atomic absorption spectroscopy (aas)

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