Principle of Atomic Absorption Spectrophotometry Mr. Charnchai Suracheep
Introduction Atomic Absorption Spectrophotometry, which are standard instruments for the determination of metal elements, are widely applied of samples, such as agriculture chemical, clinical and biochemistry, minerals, food and drugs,environmental and other.
Light absorption process of atoms Principle of Atomic Absorption Spectrophotometer Principle of the Atomic Absorption Method Atomized elements each absorb energy of a wavelength that is peculiar to that element. The atomic absorption method uses as its light source a hollow cathode lamp which emits light of a wavelength that is peculiar to each element. Elements within a solution are heated in a flame or electrically (2000K to 3000K) and subsequently determined using the fact that the degree of absorption will vary with its concentration.
Excited state E1 Excited state E1 Absorption Ground state E0 Ground state E0 e e e e Atomic Emission Spectroscopy, AES Emission Principle of Atomic Absorption Spectrophotometer Atomic Absorption Spectroscopy, AAS
Sodium (Na) energy states Electronic Transition Excited state (II) 3.6 eV Excited state (I) 2.2 eV 330.3 nm 589.0 nm Ground state 0.0 eV
Density C I0 I l Relation between light absorption and density • When light of a certain intensity is given to many atom in ground state, part of this light is absorbed by atoms.
Density C I0 I l Relation between light absorption and density Lambert-beer’s Law I = I0 e-k .l .C Abs = -logI/I0 = k .l. C k : proportional constant l : path length C : density (concentration)
Absorbance of unknown sample Concentration of unknown sample Relation between light absorption and density Calibration curve • Graph show the relation between absorbance and concentration Absorbance Concentration (ppm)
Atomization method • Atomic absorption spectrometry measures absorption of free atom. • “Free atom” means an atom not combined with other atoms. • Elements in the sample to be analyzed are not in the free state, and are combined with other elements invariably to make a so-called molecule.
Atomization method • The combination must be cut off by some means to free the atoms. • This is called “atomization” • 2 types: - Flame method - Flameless method
Flame Method With the Flame Method, the sample solution is converted into mist-form using a nebulizer, and then introduced into the flame. It is atomized by the temperature of the flame. Measurement time: A few dozen seconds Flame Atomization Method
Flame selection Flame Method • These flames vary in temperature, reducibility and transmission characteristics. • Selected according to the element being analyzed, and properties of the sample. • Argon-hydrogen : Max. temp. 1,577 0C • Air-hydrogen : Max. temp. 2,045 0C • Air-acetylene : Max. temp. 2,300 0C • Nitrous oxide-acetylene : Max. temp. 2,955 0C • (For elements are hard to combine with oxygen (Al, Si, V, Ti, etc.))
Flame Method Flame selection
Graphite holder Graphite cap Sample inlet Cooling block Aperture plate socket Graphite tube シール Seal Spring Fixing knob Eject arm Flameless Method (Graphite Furnace) Graphite tube
Flameless Method (Graphite Furnace) • Sample is injected in the formed graphite tube. • An electric current of 300 ampere (maximum) is applied to the tube.
Flameless Method (Graphite Furnace) • In an actual measurement heating is done in 3 stage. - Drying stage (100oC) • - Ashing stage (400-1000oC) - Atomizing stage (1400-3000oC)
Other atomic absorption methods • Methods having higher sensitivity than normal flame atomic absorption or electro-thermal atomic absorption • Used for special elements including arsenic, selenium and mercury. • Use chemical reactions in the process of atomization to vaporize in the form of an atom or simple molecule.
6BH4- +As3++ 3H+ 3B2H6+3H2 +AsH3(gas) Hydride Vapor Generation Technique • As, Se, Sb, Sn, Te, Bi, Hg and other metals produce a metal hydride by this method Absorption Cell Burner Head of AAS Peristaltic Pump Manifold Gas Liquid Separator Reaction Coil Drain Sample NaBH4 HCl Carrier Gas Ar Structural Diagram of Hydride Vapor Generator
253.6 nm Ho reduce SnCl2 + Hg2+ Hgo(gas) 5%KMnO4 5%H2SO4 Cold Vapor Technique SnCl2
Interference effects • Physical interference • Spectral interference • Chemical interference
Physical interference • Flame • Spray efficiency fluctuations due to difference in viscosity and surface tension between the standard and sample. • Furnace • Sample dispersion ; Measurement value fluctuations due to tube temperature distribution • Viscosity within the graphite furnace ; Adherence to sample tip causing errors in collection quantity. • Example: samples, such as blood or juice, containing numerous organic components.
Spectral interference • Spectral absorption line overlappingwith the absorption line of the target element. • Absorption and scattering by molecules
Spectral interference Spectral absorption line overlappingwith the absorption line of the target element.
Spectral interference • Absorption and scattering by molecules • Molecules absorption • Alkaline metals + Halogens = Alkali halides (Na, K)+(F, Cl, Br, I) = (Ex: NaCl, KI)
Chemical interference • Generation of non-separable compounds by coexisting matrices • Example : influence of PO4-, SO4-, SiO2 relative to Ca, Mg in flame analysis • (generation of Ca2PO4) • Generation of low boiling point compounds by coexisting matrices • Example: influence of chloride ions relative to Cd in furnace analyses • (generation of CdCl2)
Matrix modifier effect Masking of obstructing matrices Influence of phosphate on Ca is masked by La Conversion of obstructing matrices to compounds that easily undergo sublimation or evaporation Sublimation agent Example: removal of chloride ion by ammonium salt of nitric acid or phosphoric acid Conversion of measured elements to stable oxides or metallic intermediary compounds Stabilizing agent: Example: creation of measured element alloy using white metals (Pd, Pt, Rh)
Application examples of the matrix modifier method
100 ml 30 ml 20 ml 10 ml 10 ml 10 ml 10 ml 10 ml Mg concentration after filled up X X+0.1 X+0.2 X+0.3 Solvent Unknown sample 10 ml 1.0 ppm X Standard solution (ppm : mg/1000ml) Standard Addition Method No.4 No.1 No.2 No.3
Concentration of unknown sample Standard Addition Method Calibration Curve of Standard Addition Method
2-Way Background Correction is Standard Background Correction • D2 lamp method( 190-430 nm) – Molecular absorption • Self-Reversal (SR) method – Spectra interference
Elements/ wavelengths where spectral interference becomes problematic Background Correction Spectral interference
Background Correction Self-Reversal Method
Signal 100 mA 10 mA Background Background Correction Self-Reversal Method
High Performance Optical System Optical diagram of Double Beam System
Easy Switching between Flame and Furnace Flame -> Furnace: All that is involved is to remove the burner head, place the furnace unit, and fix it with the screw. No tools are required. Remove the burner head. Fit the furnace. Fit the burner head. Remove the furnace.
New Flame Atomizer For chemical resistance • Neburizer w/ Ceramic made Impact Bead • Polypropylene-made Chamber • Solid Titanium-made Burner Head
High Productivity • Full Auto ASC- Auto measurement up to 60 samples- Reagent addition 8 position - Automatic dilution • Optimize Flame analysis- Automatic search the best fuel gas flow rate- Automatic search the Optimize Flame analysis best burner height
High Temp. Burner Drain level sensor Enhanced Safety • Auto Gas Leak Check • Gas pressure monitoring to prevent flashback • Automatic flame monitoring • Automatic flame extinguish when power failure • Safety interlock for burner misuse • Auto Air/N2O flame changeover • Drain level sensor
* Select elements *Set the calibration curve and samples condition *Connect to PC *Set the spectrophotometer * Set the atomizer Wizard Software System
Automated/ Optimized Effectiveness of the automatic Line Search/Beam Balance
Burner height & Sensitivity (Cr) Effectiveness of the automatic burner height Automated/ Optimized (Cr : 4ppm standard solution used)
Search for the optimal fuel flow rate Automated/ Optimized (Cu : 4ppm standard solution used)
Calibration curve Signals in real-time Display of saved signal The 4 newest signals Screen during measurement
User Management • The Login ID and password need to be entered when the software is started up. • Records of who logged in at what time are preserved in the “Event Log”.