421-821-chapter-8-10

421-821-chapter-8-10 - Elemental Analysis - Atomic...

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Elemental Analysis - Atomic Spectroscopy A) Introduction Based on the breakdown of a sample into atoms, followed by the measurement of the atom’s absorption or emission of light. i. deals with absorbance fluorescence or emission (luminescence) of atoms or elemental ions rather then molecules - atomization : process of converting sample to gaseous atoms or elementary ions ii. Provides information on elemental composition of sample or compound - UV/Vis, IR, Raman gives molecular functional group information, but no elemental information. iii. Basic process the same as in UV/Vis, fluorescence etc. for molecules E o E 1 h ν Absorbance Fluorescence
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iv. Differences for Molecular Spectroscopy - no vibration levels much sharper absorbance, fluorescence, emission bands - position of bands are well-defined and characteristic of a given element - qualitative analysis is easy in atomic spectroscopy (not easy in molecular spectroscopy) Examples: carbon oxygen nitrogen
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B) Energy Level Diagrams energy level diagram for the outer electrons of an element describes atomic spectroscopy process. i. every element has a unique set of atomic orbitals ii. p, d, f split by spin-orbit coupling iii. Spin (s) and orbital (l) motion create magnetic fields that perturb each other (couple) - parallel higher energy; antiparallel lower energy Na Mg+ Similar pattern between atoms but different spacing Spectrum of ion different to atom Separations measured in electronvolts (eV) 1eV =1.602x10 J = 96.484 kJ mol • As number of electrons increases, number of levels increases emission spectra more complex Li 30 lines Cs 645 lines Cr 2277 lines Note slight differences in energy due to magnetic fields caused by spin
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C) Desire narrow lines for accurate identification Broadened by i. uncertainty principle Uncertainty principal: t . h E ˆ t . ∆ν l 1 t – minimum time for measurement ∆ν – minimal detectable frequency difference Peak line-width is defined as width in wavelength at half the signal intensity
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Doppler effect - emitted or absorbed wavelength changes as a result of atom movement relative to detector - wavelength decrease if motion toward receiver - wavelength increases if motion away from receiver Usage in measurement of velocity of galaxies, age of universe and big bang theory C) Desire narrow lines for accurate identification Broadened by ii. Doppler effect
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Pressure broadening : Collisions with atoms/molecules transfers small quantities of vibrational energy (heat) - ill-defined ground state energy Effect worse at high pressures : • For high pressure Xe lamps (>10,000 torr) turns lines into continua! C) Desire narrow lines for accurate identification Broadened by iii. Pressure broadening
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- temperature changes number of atoms in ground and excited states - need good temperature control Boltzmann equation N 1 and N o – are the number of atoms in excited and ground states k – Boltzmann constant (1.28x10 -23 J/K) T – temperature
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This note was uploaded on 05/09/2010 for the course CHEM 221 taught by Professor Dr.robertpowers during the Fall '07 term at San Diego.

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421-821-chapter-8-10 - Elemental Analysis - Atomic...

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