FIU CHM 4130 - CHAPTER 8_Xiao_Introduction_to_Optical_Atomic_Spectrometry_2018 (1) (45 pages)

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CHAPTER 8_Xiao_Introduction_to_Optical_Atomic_Spectrometry_2018 (1)



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Optical Atomic Spectroscopy The elements present in a sample are atomized by atomization process Optical Spectrometry Absorption Emission Fluorescence Mass Spectrometry X Ray Spectrometry An iron atom and its four unpaired electrons Orbitals and Electrons Electrons exist at distinct orbitals and have spin Motion of the electrons absorption or emission measured Energy Level Diagrams Every elements has unique set of atomic orbitals p d f levels split by spin orbit coupling Spin s and orbital l motion create magnetic fields that perturb each other couple if fields parallel slightly higher energy if fields antiparallel slightly lower energy N N N S S N vs S S Define SO coupling by J total angular momentum Na atom and Na ion Valence Electron Energy level diagrams single external electron Energy level diagram for atomic magnesium Spin orientations Spins are paired No split Spins are unpaired Energy splitting Similar pattern between atoms but different spacing Spectrum of ion different to atom Separations measured in electronvolts eV 1eV 1 602x10 19 C 1 V J C 1 602x10 19 J 96 484 kJ mol 1 As of electrons increases of levels increases Emission spectra become more complex Li 30 lines Cs 645 lines Cr 22 CEM 333 page 7 3 Atomic spectroscopy Absorption Emission Fluorescence Atomic Absorption Spectra Energy level diagrams Absorption 3s 3s 4p 3p Atomic Emission Spectra Energy level diagrams Emission 4d 3p 3p 3s Atomic Fluorescence Spectra 4p 3p 3p 589 0 nm and 589 6 nm 330 3 nm 285 2 nm 3s 3s Mg Na Atomic Fluorescence Spectra Resonance fluorescence Nonresonance fluorescence Line Broadening Desire narrow lines for accurate identification Broadened by i uncertainty principle ii pressure broadening iii Doppler effect iv electric and magnetic fields Atomic line widths Atomic line widths 1 Uncertainty Effects i Uncertainty Principle Heisenberg uncertainty principle Quantum idea places stateslimits must measure Themechanical nature of the matter on the precision for with some which mi time to certain tell two apart can be made pairsfrequencies of physical measurements One of the important forms Heisenberg uncertainty t E h principle t p156 1 t 1 minimum minimum t minimum time for measurement time for detectable minimum detectable difference in frequencies measurement difference in To determine with negligibly small uncertainty frequencies a huge measurement time is required Shows up in lifetime of excited state if lifetime infinitely long E infinitely narrow if lifetime short E is broadened 2 Pressure broadening ii Pressure broadening 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 10 1 10 2 For high pressure Xe lamps 10 000 torr 100 1000 turns lines into continua iii Doppler broadening Change in frequency produced by motion relative to detector 3 Doppler ii broadening Pressure broadening Collisions with atoms molecules transfers small quantities of Change in frequency produced by motion relative to detector vibrational energy heat ill defined ground state energy Effect worse at high pressures For low pressure hollow cathode lamps 1 10 torr 10 1 10 2 For high pressure Xe lamps 10 000 torr 100 1000 turns lines into continua iii Doppler broadening Change in frequency produced by motion relative to detector In gas broadens line symmetrically Doppler broadening increases with T At room T 10 2 10 3 3 Doppler shift The wavelength of radiation emitted or absorbed by a rapidly moving atom decreases if the motion is toward a transducer and increases if the motion is receding from the transducer c In flame Doppler broadening is much larger than natural line width Temperature Effect on Atomic Spectra T changes of atoms in ground and excite states Bolzmann equation Nj N0 gj g0 exp Nj N0 gj g0 E j E j kT exp kT Effects on AAS AFS and AES Temperature Effect on Atomic Absorption and Emission Nj No 1 72 x 10 4 172 excited atoms for each 106 atoms in ground state Sugges ng a very high popula on of the ground state even at high temperatures atoms in level kT transition energy E1 E0 N0 P0 N1absorption P1 E fluorescence Atomic emission and 23 J K 1 levels at each energy Boltzmann constant 1 38x10 exp atoms in level transition energy E1 E0 kT N0 P0 ofeach atomic is dependent the 23 J K 1 Signal levels at energyemissionBoltzmann constanton 1 38x10 atoms in level transition energy E1 E0 Important in emission measurements relying on number of atoms in the excited state thermal excitation levels at each energy Boltzmann constant 1 38x10 23 J K 1 Important in emission measurements relying on thermal excitation Na atoms at 2500 K only 0 02 atoms in first excited state Important in emission measurements relying on thermal excitation NaSignal atoms of at 2500 K only 0 02 atoms in fluorescence first excited state atomic absorption and is Less important in absorption measurements 99 98 atoms in dependent on K theonly number atoms inexcited the ground ground state Na atoms at 2500 0 02 of atoms in first state Less important in absorption measurements 99 98 atoms in state that will absorb energy ground state Less important in absorption measurements 99 98 atoms in ground state Higher temperature 1 Better sensitivities increase number of vaporized atoms 2 Increased velocities of gaseous atoms Line broadening 3 Increased number of ionized analyte Decreased number of atoms Band and ConBnuum Spectra Atomic spectroscopy Interaction of an atom in the gas phase by its electromagnetic or mass spectrum Samples are solids liquids and gases but usually not ATOMS Atomic Spectroscopy n Sample Introduction Flame Furnace ICP n Sources for Atomic Absorption Fluorescence Hollow Cathode Lamps n Sources for Atomic Emission Flames Plasmas n Wavelength Separators Slits Detectors How to get samples into the instruments Easy for gases solutions but difficult for solids How to get things to atomize Sample must be converted to atoms first 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 What is a nebulizer SAMPLE AEROSOL Concentric Tube Cross flow Fritted disk Babington IntroducBon of liquid Samples Ultrasonic nebulizers Electrothermal VaporizaBon Hydirde GeneraBon Techniques IntroducBon of Solid Samples Direct


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