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Atomic Absorption Spectroscopy
Manikandan R Megha RajasekharRakesh M Badami
Introduction
• Atomic absorption spectroscopy is quantitative method of analysis of metals and some non-metals
• The technique was introduced in 1955 by Sir Alan Walsh in Australia
• Concentrations are found in g/mL range (ppm and ppb)
Principle
• In gaseous state atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels.
• The wavelength of each transition is specific to each element – qualitative analysis
• Beer-Lambert’s law can be applied to find concentrations using a calibration graph prepared from standards
• Absorbance is directly proportional to path length and concentration
Selection criteria for Sample preparation
The selection of a preparation method is dependent upon:• The analyte •The analyte concentration level•The sample matrix •The required sample size•The environmental considerations.
Sample Preparation
• Dilution – sample is diluted in distilled water, acids or organic solvent
• Decomposition – isolation of required element from the sample by heating with/without a reagent– Wet/acid decomposition (300C)– Dry ashing (400-500C )- destroying the combustible portion
of the sample. Oxidising agents may be used – Microwave decomposition (100-200C ) – sample
decomposed at high pressures in a Teflon container • Calibration curve must be prepared using different
concentrations of the sample
Working
• The atoms of the solid are converted to gaseous state in the atomiser
• Radiation of specific wavelength is emitted by the hollow cathode lamp onto the gaseous atoms in the atomiser
• The monochromator focuses the specific wavelengths onto the detector
• The detector finds the amount of light absorbed• The concentration of atoms in the sample is directly
proportional to the absorbance
Working
Instrumentation
Hollow Cathode Lamp
• Cathode is in the form of a hollow cylinder made of the metal which has to be analysed
• Anode is made of tungsten filament• They are sealed in a tube filled with inert gas like
Neon or Argon• A large voltage across anode and cathode causes
the inert gas to ionize and form a plasma• These ions are accelerated towards the cathode
causing atoms to be sputtered off
Hollow Cathode Lamp
• The ions and metal atoms are excited due to collisions
• They give off photons of a certain wavelength when they reach ground state
Nebuliser
• The nebuliser forms a mist or aerosol of the sample
• This is done by forcing the sample at high velocities through a narrow tube
• The sample is mixed with a fuel and oxidant• Commonly used fuel-oxidant mixtures are
acetylene-air and acetylene-nitrous oxide.
Nebuliser
Atomiser
• In the atomiser the sample solutions is vaporised and the molecules are atomised
• Atomiser can be of two types– Flame atomiser – Laminar consumption or total
flow– Electro-thermal atomiser or graphite furnace
Atomiser
• Flame atomiser– The flame is usually in the form of a sheet to
increase path length and hence increase the absorbance and sensitivity.
sample mist Solid/gas aerosol
Gaseous molecules Atom
Atomiser
• Flame atomiser– Laminar flow – a mixture of sample, fuel and
oxidant is continuously introduced to the burner head
– Total consumption – sample and fuel-oxidant mixture are delivered separately to the burner head. This is a much safer method and can be easily constructed.
Atomiser• Electro-thermal atomiser
Atomiser
• Electro-thermal atomiser– Graphite rods heated by passing current– Sample goes through three phases to get vaporised– Drying - the solvent is evaporated – Pyrolysis– Sample temperature is then increased rapidly to
vaporise it – Light is then passed through the sample
Interferences
• Chemical interference– Presence of thermally stable compound that is not
totally decomposed by the energy of the flame– High flame temperature provides energy for
breakdown on interference– Addition of releasing agent which reacts with the
interference
Interferences
• Ionization interferences– Atoms of the samples are
ionized causing reduction in number of electrons and absorbance
– Addition of excess element, like alkali elements, which gets ionized easily
– Flame temperature may be reduced
Interferences
• Matrix interference– Due to viscosity, burning characteristics, surface tension of
solvent– Due to usage of different solvents in calibration and sample– Addition of diluents to reduce viscosity
• Background absorption – Light scattering by particles in flame or absorption by
undissociated molecules– This must be measured and subtracted from final results– Absorption of elements occurs as a narrow line whereas
interference occurs over a broad range
Applications
• Level of metals could be detected in tissue samples like Aluminum in blood and Copper in brain tissues
• Presence of metals as an impurity or in alloys could be found easily
• Determination of elements in the agricultural and food products
• Determination of lead in petrol• Determination of calcium and magnesium in
cement
Advantages and Disadvantages
• Advantages– High sensitivity– Easy to use– Inexpensive
• Disadvantages– Different cathode lamp for different elements– Can detect only metals and some non metals– Only one element detected
References
• Analytical Methods for Atomic Absorption Spectroscopy – Perkin-Elmer
• Sample Preparation For Flame Atomic Absorption Spectroscopy: An Overview - Nabil Ramadan Bader
• Atomic absorption spectrometry – Royal Society of Chemistry
• NMSU web notes