FIU CHM 4130 - CHAPTER 9_Xiao_AAS_and_AFS_2018 (70 pages)

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CHAPTER 9_Xiao_AAS_and_AFS_2018



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From molecular to elemental analysis there are three major techniques used for elemental analysis Optical spectrometry Mass spectrometry X ray spectrometry Chapter 9 Atomic Absorption and fluorescence Spectroscopy Both AA and AF require a light source Like Molecular Absorption Fluorescence in AA high intensity is not required in AF high intensity results in greater sensitivity Sample Atomization Atomic Absorption AA Atomization Fluorescence AF Absorption E1 E0 Molecular Absorption Emol Eelectronic Evibration Erotation Ej Ei Atomic Absorption Emol Eelectronic www docstoc com Docs Document Detail Google aspx doc id 42331615 What happens inside the flame Sample processes occurring during atomization 1 Atomiza on in Flame MgCl2 solution MgCl2 S MgO S MgCl2 S or MgO S MgCl2 g or MgO g MgCl2 g or MgO g Mg0 Cl0 O0 toc com Docs Document Detail Google aspx doc id 42331615 Evaporation Volatilization Atomization FLAMES Regions in a flame Rich in free atoms Temperature profile oxidation Variation due to the degree of oxidation for a given element A different portion of the flame should be used for the determination of each element Atomiza on cell Transport sample aerosol to flame Desolvation Atomization 1 Flame Atomizers Cheap Simple Flame stability Low temperature Flame Atomizers Disadvantages Low sampling efficiency n Only about 5 reach the flame and large portion of the sample flows down the drain n The residence time of individual atoms in the optical path in the flame is brief 10 4 s Used for atomic absorption fluorescence and emission spectroscopy Why are electroThermal atomizers Short atomization period Long average residence time of the atoms s 2 ElectroThermal Atomizers The sample is contained in a heated graphite furnace The furnace is heated by passing an electrical current through it thus it is electro thermal To prevent oxidation of the furnace it is sheathed in gas Ar usually There is no nebulziation etc The sample is introduced as a drop usually 5 20 uL slurry or solid particle rare Electrothermal Atomizers Advantages High sensitivity for small volumes of sample n Disadvantages n 5 10 relative precision n Furnace n Narrow methods are slow analytical range Used for atomic absorption fluorescence ElectroThermal Atomizers The furnace goes through several steps Drying usually just above 110 deg C Ashing up to 1000 deg C Atomization Up to 2000 3000 C Cleanout quick ramp up to 3500 C or so Waste is blown out with a blast of Ar The light from the source HCL passes through the furnace and absorption during the atomization step is recorded over several seconds This makes ETAAS more sensitive than FAAS for most elements Radiation Sources for AAS Radiation Sources for AAS Requirement of radiation source in AAS Give light at the specific wavelength Emission bandwidth is narrower than atomic absorption line 0 01 nm Sufficient intensity Hollow Cathode Lamp Conventional HCL Ne or Ar at 1 5 Torr Hollow Cathode Lamps Hollow Cathode Lamp Cont d High potential and thus high currents lead to greater intensities Doppler broadening of the emission lines from the lamp Self absorption the greater currents produce an increased number of unexcited atoms in the cloud The unexcited atoms in turn are capable of absorbing the radiation emitted by the excited ones This self absorption leads to lowered intensities particular at the center of the emission band Electrodeless Discharged Lamps Electrodeless discharge lamps EDL Constructed from a sealed quartz tube containing a few torr of an inert gas such as argon and a small quantity of the metal of interest or its salt The lamp does not contain an electrode but instead is energized by an intense field of radio frequency or microwave radiation Radiant intensities usually one or two orders of magnitude greater than the normal HCLs The main drawbacks their performance does not appear to be as reliable as that of the HCL lamps signal instability with time and they are only commercially available for some elements 15 Atomic Spectrophotometers Elimina on of Emission from Flame Problem Con nuous emission from ame Atomic line emission from atom in ame How to solve the problem Place monochromator between ame and detector modula on of radia on source Chopper AC operated source Single Beam Design Double Beam Design Note the Ref bean does not pass through the flame thus does not correct for the interferences from the flame synchronized Interferences in AAS and AFS 1 Spectral Interferences 2 Chemical Interferences Spectral Interference Spectral Interferences Overlapping Broadening absorption for air fuel mixture Scattering or absorption by sample matrix Spectral Interference Overlapping Atomic absorp on line of other elements Vanadium 308 211 nm Aluminum 309 27 nm 308 215 nm Aluminum Spectral Interference Overlapping Analyte absorp on line with matrices absorp on non speci c molecular absorp on Background Correction Two line Correction like Internal Standard not very common Use of D2 or H2 lamp Continuum Source Correction very common Source Self Reversal Smith Hieftje relatively new Zeeman Effect common for graphite furnace instruments 1 Two line Correction Reference line should have the following properties 1 Very close to analyte line 2 Not adsorbed by analyte Simple method but it is difficult to locate a suitable reference line Emission spectrum of source Sample absorption spectrum 2 D2 Background Correction AA BG by HCL and BG by D2 lamp so the difference is AA 2 D2 Background Correction The most common method but 1 Exact alignment of HCL and D2 lamps might be a problem 2 Limit the use of the technique at 350 nm 3 Addition of an extra lamp and chopper will decrease the signal to noise ratio 4 Zeeman E ect A background correction based on Zeeman effect Correction through Zeeman effect Two solutions 1 With fixed magnetic field and rotating polarizer 2 With or without magnetic field and a fixed polarizer Background Correction System Inverse transverse dc Zeeman background correction With fixed magnetic field and rotating polarizer Background Correction System Inverse transverse ac Zeeman background correction With or without magnetic field and a fixed polarizer How does Zeeman effect work Advantages and Disadvantages Advantages of the Zeeman technique Measurement of total and background absorption on the same wavelength Correction of rapid and structured background No special lamps required Correction over the entire wavelength range Better signal to noise ratio


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