Ch. 9 Atomic Absorption & Atomic Fluorescence Spectrometry 9.1 9A. Atomization Most fundamental for both techniques. Typical types 1. flame - burner type 2. Electrothermal graphite furnace 3. Specialized glow discharge hydride atom cold-vapor atomization 9A. Atomization 9.2 Process during atomization
9A-1. Flame Atomization 9.3 Fuel with air oxidant: 1700~2400 o C for easy decomposing Fuel with O 2 or N 2 O : 2500~3100 o C 9.4 Flame structure Interzonal area :Atoms are rich widely used Sec. comb. zone :oxidation to oxides Pr. Comb. zone :blue in hydrocarbon Flame, Band spectra of C 2, CH
9A-1. Flame Atomization 9.5 flame absorbance profile Oxidation of Mg to MgO Flame height must be adjusted for maximum absorption 9A-1. Flame Atomization 9.6 flame atomizers : laminar flow burners : quiet, long path length Sampling efficiency Reproducible Sensitivity better than others Weak points : sample loss by drain : residence time at beam ~ 10-4 s
9A-2. Electrothermal Atomization 1970s 9.7 : entire sample atomized in short period (~L) 9A-2. Electrothermal Atomization 9.8 Procedures Evaporation (low T) Ash (high T) Current ramping: 2000~3000 o C Atomization (~ms) Performances of ETA - High sensitivity for small volume (0.5~10L) - D.L.: 10-10 ~ 10-13 g - Relative precision nonflame flame 5~10% 1~2% - Analytical range : 2 orders of magnitude - Applicable only when flame atomization provides inadequate D.L.
9A-3. Specialized Techniques 9.9 Glow Discharge Atomization Sample: conductor or pelleted with powdered conductor (finely ground graphite or Cu) D.L. : low ppm for solids DC 250~1000V causes Ar into Ar + + e- By electric field, Ar + hits cathode (sample) SPUTTERING : Eject neutral sample atoms 100g/min 9A-3. Specialized Techniques 9.10 Hydride Atomization Heating at Quartz is necessary A hydride generation & atomization System for AAS NaBH 4 Cold Vapor Atomization only for Hg or vaporizable metal in R.T
9B. Atomic Absorption Spectrometry Instrumentation 9.11 9B-1. Radiation Sources Uses narrow line sources (0.002~0.005nm) for ex. to analyze Na (589.6nm) use same emission line from other source (sodium vapor by electrical charge) - Need separate lamp for each element 9B. Atomic Absorption Spectrometry 9.12 Hollow Cathode Lamp (HCL) ~300V : made with same metal to be analyzed Gaseous cations from inert gas (Ar + ) accelerated to cathode and detach metal atoms from cathode - sputtering Then Excited atom emits characteristic radiation
9B. Atomic Absorption Spectrometry 9.13 Electrodeless Discharge Lamp (EDL) Intensities 10~100 times larger than HCL -Ionization of gas by Radiowave or microwave - 15 or more elements - not as reliable as HCL 9B. Atomic Absorption Spectrometry 9.14 Source Modulation : to eliminate interferences caused by flame emission high pass RC filter or metal disk or chopper rotation : source is switched on and off at constant freq.
9B-2. Spectrometers 9.15 Typical flame spectrophotometers Single beam Double beam 9C. Interferences in AAS 9.16 Spectral interferences: absorption or emission of interfering species are so close in resolution Chemical interferences: from various chemical processes during atomization that alter absorption
9C-1. Spectral Interferences 9.17 i) Spectral overlapping when analyzing V (3082.11A o ) with Al (3082.15 A o ) choose 3092.7A o of Al ii) the presence of combustion products (broad band absorption) from fuel or oxidant causes scattering by particulate materials --- blank measurement 9C-1. Spectral Interferences 9.18 iii) from matrix In case of Ba in matrix (containing Ca)- CaOH -- use N 2 O iv) Scattering by refractory oxides : Ti, Zr, W in flame method not common -- by varying temperature (change fuel-ox. ratio)
9C-1. Spectral Interferences 9.19 Matrix interference problem is severe to reduce or correct for the interferences Two-line correction method using a spectrum line close to analyte s line as a reference line but not be absorbed by analyte : reference line exhibit absorption or scattering by the matrix product Then subtract the signal from the measurement 9C-1. Spectral Interferences 9.20 Continuum-source background correction : use continuum light source alternatively with HCL : attenuation of the continuum (D 2 ) reflects only the background absorption or scattering Error sources -S/N degradation by lamp, chopper -Lamp alignment positive or negative error -In visible region (>350nm) D 2 output is low
9C-1. Spectral Interferences 9.21 Background correction based on Zeeman Effect Zeeman effect : under magnetic field (~10kG) atomic E levels split (~0.01nm) - lines : singlet, central, at original only absorbs radiation polarized in parallel direction to magnetic field -2 lines : slightly longer and shorter to orig. :absorbs only radiation polarized at 90 o to the field : only show broad-band molecular absorption or scattering Subtract sigma from pi absorption Good for electrothermal atomizers 9C-1. Spectral Interferences 9.22 Background correction based on Zeeman Effect
9C-2. Chemical Interferences 9.23 Formation of compounds of low volatility anions: SO 4 2-, PO 4 3-, Ca 2+ cations: Al Al 2 O 3 Mg - MgO solve by using - High T - Releasing agent i.e. St, La - react preferentially with interf. Anions phosphate in determining Ca. - protective agent by forming stable but volatile species with analyte EDTA, 8-hydroxyquinoline in case of Al, Si, phosphate, sulfate in deter. Ca 9C-2. Chemical Interferences 9.24 Dissociation Equilibria Using high T to decompose and convert to gaseous M Ionization Equilibria Under air oxidant ionization negligible In high temp. (O 2 or N 2 O used) free e - formed M M e [ M ][ e K [ M ] 2 ] P 1 : fraction of ionzed M P: partial pressure of metal in gaseous solvent before ionization
9C-2. Chemical Interferences 9.25 In case of ionization of other metals free electrons lower ionization When B, M metal B [ e B ] [ B e ] [ M Ionization suppressor suppress ionization of analyte by adding ions of high ionization potential K. ] 9D. Analytical Techniques 9.26 1) Sample prep. decomposition & solution step Hot mineral acids Oxidation with strong acid Combustion in an O 2 bomb Ashing High Temp 2) Organic solvents. signal enhanced with low MW solvent (MeOH, esters, ketones) due to increased nebulization (lower surface tension smaller droplets) rapid solution evaporation 3) Calibration --- std. addition method for matrix effect 4) Application ~60 metals D.L. flame 1~20 ng/ml or 0.001 ~ 0.020 ppm EA 0.002~0.01 ng/ml or 2x10-6 ~ 1x10-5 ppm Accuracy flame 1~2% in Relative error EA 5~10 times
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