AAS.pdf atomic absorption spectro scopy c

AAS.pdf atomic absorption spectro scopy c

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  ATOMIC ABSORPTION SPECTROMETER  Introduction Invention  Working Principle of AAS  Instrumentation  Interferences & Correlation Methods  Applications
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  INVENTION  Introduced in1955 by Alan Walsh in Australia  Firstly used for mining, medical treatment&agriculture  Alan Walsh(1916-1998) http://www.science.org.au/academy/memoi rs/walsh2.html
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  PROPERTIES OF AAS The most widely used method in analysisof elements  Based on the absorption of radiation  So sensitive (ppb)  Quantitative analysis
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  Qualitative & Quantitative Determine how much of certain elements are in a sample.  It uses the principlethat atoms (and ions) can absorb light at a specific, unique wavelength. When this specificwavelengthof light is provided, the energy (light) is absorbed by the atom. Electrons in the atom move from the ground state to an excited state.  The amount of light absorbed is measured  The concentration of the element in the sample can be calculated
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  An electron isexcitedfrom the ground state to higher energy level by absorbing energy (light) at a specific wavelength. In atomic absorption spectroscopy, the wavelength of absorbed light is determined by the type of atom (which element it is) and the energy levels the electrons are moving to. How much light is absorbed is determined by the concentrationof the element in the sample.
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  WORKING PRINCIPLE OFAAS  Electrons promote to higher orbitals for a short amount of time by absorbing a energy  M + hv → M*  Relises on Beer-Lambert Law A= a.b.c
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  Electron Energy Transitions Energyfrom the heatof the atomizercauses atomsto freely dissociate.  Theamount of energyrequired for theelectronsto move between energylevelscorrespondsto specific wavelengthsof light.  Moving an electron from the ground stateof a Pb atomto the first energylevel (E1) requires energyequivalentto lightat 283.3 nm.
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   It requiresmore energy to move an electron from the ground state to the second energy level (which is further away from the nucleus). For AAS analysis, the wavelengthof the ground state to the E1 level is frequently of most interest, as it is the most intense. A strong absorbance band gives the best (lowest) detection limits. In sampleswhere the concentration of the element is higher, an alternate wavelengthcan be used.
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   For Pbanalysis, a thin beam of light is passed through the flamecontaining the analyte. The beam contains light at 283.3 nm. The light is absorbed by Pb atoms as excitation of electrons from the Pb ground state to the first energy level occurs. The amount of absorption allowsa calculation of the concentration of Pb in the sample to be determined. Only free, ground state Pb atoms in the flame will absorb at 283.3 nm. Electrons moving between other energy levels in the Pb atoms will absorb light at different wavelengths.
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  Calibration graph forPb  This solution is called the ‘blank’ and determines the baseline absorption measurement. For example, 5, 10, 15, and 20 mg/L of Pb. The calibration curve determines the relationship between the absorbance of the light and the concentration of the element in the solution. Thiscurve follows the Beer-Lambert Law.
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  Working of AAS Creating a steady state of freely dissociated ground state atoms using a atomizer  Passing light of a specific wavelength through the flame. The wavelength corresponds to the amount of energy required to excite an electron from (typically) the ground to first excited state for a specific element.  Measuring the amount of the light absorbed by the atoms as they move to the excited state (the atomic absorption).  Using the measured absorbance to calculate the concentration of the element in a solution, based on a calibration graph.
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  A sample introductionsystem The burner (flame) and its associated gas supplies: air-acetylene or nitrous oxide- acetylene A light source, the hollow cathode lamp (HCL) A monochromator (the opticalcomponents inside the box in the diagram) An opticaldetector (photomultiplier tube or PMT) Computerizedinstrument control, data collection, and analysis.
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  Sample Introduction System The liquid sample is transported via capillary tubing into the nebulizer.  The pneumatic nebulizer →Venturi effect*  Sample then impacts a glass bead → fine spray of droplets (aerosol)  Larger droplets → to waste, while the fine aerosol → spray chamber  Equipped with mixing paddles → further remove large droplets → maintain a homogenous flow of fine droplets into the spray chamber and burner.  Mixing paddles → minimizing burner blockage ; ensure thorough mixing of the oxidant/acetylene gases with the sample droplets *the principle that fluid flows at a highervelocity through a narrowertube, to acceleratethe solution stream.
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  Drain trap tobe partially filled with liquid Float → to ensure that the liquid level is maintained. Spray chamber bung → safety device → safelyreleaseupon anyabnormal build-upof gases in the sample introduction system
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  ATOMIZATION  Compoundsmaking upthe sample are broken into free atoms.  High temperature is necessary  Basic two types -Flame atomizer -Electrothermal atomizer
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  TYPES OF ATOMIZERS FLAMEATOMIZER  Simplestatomization  Converts analyte into free atoms of vaporphase  Flammable &caustic gases  Not has an inert medium (−)  Short analysis time (−)
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  Flame AA spectrometerburner  Burner converts the aerosol/gas mixture created by the spray chamberand nebulizer, into free, ground state atoms.  There are two common gas mixtures that are burnt to fuel the flame.  air-acetylene → flame around 2300 °C, air-acetylene is suitable for most elements  nitrous oxide-acetylene → around 2700 °C, more reducing environment, suitable for elements that are prone to form oxides.
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  Main Stages ofSample Aerosol  Desolvation, or drying. The solvent is evaporated, resulting in dry nanoparticles of the sample remaining.  Vaporization. The particles are converted to the gaseous phase  Atomization. The key stage at which the population of ground state, freely dissociated atoms is created. Ground state atoms are the target for AAS analysis.  Ionization. Some, but not all, free atoms will be converted to ions. This will depend on the flame conditions (gas mix) and the ionization potential of the analytes on solution.
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  Features of Burner The burner is specially designed with a thin slit, that is 5–10 cm long, depending on the type of burner used.  The slit defines the length of the flame in the spectrometer where the population of free ground state atoms exist.  The length of the flame determines the light path passing through the atoms, which determines the sensitivity, according to the Beer-Lambert Law.  Burner height is also able to be raised or lowered → allows the light source to be passed through the area of the flame that provides the best sensitivity for the selected analyte.
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  Drawback of flameatomization  Inadequate sampling mechanism → Only a little portion of the sample aspirated through the atomization device (approximately 10%) reaches the flame  Sample is diluted with an enormous amount of gas, which transports the aerosol into the flame.  Many variables influence the production of atoms in the ground state, including flame temperature, interactions between flame gases, matrix components, and analyte, chemical interferences, and the extent to which the analyte molecular species are dissociated.  Free atoms are only present in the light path for a short period typically 10-4 seconds.  The residence time is proportional to the velocity of the flame gases
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  LIGHT SOURCES Hollow CathodeLamps  Anode-Tungsten wire  Cathode made from the element of interest (Na,K,Ca..)  Argon or neon gas http://www.safir.be/AAS.ht
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  Hollow Cathode Lamps Filled with an inert ‘filler’ gas at low pressure, usually argon or neon.  A metal cathode, coated in the element of interest, is positioned oppositean anode.  A high voltage is applied,across the two electrodes →which ionizes the filler gas accelerating ions toward the cathode.  The cathode is bombarded by these ions with enough energy that metal atoms from the cathode material are ejected or “sputtered” creating an atom plume.
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  HCL continued  Insidethe atom plume, further collisions between metal atoms occurs raising them to an excitation state.  When the atoms return to their preferred ground state, radiation is emitted, as light, at the characteristic wavelengths of that specific element.
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  Multielement Hollow Cathode Lamps The cathode of multielement lamps is made from alloying compatibleelements without overlapping line spectra. Examplesof such lengths are Ca-Mg,Cu-Fe- Ni, Cu-Fe-Mn-Zn,etc.  All elements of multielement hollow cathode lamps can be determined sequentiallywithout need for change of lamps in between. Multielementlamps provide advantagesof cost, speed of analysis but the sensitivity is lower in comparison to individual element determination bysingle element lamp
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  Limitations of HollowCathode Lamps  Hollow cathode lamps have a shelf life  With the exception of multi-element lamps the lamp needs to be changed for determination of different elements  Sputtering deposits metal atoms on sides and end windowswhich affects lamp life and more so for volatileelements  Some cathode materials liberate hydrogen on heating which contributes to continuum background emission
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  LIGHT SOURCES Electrodeless DischargeLamp  Typically argon gas at low pressure  Narrower line width  Not prefered www.freepatentsonline.com
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  Electrodeless Discharge Lamps In case of volatile elements reduced lamp lifeand low intensity can be overcome by use of high energy throughput electrodeless discharge lamps.  Electrodeless discharge lamps are commonly available for Sb, As, Bi, Cd, Cs, Pb, Hg, K, Rb, Sn, Te, etc. Electrodeless Discharge Lamp Schematic Diagram Electrodeless Discharge Lamp
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   An EDLconsists of a quartz bulb filled with an inert gas containing the element or a salt of the element for which the lamp is to be used.  The bulb is placed inside a ceramic cylinder on which antenna for a RF generator is coiled. When an RF field is appliedto the bulb, the inert gas is ionised and the coupled energy excites the vaporizedatoms inside the bulb and causes emission of characteristic light.  EDL’s offeradvantageof lower detection limits.  The useful life of an EDL is considerably longer than that of a hollow cathode lamp of same element.
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  TYPES OF ATOMIZERS ELECTROTHERMALATOMIZER A cylindirical graphite tube  Inert gas medium (Argon gas)  Longer anlaysis time than flame  Superiorsensitivity, high accuracy
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  Graphite Furnace Atomization Analyticalsensitivity can be greatly improved by atomizing the entire sampleat once  allowing free atoms to stay in the optical path for a greater duration of time.  These advantagesare provided through graphite furnace atomization.  A graphite furnace atomizer is also known as an electrothermal atomizer
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  Electrothermal Atomizer  Electricallyheated graphite tube in an argon chamber  Argon gas protects the graphite tubes from  oxidizing too quickly at high operating T  aids in the removal of matrix components  Removal of interfering species from the light path during the drying and ashing processes
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  1) externalgas flowinlet, (2) the externalgas flow outlet, (3) the internal gas flow outlet, (4) the internal gas flow inlet, (5) the light beam Graphite furnaceatomization (electrothermal atomization)
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  Stages of Atomization Drying: After injecting the sample into the graphite tube, it is dried at or near the boiling point of the solvent (typically between 80 and 200oC).  Charring/ashing: Temperature is raised to remove as much of the matrix material as possible without losing the analyte.  Atomization: furnace is rapidly heated to a high temperature to evaporate the ashing stage residues. This results in the formation of a cloud of free atoms in the optical path. During this stage, the absorbance is measured. The temperature of atomization is determined by the element’s volatility.
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  First dries thesampleand evaporates muchexcess solventand contaminants then atomizes it, and then rises it to an extremelyhigh temperatureto clean the graphite tube
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  Platform Atomization  Platformis mounted on the graphite tube only supported within the tube at the edges  Little physical contact between the tube and the platform  delay sampleatomization until the graphite tube has attained a stable (high) temperature.  Atomization of the analyte from the platform occurs in an high-temperature environment  One of the platform’sadvantagesallows for more freedom from interferences and background for volatileelements
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  Specialized Atomization Techniques  Afew elements are atomized by using a chemical reaction to produce a volatile product.  Elements such as As, Se, Sb, Bi, Ge, Sn, Te, and Pb, for example, form volatile hydrides when they react with NaBH4 in the presence of acid.  An inert gas carries the volatile hydride to either a flameor to a heated quartz observationtube situated in the optical path.
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  Cold-vapor method AAS Mercury is determined by the cold-vapormethod in which it is reduced to elemental mercury with SnCl2. The volatile Hg is carried by an inert gas to an unheated observationtube situated in the instrument’s optical path.
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  MONOCHROMATOR  Also calledwavelengh selector  Select the specific wavelenght  Polychromatic light →monochromatic light  Simple one is enough for AAS
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  DETECTOR  Electromagnetic waves→ electric current  Photomultiplier tube  Have fast response times www.answers.com
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  CALIBRATION TECHNIQUES • Twomain techniques ➢Calibration curve method ➢Standart addition method
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  CALIBRATION TECHNIQUES CALIBRATIONCURVE METHOD Draw a graph  Have two or more variables -One is set at known values -One is measured response  Most convenient for a large number of similar samples analysis.
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  CALIBRATION TECHNIQUES An exampleof calibration curve method
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  CALIBRATION TECHNIQUES STANDARTADDITION METHOD To measure the analyte concentration in a complex matrix.  Most convenient for small numberof samplesanalysis  Prevent effect of chemical & spectral interferences
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  INTERFERENCES  Causes higheror lower absorbancevalue  Two major groups ➢Chemical Interferences ➢Spectral Interferences
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  CHEMICAL INTERFERENCES  Themost common one in flame atomizer.  Consequenceof chemical reactions.  Reduce amount of oxygen in flame to overcome
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  SPECTRAL INTERFERENCES  Absorptionoremission of the radiation at the same wavelength  Radiation which is absorbed→pozitiveerrors  Radiation which is emmitted→negative errors
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  SPECTRAL CORRELATION METHODS TWO-LINECORRELATIONMETHOD  Select two line ➢characteristic wavelengthof analyte ➢very close to analyte line but not absorbed by analyte  Measure the difference between two lines
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  SPECTRAL CORRELATION METHODS CONTINUUMSOURCECORRELATION METHOD  Select two lamps ➢Deuterium lamp & hollow cathode lamp  When hollow cathode lamp is used total absorbace is measured  When deuterium lamp is used only background absorption is measured  Measure the difference between two lines.
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  SPECTRAL CORRELATION METHODS ZEEMANEFFECT CORRELATION METHOD  Presence of magnetic field.  Splitting of spectral lines.
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  APPLICATIONS OF AAS Water analysis (e.g. Ca, Mg, Fe, Si, Al, Ba content)  Food analysis  Analysisof animal feedstuffs (e.g. Mn, Fe, Cu, Cr, Se,Zn)  Analysisof soils  Clinical analysis (blood samples: whole blood, plasma,serum; Ca, Mg, Li, Na, K, Fe)