BCHM 3100 Chapter 8 _02032020.pptx!!!111

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1. Chapter 8 Optical Atomic Spectrometry

2. Interactions of Radiation Matter Chemiluminescence Spectroscopy Absorption Methods Photon Emission Nonradioactive excitation Photoiuminescence Methods Fluorescence and phosphorescence Raman Spectroscopy (Virtual state) 1 & 0  “vibrational states” Incident radiation of frequency (v ex ) v ex v ex v ex v ex V v = vibrational frequency Luminescence

3. Optical Atomic Spectroscopy Optical Spectrometry Absorption Emission Fluorescence Mass Spectrometry X-Ray Spectrometry Used for elemental analysis: identification and quantitation. Need free atom in gas phase for atomic analysis. Elements in the sample are converted to gas “gaseous atoms” or “elementary ions”  atomization Atomic species in vapor is measured.

4. Atomic Spectra (Energy Level Diagrams): Single External Electron * * Slightly different in energy Electron volts (eV) : the amount of energy gained (or lost) by a single electron moving across an electron potential of one volt 1 eV = 1.602 x 10 -19 J

5. Atomic Spectra: Singlet to Triplet Transition

6. Spin Orientations in Singlet Ground and Excited States and Triplet Excited States Spins Are Paired No Split Spins are Unpaired Energy Splitting Quantum mechanical selection rule : radiation producing transitions are observed only between certain energy states

7. Spin Multiplicity Multiplicity = 2 S + 1, where S is the angular spin momentum. Multiplicity is the number of possible orientations of spin angular momenta..so … S = ½ (no. of unpaired electrons in a shell) 1) Singlet: all electrons are paired, S = 0, therefore multiplicity = 2(0) + 1 = 1 2) Doublet: a molecule has 1 unpaired electron, S = ½ therefore, 2S + 1 = 2 3) Triplet: two unpaired electrons, S = 1 therefore, 2 S + 1 = 3 …and so on.

8. More spin momentum goodness…. Both the spin and the orbital motions create magnetic fields, owing to rotation of charge carried by the electron. When e - spin is parallel to orbital motion (L + S), E is High ( repulsive interaction b/w the fields ) When e - spin is opposite to orbital motion (L - S), E is Low ( attractive interaction b/w the field ) The magnitudes of such splitting for d and f orbitals are small A doublet line is observed for species containing single e- : Na, Mg 1+ , Al 2+ Higher no. of e-, complex spectra (e.g., Fe) have hundreds of such electron transitions as shown in the simple Na.

9. Types of Optical Atomic Spectra/ Energy Level Diagrams Atomic Emissions Spectra Atomic Absorption Spectra Atomic Fluorescence Spectra (three mechanisms)

10. Atomic Emissions Spectra 589.0 and 589.59 nm (3p 3s) Resonance lines: transitions between and excited and ground states. Na Unresolved doublet Partially resolved doublet Nearly complete doublet

11. Atomic Absorption Spectra Absorption spectrum consists of resonance lines, indicate transition from ground to upper levels. * *

12. Atomic Fluorescence Spectra Resonance fluorescence (Mg: 285.2 nm) Radiationless , then fluorescence Na: 330.3 nm  589.0, 589.6 nm) Fluorescence, then Ratiationless (Thallium: two fluorescence lines) Energy level diagram for thallium.

13. Line Broadening from the Uncertainty Effect Uncertainty Effects Heisenberg uncertainty principle The nature of the matter places limits on the precision with which certain pairs of physical measurements can be made. One of the important forms Heisenberg uncertainty principle: t ≥ 1 To determine  with negligibly small uncertainty, a huge measurement time is required. In flame, Doppler broadening is much larger than natural line width See Example 8-1

14. Line Broadening: Doppler Shift Atom moves toward photon detector and emits radiation, the detector sees wave crests more frequently and detects at a higher frequency… Atom moves away photon detector and emits radiation, the detector sees wave crests less frequently and detects at a lower frequency…

15. Pressure/Collision Broadening Caused by collisions of the emitting or absorbing species with other ions or atoms Example above: increase in pressure on CO 2 Additional example: High pressure Hg and xenon lamps, continuum spectra P

16. Temperature Effects on Atomic Spectra Boltzmann equation : Relates excited state Population/Ground State Population Ratios to Energy, Temperature, and Degeneracy Emission : requires close control of atomization methods (measurements are made on excited atoms). Absorption and Florescence : Theoretically less dependent on temperature. N j = no. of atoms in excited state N = no. of atoms in ground state k= 1.38 x10 -16 J/K (Boltzmann’s constant) Ej = btw ground and excited states g = statistical weight factors g= 2J+1 (J= L±S) N j /N o is exponentially related to T See Example 8-2

17. Band and Continuous Spectra Assoc. with Atomic Spectra Generally, when atomic line spectra are generated, both band and continuum radiation are produced as well.

18. How to get analytes to atomize?

19. How to get samples into the instruments?

20. Sample Introduction Pneumatic Nebulizers Break the sample solution into small droplets. Solvent evaporates from many of the droplets. Most (>99%) are collected as waste The small fraction that reach the plasma have been de-solvated to a great extent. Concentric Tube Cross-flow Babington Fritted-disk

21. Continuous Sample-Introduction Methods Nebulizer continually introduces samples into flames or plasma. Samples can be introduced into nebulizer by means of FIA or HPLC. In some systems, sample is introduced to flame by vapor generator (e.g. hydride generator or electrothermal vaporizer). FIA: Flow Injection Analysis HPLC: High-performance Liquid Chromatography

22. Atomization by Nebulizer is a Complex Process

23. Glow-Discharge Atomizer for Solid Samples

24. Suggested Questions/Problems 8-1, 8-2, 8-4 to 8-8 to 8-11