1. Introduction to Optical Atomic spectrometry
Yongsik Lee

2. introduction Three methods for atomic spectrometry
Optical spectroscopy
Mass spectrometry
X-ray spectroscopy

3. Preparation of atomic samples
In optical methods
Sample is atomized
gas phase atoms or ions
Emission/absorption/luminescence
In mass spectrometry
Cation and m/e measured
X-ray
Atomic intrinsic data
Direct measurement

4. 8A Optical Atomic Spectra
Energy level diagram
Atomic Line width
Thermal effect
Band/continuous spectrum

5. Atomic Energy levels Energy levels
Every elements has unique set of atomic orbital levels
Hydrogen atom has a simple energy levels
One proton + one electron (two body problem)
Other elements have complex atomic orbital (p,d,f...) levels
Levels split by spin-orbit (SO) coupling
Energy level diagram
A picture of relative energy levels of an atom

6. Energy levels and atomic spectra

7. 복잡한 “커플링”은 에너지 준위를 degenerate

8. Spin-orbit coupling
Spin (s) and orbital (l) motion create magnetic fields that perturb each other
if fields parallel - slightly higher energy
if fields anti-parallel - slightly lower energy
Define Spin-Orbit coupling by J
total angular momentum
J=L+S (L = Sl , S = Ss)
positive values only

9. Vector sum of angular momenta
P.W. Atkins,
Physical Chemistry
Atomic structure and atomic spectra
s electron (l=0, s=+1/2 or -1/2)
J=0+1/2=1/2
p electron (l=1, s=+1/2 or -1/2)
J= 1+1/2 = 3/2 (higher energy) or
J= 1-1/2 = 1/2 (lower energy)

10. Electronic Term Symbol
2S+1LJ
L written as letter (S, P, D...) instead of number!
Li =1s2 2s1 L = 0, S = ±1/2 2S1/2
Li* =1s2 2p1 L = 1, S = ±1/2 2P1/2, 3/2
Be =1s2 2s2 L = 0, S = 0 1S0
Be* =1s2 2s12p1 L =1, S = 1, 0 3P2 , 1P0

11. Sodium D lines

12. Energy Level Diagram with SO coupling

13. Allowed/forbidden transition
Similar pattern between atoms but different spacing
Spectrum of ion different to atom
Separations measured in electron volts (eV)
Figure 8-2
Electron volt
Energy unit
1 eV =1.602x10-19 C X 1V (J/C) = 1.602x10-19 J
1 eV = kJ/mol
As # of electrons increases
# of levels increases
Emission spectra become more complex
Using selection rule (Quantum mechanics) – allowed/forbidden
Li 30 lines, Cs 645 lines, Cr 2277 lines

14. Review of atomic spectrometry
Three methods for atomic spectrometry
Optical spectroscopy
Mass spectrometry
X-ray spectroscopy

15. Three Types of Atomic optical spectroscopy
Atomic emission
Arc or spark (heating)
H2-O2 flame
Atomic absorption
Resonance absorption lines
Atomic fluorescence
Resonance fluorescence
Non-resonance fluorescence

16. Atomic line widths Desire narrow lines for accurate identification
Broadened by 4 mechanism
(i) uncertainty principle
Natural line width (about 1/10000 Å)
(ii) Doppler effect (about 1/100 Å)
(iii) pressure broadening (about 1/100-1/10 Å)
(iv) electric and magnetic fields
Zeeman effect
Chapter 9C-1

17. Uncertainty Principle
Why?
Quantum mechanical idea states must measure for some minimum time to tell two frequencies apart
How?
Shows up in lifetime of excited state
if lifetime infinitely long, DE infinitely narrow
if lifetime short, DE is broadened

18. Calculation of uncertainty broadening

19. Pressure broadening Why? For low pressure hollow cathode lamps
Collisions with atoms/molecules transfers small quantities of vibrational energy (heat)
ill-defined ground state energy
Effect worse at high pressures
For low pressure hollow cathode lamps
1-10 torr
Dl = 0.1 – 0.01 Å
For high pressure Xe lamps
>10,000 torr
Å (turnslines into continua!)

20. Doppler broadening
Change in frequency produced by motion relative to detector
In a flame, Doppler broadening is 100 times of natural broadening
Red shift is observed by Edwin Hubble (1920)
Big bang theory

21. Compression/expansion

22. Doppler broadening of Gas
In gas, broadened line is symmetric shape
Average speed of gas atoms
Doppler effect to the detector
Maxwell-Boltzmann Distribution of speed
Super sonic jet nozzle (molecular beam)
Doppler broadening increases with (T)1/2

23. Sub-millimeter supersonic jet Spectrometer at Texas A&M
(Instrumental line width <2 kHz, 6.8 x 10-8 cm-1)

24. Atomic linewidths Broadened by 4 mechanism (i) uncertainty principle
Natural line width (about 1/10000 Å)
(ii) Doppler effect
Flames - about 1/100 Å
(iii) pressure broadening
about 1/100-1/10 Å
(iv) electric and magnetic fields
Zeeman effect
Chapter 9C-1

25. Other Effects of Temperature on Atomic Spectrometry
Boltzmann equation
Important in emission measurements
relying on thermal excitation
Na atoms at 2500 K, only 0.02 % atoms in first excited state!
Less important in absorption measurements
99.98 % atoms in ground state!

26. Best method? ABS/FL Absorption FL More sample atoms than Emission
Difference measurement (log Po – log P)
Big Background and big signal
Cancels out the advantage FL
For the same number of atoms, FL is best

27. 8B Methods for atomizing
Sample must be converted to atoms first

28. 8C Sample Introduction method
transfer sample to atomizer
Achilles heel of atomic spectroscopy
easy for gases /solutions
but difficult for solids

29. Sample phase and introduction methods
Solution
Or slurry (suspension)
Solid
Solid or fine powder
Less producible
More error

30. Solution phase pnuematic nebulization Supersonic nebulization
Sample in aq solution
Figure 8-9
Bernoulli aspiration
Supersonic nebulization
20 kHz – MHz peizoelectric surface
Electrothermal vaporizer (ETV)
Non-continuous signal
Hydride generation

32. Hydride generation
For As, Sb, Sn, Se, Bi, Pb containing samples to vaporize
Detection limit times increase
Some of them are toxic!
Generation method
Prepare acidic sample solution
Add to 1% NaBH4 solution
Metal hydride (g) generated

33. Solid sample Direct insertion Electothermal vaporization
Powder on the probe
Metal samples as an electrode
Electothermal vaporization
Spark and arc ablation
Vaporized samples moved by inert gas
Conductive (or mixture of) samples
Laser ablation
Various kinds of sample
Glow discharge (GD) sputtering
Figure 8-10
Introduction and atomization at the same time
Ar+ ion collision (1-10 torr Ar used for VDC)
Samples on cathode (which contains the sample)

34. Homework
8-8, 9, 10