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Atomic Absorption and Atomic Fluorescence Yongsik Lee Slide 2 Introduction AAS and AFS similarity Sample introduction Atomization AFS has not gained widespread general use for routine elemental analysis Slide 3 9A. Sample atomization tech Flame atomization Electrothermal atomization Specialized atomization techniques Glow discharge atomization Hydride atomization Cold vapor atomization Slide 4 Flame Atomization Neubulization the sample solution is dispersed into tiny droplets. Desolvation the solvent of the solution is evaporated. The finely divided solid aerosol is mixed with a fuel and an oxidant. common fuels: natural gas, hydrogen, acetylene common oxidants: air, oxygen, nitrous oxide Volatilization The sample is burned in a flame produced by the fuel and oxidants to form gaseous molecules. The sample is burned in a flame produced by the fuel and oxidants to form gaseous molecules. Temperatures ranging from 1700 C to 3150 C are produced depending on the fuel/oxidant combination. At such a high temperature, the gaseous molecule (MX) can be atomized (MX --> M) and ionized (M --> M+). and ionized (M --> M+). Energy of a particular wavelength will be used to excite the molecule, atom, and ions. Changes in the energy level can be measured for quantitative determination. Slide 5 Scheme of flame atomization Slide 6 Types of Flames Fuel Natural gas Hydrogen Acetylene Oxidant Air 1700-2400 Oxygen 2500-3100 Nitrous oxide 2500-3100 Table 9-1 Properties of Flames Burning velocity important for flame stability Flashback flame propagate back into the burner Blowing off the burner at higher flow rates Slide 7 Flame structure Flame profile Figure 9-2 & 9-3 Primary combustion zone Blue luminescence of C2, CH, and other radicals Internal region Rich in free atoms Secondary combustion zone Products of the inner core are converted to stable molecular oxides Slide 8 Temperature profile Figure 9-3 Max temperature Location about 1 cm above the primary combustion zone Optical focus to this region Slide 9 Flame absorbance profiles Ag (not readily oxidized) Continuous increase from the flame Cr (forms stable oxides) Continuous decrease Oxide formation dominant Mg Have a maximum Use proper portion of the flame for maximum absorbance Slide 10 Flame atomizer laminar flow burner Slide 11 Multiple-slot premixed CH4/air burner Slide 12 Flame Diagnostics Laser-induced fluoresence measurements of OH, CH, CN and H2CO radicals in low and atmospheric pressure flames rag.web.psi.ch/htdz/Www_Homepage/ Comb_Diag_Exp.ht Slide 13 Performance Characteristics Reproducible behavior Best to all other methods for liquid samples Sampling efficiency (= Sensitivity) bad Sample flows down the drain Residence time in the optical path in the flame is brief (= 1/10000 s) Slide 14 Slide 15 Electro-thermal Atomization An Electro-thermal Atomizer consists a cylindrical graphite tube connected to an electrical power supply. An Electro-thermal Atomizer consists a cylindrical graphite tube connected to an electrical power supply. A small amount (0.5 to 10mL) of sample is introduced and heated electrically. Two stage heating: At lower temperature: the sample is evaporated and ashed Rapid increase in temperature: volatilization and atomization Used in ICP, AAS, AFS Slide 16 Electrothermal atomization Advantage less reproducible results than flame atomization Atomizie in short time residence time in the optical path is long( Electrothermal Graphite Furnace Gas flow external Ar gas prevents tube destruction internal Ar gas circulates gaseous analyte Three step sample preparation for graphite furnace Dry - evaporation of solvents (10->100 s) Ash - removal of volatile hydroxides, sulfates, carbonates (10-100 s) Fire/Atomize - atomization of remaining analyte (1 s) Slide 20 ETA output signal sample Canned orange juice 2 L Drying 20 s Ashing 60 s Standards lead High speed data acquisition possible Rapid (