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Atomic spectra are a result of energy level diagrams - quantum theory

Atomic spectra are a result of energy level diagrams - quantum theory. Absorption vs. Emission spectra. Atomic Line Width. Although theoretically, the line width of an atomic line should be very narrow - 0.00001 nm (it’s quantized!) But…

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Atomic spectra are a result of energy level diagrams - quantum theory

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  1. Atomic spectra are a result of energy level diagrams - quantum theory

  2. Absorption vs. Emission spectra

  3. Atomic Line Width • Although theoretically, the line width of an atomic line should be very narrow - 0.00001 nm (it’s quantized!) • But… • Many environmental effects cause these lines to be broadened - 0.003 nm • Uncertainty Principle • Collisions with other atoms in gas phase • Doppler Effect • Zeeman Effect • The width of the line affects the LOD.

  4. Doppler and Zeeman • Doppler shift - frequency shifts due to movement of atom with respect to the detector • Zeeman effect - splitting of electronic states due to external EM field

  5. Hollow cathode lamp • Cathode made of specified metal (solid) - atoms are sputtered off and excited. • Bulb contains low pressure (1-5 Torr) inert gas.

  6. Line width and LOD • The line width from your HCL must be more narrow than the line width of your sample atoms.

  7. Boltzmann Equation • We use heat (flame, furnace) to atomize our samples. In emission spectrometry, this is also our means of excitation. How does the T of the flame affect the intensity of the emission? Nj = Pj e (-Ej/kT) N0 P0 • Nj = # atoms in excited state, N0 = # in ground state • Pj = # quantum states in excited state, P0 = # in ground state. i.e. for p orbitals, P = 6; for s orbitals, P = 2 • Ej = energy difference between ground and excited states (J) • k = Boltzmann constant = 1.28 x 10-23 J/K • T = Temperature (K)

  8. Element to be measured must be… • Vaporized • Nebulizer (solution) • Electrothermal vaporization, ETV (solution) • Can use microliters of solution instead of several mL • Hydride Generation (only certain elements who form easily vaporized hydrides) • Cold vapor (mercury) • Direct insertion (solid) • Laser ablation (solid)

  9. After it is vaporized it must be… • Atomized/Excited - must be in free atom state to excite the valence electrons • Hot flame (1700 - 3150 C) • ETV (1200 - 3000 C) • Plasmas - the fourth state of matter • Inductively coupled plasma, ICP (4000-6000 C) • Other plasmas - microwave-induced, DC current • Electric spark - 40,000 (?) - used in metallurgy

  10. Continuous vs. Discrete • Flame AAS results in continuous signal. • ETV AAS results in discete signal.

  11. Atomic spectroscopy is selective. • You know what you are looking for and you measure that particular wavelength.

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