbidden transitionsForbidden transitions

-low probability, weak intensity, non-electric -low probability, weak intensity, non-electric dipole interactiondipole interaction

Selection rules for allowed transitions:Selection rules for allowed transitions:* The parity of the upper and lower level must be * The parity of the upper and lower level must be different. (The parity is even if different. (The parity is even if llii is even. The is even. The parity is parity is

odd if odd if llii is odd.) is odd.)

* * ll = ±1 = ±1* * JJ = 0 or ±1, but = 0 or ±1, but JJ = 0 to = 0 to JJ = 0 is forbidden. = 0 is forbidden.

Additional Splitting EffectsAdditional Splitting Effects

•Hyperfine splittingHyperfine splitting due to magnetic coupling of spin and orbital due to magnetic coupling of spin and orbital motion of electrons with the nuclear spin.motion of electrons with the nuclear spin.

•Isotope shift. Sufficient to determine isotope ratios.Isotope shift. Sufficient to determine isotope ratios.

•Splitting in an electric field (Stark effect): Relevant for arc and Splitting in an electric field (Stark effect): Relevant for arc and spark techniques.spark techniques.

•Splitting in a magnetic field (Zeeman effect):Splitting in a magnetic field (Zeeman effect):

* In absence of a magnetic field, states that differ * In absence of a magnetic field, states that differ only by their only by their MMJJ values are degenerate, i.e., they values are degenerate, i.e., they have have equivalent energies.equivalent energies.

* In presence of a magnetic field, this is not true * In presence of a magnetic field, this is not true anymore.anymore.

* Can be used for background correction.* Can be used for background correction.

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

Pretsch/Buhlmann/Affolter/Badertscher,Pretsch/Buhlmann/Affolter/Badertscher,Structure Determination of Structure Determination of Organic CompoundsOrganic Compounds

Stark SplittingStark Splitting

www.wikipedia.orgwww.wikipedia.org

For H:For H:split split E E

For others:For others:split split (E) (E)22

Zeeman SplittingZeeman Splitting

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

MMJJ – Resultant total – Resultant total

magnetic quantummagnetic quantumnumbernumber

MMJJ = J, J-1, …, -J = J, J-1, …, -J

2J +1 possible values2J +1 possible values

NormalNormal AnomalousAnomalous

Sample Introduction and AtomizationSample Introduction and Atomization

Atomization:Atomization:Convert solution Convert solution → vapor-phase free atoms→ vapor-phase free atoms

Measurements usually made in hot gas or enclosed furnace:Measurements usually made in hot gas or enclosed furnace:

•flamesflames•plasmasplasmas•electrical discharges (arcs, sparks)electrical discharges (arcs, sparks)•heated furnacesheated furnaces

Free Free AtomsAtoms

Free Free AtomsAtoms

IonsIonsIonsIonsMole-Mole-culescules

Mole-Mole-culescules

NebulizationNebulization

DesolvationDesolvation

VolitalizationVolitalization

Adapted from Ingle and CrouchAdapted from Ingle and Crouch

Atomic Emission Spectroscopy (AES)Atomic Emission Spectroscopy (AES)

See also: Fundamental reviews in See also: Fundamental reviews in Analytical ChemistryAnalytical Chemistry e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal. Anal. Chem. Chem. 20022002, , 7474, 2691-2712 (“Atomic Spectroscopy”), 2691-2712 (“Atomic Spectroscopy”)

•Beginning 19th century: alcohol flame (Brewster, Herschel, Talbot, Beginning 19th century: alcohol flame (Brewster, Herschel, Talbot, Foucault)Foucault)

•mid 1800s: Discovery of Cs, Tl, In, Ga by atomic spectroscopy mid 1800s: Discovery of Cs, Tl, In, Ga by atomic spectroscopy (Bunsen, Kirchhoff)(Bunsen, Kirchhoff)

•1877: Gouy designs pneumatic nebulizer1877: Gouy designs pneumatic nebulizer

•1920s: Arcs and sparks used for AES1920s: Arcs and sparks used for AES

•1930s: First commercial AES spectrometer (Siemens-Zeiss)1930s: First commercial AES spectrometer (Siemens-Zeiss)

•1960s: Plasma sources (commercial in 1970s)1960s: Plasma sources (commercial in 1970s)

Atomic Emission Spectroscopy (AES)Atomic Emission Spectroscopy (AES)2S1/2

22DD3/2, 5/23/2, 5/222PP3/23/2

22PP1/21/222SS1/21/2

At RT, nearly allAt RT, nearly allelectrons in 3selectrons in 3sorbitalorbital

Excite with flame, Excite with flame, electric arc, or electric arc, or sparkspark

Common electronicCommon electronictransitionstransitions

http://raptor.physics.wisc.edu/data/e_sodium.gifhttp://raptor.physics.wisc.edu/data/e_sodium.gif

An Ideal AES SourceAn Ideal AES Source

1. complete atomization of all elements2. controllable excitation energy3. sufficient excitation energy to excite all elements4. inert chemical environment5. no background6. accepts solutions, gases, or solids7. tolerant to various solution conditions and solvents8. simultaneous multi-element analysis9. reproducible atomization and excitation conditions10. accurate and precise analytical results11. inexpensive to maintain12. ease of operation

Flame AESFlame AES

•Background signals due to flame fuel and oxidants – line spectra:Background signals due to flame fuel and oxidants – line spectra:

•OHOH•• 281.1, 306.4, 342.8 nm 281.1, 306.4, 342.8 nm from O + Hfrom O + H22 H + OH H + OH

H + OH + O22 O + OH O + OH

•OO22 250, 400 nm250, 400 nm

•CHCH 431.5, 390.0, 314.3 nm431.5, 390.0, 314.3 nm•COCO bands between 205 to 245 nmbands between 205 to 245 nm•CN, CCN, C22, CH, NH bands between 300 to 700 nm, CH, NH bands between 300 to 700 nm

Unlike bands of atomic origin, these molecular bands are fairly broad.Unlike bands of atomic origin, these molecular bands are fairly broad.

•Continuum emission from recombination reactionsContinuum emission from recombination reactions e.g. H + OH e.g. H + OH H H22O + hO + h CO + O CO + O CO CO22 + h + h

Flames used in AES nowadays only for few elements. Cheap Flames used in AES nowadays only for few elements. Cheap but but limited. {Flame AES often replaced by flame AAS.}limited. {Flame AES often replaced by flame AAS.}

Inductively Coupled Plasma AESInductively Coupled Plasma AES•Spectral interference more likely for plasma than for flame due to larger Spectral interference more likely for plasma than for flame due to larger population of energetically higher states.population of energetically higher states.

•Modern ICP power: 1–5 kW (4 to 50 MHz)Modern ICP power: 1–5 kW (4 to 50 MHz)

•4000 to 10,000 K: Very few molecules4000 to 10,000 K: Very few molecules

•Long residence time (2–3 ms)Long residence time (2–3 ms) results in high desolvationresults in high desolvation and volatilization rateand volatilization rate

•High electron density suppresses High electron density suppresses ionization interference effectsionization interference effects

•Background: Ar atomic lines and,Background: Ar atomic lines and, in hottest plasma region, in hottest plasma region, Bremsstrahlung (continuum radiationBremsstrahlung (continuum radiation from slowing of charged particles) from slowing of charged particles)

•Price > $ 50 kPrice > $ 50 k

•Operating cost relatively high dueOperating cost relatively high due to Ar cost (10–15 mL/min) andto Ar cost (10–15 mL/min) and training.training. www.wikipedia.org,www.wikipedia.org, Ingle and CrouchIngle and Crouch

Microwave Plasma AESMicrowave Plasma AES•Power 25 to 1000 W (ICP 1000–2000 W)Power 25 to 1000 W (ICP 1000–2000 W)

•Frequency 2450 MHz (ICP 4 to 50 MHz)Frequency 2450 MHz (ICP 4 to 50 MHz)

•Argon, helium or nitrogenArgon, helium or nitrogen

•Thermodynamic equilibrium typically not reached (temperature Thermodynamic equilibrium typically not reached (temperature estimated to be around 2000 to 3000 K)estimated to be around 2000 to 3000 K)

•Low temperature causes problems with liquidsLow temperature causes problems with liquids

•Useful for gases: GC–microwave plasma AESUseful for gases: GC–microwave plasma AES

AES: Figures of MeritAES: Figures of Merit•Linearity over 4 to 5 Linearity over 4 to 5 concentration decadesconcentration decades

Reasons for deviationsReasons for deviations from linearity:from linearity:

- Self-absorption- Self-absorption

- Extent of ionization- Extent of ionization affected by sampleaffected by sample- Flow rate- Flow rate- Atomization efficiency- Atomization efficiency

Ingle and CrouchIngle and Crouch

AES: Figures of MeritAES: Figures of Merit•Linearity over 4 to 5 concentration decadesLinearity over 4 to 5 concentration decades

•Precision: Typically a few % (lower in calibration solutions)Precision: Typically a few % (lower in calibration solutions)

Limited by stability of source and random electrical noiseLimited by stability of source and random electrical noise

•Accuracy: An optimized spectrometer should be capable of Accuracy: An optimized spectrometer should be capable of precision-limited accuracyprecision-limited accuracy

Limited in ICP AES by spectral overlapLimited in ICP AES by spectral overlap

•Applicability: 3/4 of all elements (ICP)Applicability: 3/4 of all elements (ICP)

•Limitations in detection limits:Limitations in detection limits: * Major transitions in UV* Major transitions in UV* Temperature too high * Temperature too high

for alkali for alkali metals (ion emission metals (ion emission in in UV as they UV as they have fully occupied have fully occupied electron shells) electron shells)

Detection Limits for Flame AESDetection Limits for Flame AES


Detection Limits for ICP AESDetection Limits for ICP AES


AES: Instrumental AspectsAES: Instrumental Aspects

Ingle and Crouch,Ingle and Crouch,Spectrochemical AnalysisSpectrochemical Analysis

c – 1s 2 2s 2 2p 2 step 1:consider two valence p electrons 1 st 2p electron has n = 2, l = 1, m l...

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