HISTORY

• The technique was introduced in 1955 by Alan Walsh in Australia ( 1916 – 1998 ).

• The first commercial atomic absorption spectrometer was introduced in 1959.

The application of atomic absorption spectra to chemical

analysis

PRINCIPLEAtomic absorption spectroscopy is a method of

elemental analysis. It is particularly useful for determining trace metals in liquids and is most independent of molecular form of the metal in sample.

It is the absorption phenomenon.When a sample solution is aspirated into the

flame, the solvent is vaporised leaving particles of the solid salt.

The salt burns and converted into the gasses form which is dissociated into free neutral atoms and some of them excited by the flame heat. Remaining atoms in the ground state absorbs light of specific wavelength emitted by the lamp of same element to be determined and the intensity of the light absorption is measured.

Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals.It is very reliable and simple to use.It can analyze over 62 elements.

Elements detectable by atomic absorption are highlighted in pink in this periodic table

BOLTZMANN DISTRUBUTION EQUATION

N*/No = e - ∆E/kT

N* – number of excited atomsN

o – number of unexcited atoms∆E – difference in energies of two levelsK – Boltzmann constantT – temperature of the flame

INSTRUMENTATION Radiation source Chopper Atomiser Monochromators Detector Amplifier Read out device

Types of AAS

Radiation source1. Hollow Cathode Lamp (HCL)

The cathode consists of a hollow cup. In the cup is the element which is determined.

The anode is tungsten wire. The two electrodes keep in a tube containing an inert gas (Ne or Ar).

The lamp window is constructed of quartz, silica, or glass.

When a potential (300-500 V) is applied between two electrodes, The electric discharge ionizes the inert gas (Ne or Ar usually) atoms filled in the hollow tube, which in turn, are accelerated into the cathode with high velocity and sputter metal atoms into the gas phase. Metal atoms become excited and after returning to ground state give rise to metal emission spectrum.

The neon or helium gas in the hollow cathode lamp performs three functions

It dislodges atoms from the surface of the cathode responsible for excitation of the ground state metal atoms It is main source of current carrying capacity in the HCL.

• The pressure maintained in the lamp is 1 to 5 torr.• The spectral lines produced by the HCL are so narrow

that they are completely absorbed by the atoms.• Each hollow cathode lamp emits the spectrum

of the metal which is used in the cathode, for example copper cathode emits copper spectrum which is absorbed by copper atoms.

2. Electrodeless Discharge Lamp (EDL) It is difficult to make stable hollow cathode

lamp from certain elements particularly those that are volatile, such as arsenic, germanium.

An alternative light has been developed in the EDL. It consists of an evacuated tube in which the metal of interest is placed. The tube is filled with argon at low pressure and sealed off. The sealed tube is then placed in microwave discharge cavity.

Under these conditions the argon becomes a plasma and cause excitation of the metal sealed inside the tube. The emission from the metal is that of its spectrum.

CHOPPER A rotating wheel is interposed between the hollow

cathode lamp and flame .This rotating wheel is known as chopper.

It is interposed to break the steady light coming from the lamp into pulsating light which is used to measure the intensity of light absorbed by elements without interference by radiation from the flame itself.

Pulsating light gives pulsating current in photocell. There is also steady current caused by light which is emitted by flame. But only pulsating current is amplified and recorded.

ATOMISER Atomization is separation of particles into

individual molecules and breaking molecules into atoms .This is done by exposing the analyte to high temperatures in a flame or graphite furnace

Atomiser converts the liquid into small droplets which are easily vaporised.

Types of atomisers :- 1. Flame atomiser:- a.) Total consumption burner b.) premixed burner 2. Non-flame atomiser

a). Total consumption burner In this whole sample is atomised into the flame,

hence named as total consumption burner.

Disadvantages Noisy and hard to use.

In this burner, the sample solution, the fuel, and oxidizing gases are passed through separate passages to meet at the opening of the base of flame. Then the flame breaks the sample in liquid form into the droplets which are evaporated and burns. Leaving the residue which is reduced to atoms. Fuel used – H2

/acetylene Oxidant – O 2

b). Premixed burner It is most widely used because of uniformity in flame

intensity. In this the sample solution ,fuel and oxidant are

mixed before they reach the tip. The fine droplets get carried out along with the fuel

gas at outlet, the large drops of sample get collected in chamber and are drained out.Advantages

Non-turbulent Noiseless StableDisadvantagesOnly 5% sample reaches to the flame and rest 95% is wasted.

Temperature of some flamesFuel oxidant Temperature (K)H2 Air 2000-2100

C2H2 Air 2100-2400H2 O2 2600-2700

C2H2 N2O 2600-2800

For some elements that form refractory oxides (molecules hard to break down in the flame) nitrous oxide (N2O) needs to be used instead of air (78% N2 + 21% O2) for the oxidant. In that case, a slightly different burner head with a shorter burner slot length is used.

Non flame atomiser The graphite furnace is an electro thermal

atomiser system that can produce temperatures as high as 3,000°C. The heated graphite furnace provides the thermal energy to break chemical bonds within the sample held in a graphite tube, and produce free ground state atoms.

The ground-state atoms are capable of absorbing energy, in the form of light, and are elevated to an excited state. The amount of light energy absorbed increases as the concentration of the selected element increases.

Non flame atomiser

GRAPHITE TUBE ATOMIZER:

• uses a graphite coated furnace to vaporize the sample.

• ln GFAAS sample, samples are deposited in a small graphite coated tube which can then be heated to vaporize and atomize the analyte.

• The graphite tubes are heated using a high current power supply.

NEBULIZATION

Before the liquid sample enters the burner ,it is converted into droplets this method a formation of small droplets its called nebulization

Common method of nebulization is by use of gas moving at high velocity, called pneumatic nebulization.

MONOCHROMATORSThis is a very important part in an AA

spectrometer. It is used to separate out all of the thousands of lines.

Without a good monochromator, detection limits are severely compromised.

A monochromator is used to select the specific wavelength of light which is absorbed by the sample, and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others. They are of two types:1) Prism 2) Grating

Prism monochromator :- Quartz material is used for making prism, as quartz is transparent over entire region

Grating monochromator :- it consists of a series of parallel straight lines cut into a plane surface

DETECTORS

READ OUT DEVICE

• In the most of AAS measurement, chart recorders are used as read out device. A chart recorder is a potentiometer.

INTERFERENCE Interference is a phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude. Interference decrease the intensity of absorption of light . Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.Types of interferences1) Spectral interference2) Chemical interference3) Physical interference

Spectral interferences Spectral interferences arise when the absorption or

emission of an interfering species either overlaps or lies so close to the analyte absorption or emission that resolution by the monochromator becomes impossible. ex:-Manganese triplet (4031,4033,4035Å) overlapped by gallium line(4033Å).

This interference can be corrected by amplitude modulation of the source.

Chemical interference Occurs due to incomplete dissociation of compounds in

the flame when the concentration of compound is more. Removed by use of higher flame temperature. Chemically –by addition of more thermally stable

compound. ex-addition of lanthanum to the aluminium and magnesium for detection of magnesium

Physical interference :-viscosity -solvent -ionization1) Viscosity – viscosity is invertionaly proportional to the

intensity of absorption.2) Solvent – organic solvent increases the intensity and

aqueous solvent decreases the intensity of absorption.3) Ionization – occurs due to high flame temperature. A

number of vaporized atoms become ionized by the flame. Resulting ions absorb at a different wavelength than the vaporized atoms the new wavelength will not be selected by the monochromator and low results occurs.

Na Na+ + e-

Overcome by addition of more easily ionizable element Ex- ionization interference of Na is corrected by the addition of Potassium to the Sodium .

APPLICATIONClinical analysis: Analysing metals in biological fluids such as blood and urine. Environmental analysis: Monitoring our environment – eg finding out the levels of various elements in rivers, seawater, drinking water, air, petrol and drinks such as wine, beer and fruit drinks. Pharmaceuticals: In some pharmaceutical manufacturing processes, minute quantities of a catalyst used in the process (usually a metal) are sometimes present in the final product. By using AAS the amount of catalyst present can be determined. Industry: Many raw materials are examined and AAS is widely used to check that the major elements are present and that toxic impurities are lower than specified – eg in concrete, where calcium is a major constituent, the lead level should be low because it is toxic. Mining: By using AAS the amount of metals such as gold in rocks can be determined to see whether it is worth mining the rocks to extract the gold.

Calibration Curve

• A calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The absorbance of each known solution is measured and then a calibration curve of concentration vs absorbance is plotted.

• The sample solution is fed into the instrument, and the absorbance of the element in this solution is measured .The unknown concentration of the element is then calculated from the calibration curve