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Introduction to Optical Atomic spectrometry

Introduction to Optical Atomic spectrometry. 2004. 4 Yongsik Lee. introduction. Three methods for atomic spectrometry Optical spectroscopy Mass spectrometry X-ray spectroscopy. Preparation of atomic samples. In optical methods Sample is atomized gas phase atoms or ions

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Introduction to Optical Atomic spectrometry

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  1. Introduction to Optical Atomic spectrometry 2004. 4 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 http://radio.weblogs.com/0101365/categories/periodic/2002/11/01.html

  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 = 96. 484 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? • 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 • 100-1000 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) http://www.chem.tamu.edu/rgroup/bevan/activity.html

  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 • More sample atoms than Emission • Absorption • 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 • Sample in aq solution • Figure 8-9 • Bernoulli aspiration • Supersonic nebulization • 20 kHz – MHz peizoelectric surface • Electrothermal vaporizer (ETV) • Non-continuous signal • Hydride generation

  31. Hydride generation • For As, Sb, Sn, Se, Bi, Pb containing samples to vaporize • Detection limit 10-100 times increase • Some of them are toxic! • Generation method • Prepare acidic sample solution • Add to 1% NaBH4 solution • Metal hydride (g) generated

  32. Solid sample • Direct insertion • 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 250-1000 VDC) • Samples on cathode (which contains the sample)

  33. Homework • 8-8, 9, 10

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