Transcript Document

Atomic spectra are a result of energy level
diagrams - quantum theory
Absorption vs. Emission
spectra
Atomic Line Width
•
1.
2.
3.
4.
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.
•
•
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
Hollow cathode lamp
• Cathode made of specified metal (solid) - atoms are
sputtered off and excited.
• Bulb contains low pressure (1-5 Torr) inert gas.
Line width and LOD
• The line width from your HCL must be
more narrow than the line width of
your sample atoms.
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
=
N0
Pj e (-Ej/kT)
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)
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)
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
Continuous vs. Discrete
• Flame AAS results in continuous
signal.
• ETV AAS results in discete signal.
Atomic spectroscopy is
selective.
• You know what you are looking for and you
measure that particular wavelength.